[v4,1/6] Documentation: arm: define DT idle states bindings

On Fri, Jun 13, 2014 at 06:33:35PM +0100, Nicolas Pitre wrote:
> On Fri, 13 Jun 2014, Lorenzo Pieralisi wrote:
> 
> > On Wed, Jun 11, 2014 at 07:15:16PM +0100, Nicolas Pitre wrote:
> > > Let's illustrate the different periods on a time line to make it clearer
> > > (hmmm let's see how this can be managed on a braille display :-O ):
> > > 
> > > EXEC:	Normal CPU execution.
> > > 
> > > PREP:	Preparation phase before committing the hardware to idle mode
> > > 	like cache flushing. This is abortable on pending wake-up 
> > > 	event conditions. The abort latency is assumed to be negligible 
> > > 	(i.e. less than the ENTRY + EXIT duration). If aborted, we go 
> > > 	back to EXEC. This phase is optional. If not abortable, this 
> > > 	should be included in the ENTRY phase instead.
> > > 
> > > ENTRY:	The hardware is committed to idle mode. This period must run to
> > > 	completion up to IDLE before anything else can happen.
> > > 
> > > IDLE:	This is the actual power-saving idle period. This may last 
> > > 	between 0 and infinite time, until a wake-up event occurs.
> > > 
> > > EXIT:	Period during which the CPU is brought back to operational
> > > 	mode (EXEC).
> > > 
> > > ...__[EXEC]__|__[PREP]--|__[ENTRY]__|__[IDLE]__|___[EXIT]_--|__[EXEC]__...
> > >              |          |           |          |            |
> > > 
> > >              |<-- entry-latency --->|
> > > 
> > >                                                |<- exit-  ->|
> > >                                                |  latency   |
> > > 
> > >              |<-------------- min-residency --------------->|
> > > 
> > >                         |<----- worst_wakeup_latency ------>|
> > > 
> > > entry-latency: Worst case latency required to enter the idle state.  The 
> > > exit_latency may be guaranteed only after entry-latency has passed.
> > > 
> > > min-residency: Minimum period, including preparation, entry and exit, 
> > > for a given power mode to be worthwhile energy wise.  It must be at 
> > > least equal to entry_latency + exit_latency.

Ok, a minor tweak to the diagram above, min-residency should include
energy costs related to idle entry and exit, but not the exit-latency
itself, as long as the energy costs implied by exiting the state are
factored out in the min-residency-us property.

Hence, to sum it up, I attached below the updated bindings patch:

I think we are close to an agreement, if anyone disagrees please shout
as soon as possible so that we can still integrate changes.

Thanks,
Lorenzo

-- >8 --
Subject: [PATCH] Documentation: arm: define DT idle states bindings

ARM based platforms implement a variety of power management schemes that
allow processors to enter idle states at run-time.
The parameters defining these idle states vary on a per-platform basis forcing
the OS to hardcode the state parameters in platform specific static tables
whose size grows as the number of platforms supported in the kernel increases
and hampers device drivers standardization.

Therefore, this patch aims at standardizing idle state device tree bindings for
ARM platforms. Bindings define idle state parameters inclusive of entry methods
and state latencies, to allow operating systems to retrieve the configuration
entries from the device tree and initialize the related power management
drivers, paving the way for common code in the kernel to deal with idle
states and removing the need for static data in current and previous kernel
versions.

Reviewed-by: Sebastian Capella <sebcape@gmail.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
---
 Documentation/devicetree/bindings/arm/cpus.txt     |   8 +
 .../devicetree/bindings/arm/idle-states.txt        | 561 +++++++++++++++++++++
 2 files changed, 569 insertions(+)
 create mode 100644 Documentation/devicetree/bindings/arm/idle-states.txt

On Wed, Jun 18, 2014 at 10:36 AM, Lorenzo Pieralisi
<lorenzo.pieralisi@arm.com> wrote:
> On Fri, Jun 13, 2014 at 06:33:35PM +0100, Nicolas Pitre wrote:
>> On Fri, 13 Jun 2014, Lorenzo Pieralisi wrote:
>>
>> > On Wed, Jun 11, 2014 at 07:15:16PM +0100, Nicolas Pitre wrote:
>> > >
>> > > ...__[EXEC]__|__[PREP]--|__[ENTRY]__|__[IDLE]__|___[EXIT]_--|__[EXEC]__...
>> > >              |          |           |          |            |
>> > >              |<-- entry-latency --->|
>> > >                                                |<- exit-  ->|
>> > >                                                |  latency   |
>> > >              |<-------------- min-residency --------------->|
>> > >                         |<----- worst_wakeup_latency ------>|
>> > >
>> > > entry-latency: Worst case latency required to enter the idle state.  The
>> > > exit_latency may be guaranteed only after entry-latency has passed.
>> > >
>> > > min-residency: Minimum period, including preparation, entry and exit,
>> > > for a given power mode to be worthwhile energy wise.  It must be at
>> > > least equal to entry_latency + exit_latency.
>
> Ok, a minor tweak to the diagram above, min-residency should include
> energy costs related to idle entry and exit, but not the exit-latency
> itself, as long as the energy costs implied by exiting the state are
> factored out in the min-residency-us property.

This makes sense to me..

It includes accounting for the energy cost vs WFI of prep/entry/exit,
but timing is from the end of the previous exec, until the event is
expected to trigger.

Thanks!

Sebastian
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On Wednesday 18 June 2014 01:36 PM, Lorenzo Pieralisi wrote:
> On Fri, Jun 13, 2014 at 06:33:35PM +0100, Nicolas Pitre wrote:

[..]
> Ok, a minor tweak to the diagram above, min-residency should include
> energy costs related to idle entry and exit, but not the exit-latency
> itself, as long as the energy costs implied by exiting the state are
> factored out in the min-residency-us property.
> 
> Hence, to sum it up, I attached below the updated bindings patch:
> 
> I think we are close to an agreement, if anyone disagrees please shout
> as soon as possible so that we can still integrate changes.
> 

[..]

> 
> -- >8 --
> Subject: [PATCH] Documentation: arm: define DT idle states bindings
> 
> ARM based platforms implement a variety of power management schemes that
> allow processors to enter idle states at run-time.
> The parameters defining these idle states vary on a per-platform basis forcing
> the OS to hardcode the state parameters in platform specific static tables
> whose size grows as the number of platforms supported in the kernel increases
> and hampers device drivers standardization.
> 
> Therefore, this patch aims at standardizing idle state device tree bindings for
> ARM platforms. Bindings define idle state parameters inclusive of entry methods
> and state latencies, to allow operating systems to retrieve the configuration
> entries from the device tree and initialize the related power management
> drivers, paving the way for common code in the kernel to deal with idle
> states and removing the need for static data in current and previous kernel
> versions.
> 
> Reviewed-by: Sebastian Capella <sebcape@gmail.com>
> Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
> ---
Nice work Lorenzo !!
I have few comments/questions.

>  Documentation/devicetree/bindings/arm/cpus.txt     |   8 +
>  .../devicetree/bindings/arm/idle-states.txt        | 561 +++++++++++++++++++++
>  2 files changed, 569 insertions(+)
>  create mode 100644 Documentation/devicetree/bindings/arm/idle-states.txt
> 
> diff --git a/Documentation/devicetree/bindings/arm/cpus.txt b/Documentation/devicetree/bindings/arm/cpus.txt
> index 1fe72a0..a44d4fd 100644
> --- a/Documentation/devicetree/bindings/arm/cpus.txt
> +++ b/Documentation/devicetree/bindings/arm/cpus.txt
> @@ -215,6 +215,12 @@ nodes to be present and contain the properties described below.
>  		Value type: <phandle>
>  		Definition: Specifies the ACC[2] node associated with this CPU.
>  
> +	- cpu-idle-states
> +		Usage: Optional
> +		Value type: <prop-encoded-array>
> +		Definition:
> +			# List of phandles to idle state nodes supported
> +			  by this cpu [3].
>  
>  Example 1 (dual-cluster big.LITTLE system 32-bit):
>  
> @@ -411,3 +417,5 @@ cpus {
>  --
>  [1] arm/msm/qcom,saw2.txt
>  [2] arm/msm/qcom,kpss-acc.txt
> +[3] ARM Linux kernel documentation - idle states bindings
> +    Documentation/devicetree/bindings/arm/idle-states.txt
> diff --git a/Documentation/devicetree/bindings/arm/idle-states.txt b/Documentation/devicetree/bindings/arm/idle-states.txt
> new file mode 100644
> index 0000000..c9e1ec6
> --- /dev/null
> +++ b/Documentation/devicetree/bindings/arm/idle-states.txt
> @@ -0,0 +1,561 @@
> +==========================================
> +ARM idle states binding description
> +==========================================
> +
> +==========================================
> +1 - Introduction
> +==========================================
> +
> +ARM systems contain HW capable of managing power consumption dynamically,
> +where cores can be put in different low-power states (ranging from simple
> +wfi to power gating) according to OSPM policies. The CPU states representing
s/OSPM/OS PM ?
> +the range of dynamic idle states that a processor can enter at run-time, can be
> +specified through device tree bindings representing the parameters required
> +to enter/exit specific idle states on a given processor.
> +
> +According to the Server Base System Architecture document (SBSA, [3]), the
> +power states an ARM CPU can be put into are identified by the following list:
> +
> +- Running
> +- Idle_standby
> +- Idle_retention
> +- Sleep
> +- Off
> +
> +The power states described in the SBSA document define the basic CPU states on
> +top of which ARM platforms implement power management schemes that allow an OS
> +PM implementation to put the processor in different idle states (which include
> +states listed above; "off" state is not an idle state since it does not have
> +wake-up capabilities, hence it is not considered in this document).
> +
> +Idle state parameters (eg entry latency) are platform specific and need to be
> +characterized with bindings that provide the required information to OSPM
Ditto
> +code so that it can build the required tables and use them at runtime.
> +
> +The device tree binding definition for ARM idle states is the subject of this
> +document.
> +
> +===========================================
> +2 - idle-states node
> +===========================================
> +
> +ARM processor idle states are defined within the idle-states node, which is
> +a direct child of the cpus node [1] and provides a container where the
> +processor idle states, defined as device tree nodes, are listed.
> +
> +- idle-states node
> +
> +	Usage: Optional - On ARM systems, is a container of processor idle
s/is/it is ?
> +			  states nodes. If the system does not provide CPU
> +			  power management capabilities or the processor just
> +			  supports idle_standby an idle-states node is not
> +			  required.
> +
> +	Description: idle-states node is a container node, where its
> +		     subnodes describe the CPU idle states.
> +
> +	Node name must be "idle-states".
> +
> +	The idle-states node's parent node must be the cpus node.
> +
> +	The idle-states node's child nodes can be:
s/idle-states/idle-state
> +
> +	- one or more state nodes
> +
> +	Any other configuration is considered invalid.
> +
> +	An idle-states node defines the following properties:
> +
> +	- entry-method
> +		Usage: Required
> +		Value type: <stringlist>
> +		Definition: Describes the method by which a CPU enters the
> +			    idle states. This property is required and must be
> +			    one of:
> +
> +			    - "arm,psci"
> +			      ARM PSCI firmware interface [2].
> +
> +			    - "[vendor],[method]"
> +			      An implementation dependent string with
> +			      format "vendor,method", where vendor is a string
> +			      denoting the name of the manufacturer and
> +			      method is a string specifying the mechanism
> +			      used to enter the idle state.
> +
> +The nodes describing the idle states (state) can only be defined within the
> +idle-states node, any other configuration is considered invalid and therefore
> +must be ignored.
> +
> +===========================================
> +3 - state node
> +===========================================
> +
> +A state node represents an idle state description and must be defined as
> +follows:
> +
> +- state node
> +
> +	Description: must be child of the idle-states node
> +
> +	The state node name shall follow standard device tree naming
> +	rules ([5], 2.2.1 "Node names"), in particular state nodes which
> +	are siblings within a single common parent must be given a unique name.
> +
> +	The idle state entered by executing the wfi instruction (idle_standby
> +	SBSA,[3][4]) is considered standard on all ARM platforms and therefore
> +	must not be listed.
> +
> +	To correctly specify idle states timing and energy related properties,
> +	the following definitions identify the different execution phases
> +	a CPU goes through to enter and exit idle states and the implied
> +	energy metrics:
> +
> +	..__[EXEC]__|__[PREP]__|__[ENTRY]__|__[IDLE]__|__[EXIT]__|__[EXEC]__..
> +		    |          |           |          |          |
> +
> +		    |<------ entry ------->|
> +		    |       latency        |
> +						      |<- exit ->|
> +						      |  latency |
> +		    |<-------- min-residency -------->|
> +			       |<-------  wakeup-latency ------->|
> +
I don't know the wakeup latency makes much sense and also correct.
Hardware wakeup latency is actually exit latency. Is it for failed
or abort-able ilde case ? We are adding this as a new parameter
at least from idle states perspective. I think we should just
avoid it.

> +	EXEC:	Normal CPU execution.
> +
> +	PREP:	Preparation phase before committing the hardware to idle mode
> +		like cache flushing. This is abortable on pending wake-up
> +		event conditions. The abort latency is assumed to be negligible
> +		(i.e. less than the ENTRY + EXIT duration). If aborted, CPU
> +		goes back to EXEC. This phase is optional. If not abortable,
> +		this should be included in the ENTRY phase instead.
> +
> +	ENTRY:	The hardware is committed to idle mode. This period must run
> +		to completion up to IDLE before anything else can happen.
> +
> +	IDLE:	This is the actual energy-saving idle period. This may last
> +		between 0 and infinite time, until a wake-up event occurs.
> +
> +	EXIT:	Period during which the CPU is brought back to operational
> +		mode (EXEC).
> +
> +	With the definitions provided above, the following list represents
> +	the valid properties for a state node:
> +
> +	- compatible
> +		Usage: Required
> +		Value type: <stringlist>
> +		Definition: Must be "arm,idle-state".
> +
> +	- logic-state-retained
> +		Usage: See definition
> +		Value type: <none>
> +		Definition: if present logic is retained on state entry,
> +			    otherwise it is lost.
> +
> +	- cache-state-retained
> +		Usage: See definition
> +		Value type: <none>
> +		Definition: if present cache memory is retained on state entry,
> +			    otherwise it is lost.
> +
> +	- entry-method-param
> +		Usage: See definition.
> +		Value type: <u32>
> +		Definition: Depends on the idle-states node entry-method
> +			    property value. Refer to the entry-method bindings
> +			    for this property value definition.
> +
> +	- entry-latency-us
> +		Usage: Required
> +		Value type: <prop-encoded-array>
> +		Definition: u32 value representing worst case latency in
> +			    microseconds required to enter the idle state.
> +			    The exit-latency-us duration may be guaranteed
> +			    only after entry-latency-us has passed.
> +
> +	- exit-latency-us
> +		Usage: Required
> +		Value type: <prop-encoded-array>
> +		Definition: u32 value representing worst case latency
> +			    in microseconds required to exit the idle state.
> +
> +	- min-residency-us
> +		Usage: Required
> +		Value type: <prop-encoded-array>
> +		Definition: u32 value representing minimum residency duration
> +			    in microseconds, inclusive of preparation and
> +			    entry, for this idle state to be considered
> +			    worthwhile energy wise.
> +			    The residency time must take into account the
> +			    energy consumed while entering and exiting the
> +			    idle state and is therefore expected to be
> +			    longer than entry-latency-us.
> +
> +	- wakeup-latency-us:
> +		Usage: Optional
> +		Value type: <prop-encoded-array>
> +		Definition: u32 value representing maximum delay between the
> +			    signaling of a wake-up event and the CPU being
> +			    able to execute normal code again. If omitted,
> +			    this is assumed to be equal to:
> +				entry-latency-us + exit-latency-us
> +
Rest of the patch looks fine by to me.

regards,
Santosh

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On Wed, 18 Jun 2014, Santosh Shilimkar wrote:

> On Wednesday 18 June 2014 01:36 PM, Lorenzo Pieralisi wrote:
> [..]
> > +	To correctly specify idle states timing and energy related properties,
> > +	the following definitions identify the different execution phases
> > +	a CPU goes through to enter and exit idle states and the implied
> > +	energy metrics:
> > +
> > +	..__[EXEC]__|__[PREP]__|__[ENTRY]__|__[IDLE]__|__[EXIT]__|__[EXEC]__..
> > +		    |          |           |          |          |
> > +
> > +		    |<------ entry ------->|
> > +		    |       latency        |
> > +						      |<- exit ->|
> > +						      |  latency |
> > +		    |<-------- min-residency -------->|
> > +			       |<-------  wakeup-latency ------->|
> > +
> I don't know the wakeup latency makes much sense and also correct.
> Hardware wakeup latency is actually exit latency. Is it for failed
> or abort-able ilde case ? We are adding this as a new parameter
> at least from idle states perspective. I think we should just
> avoid it.

I explained the rationale for this parameter in a previous email but 
Lorenzo didn't carry it over. To be clearer, this should be "worst case 
wake-up latency".  It is of interest for PMQOS.  This is the maximum 
delay that can be expected from the moment a wake-up event is signaled 
and the moment the CPU is back operational.  This is more than just exit 
latency.  By default this is entry_latency + exit_latency but when there 
is an abortable PREP phase then it may be shorter than that.


Nicolas
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On Wednesday 18 June 2014 04:51 PM, Nicolas Pitre wrote:
> On Wed, 18 Jun 2014, Santosh Shilimkar wrote:
> 
>> On Wednesday 18 June 2014 01:36 PM, Lorenzo Pieralisi wrote:
>> [..]
>>> +	To correctly specify idle states timing and energy related properties,
>>> +	the following definitions identify the different execution phases
>>> +	a CPU goes through to enter and exit idle states and the implied
>>> +	energy metrics:
>>> +
>>> +	..__[EXEC]__|__[PREP]__|__[ENTRY]__|__[IDLE]__|__[EXIT]__|__[EXEC]__..
>>> +		    |          |           |          |          |
>>> +
>>> +		    |<------ entry ------->|
>>> +		    |       latency        |
>>> +						      |<- exit ->|
>>> +						      |  latency |
>>> +		    |<-------- min-residency -------->|
>>> +			       |<-------  wakeup-latency ------->|
>>> +
>> I don't know the wakeup latency makes much sense and also correct.
>> Hardware wakeup latency is actually exit latency. Is it for failed
>> or abort-able ilde case ? We are adding this as a new parameter
>> at least from idle states perspective. I think we should just
>> avoid it.
> 
> I explained the rationale for this parameter in a previous email but 
> Lorenzo didn't carry it over. To be clearer, this should be "worst case 
> wake-up latency".  It is of interest for PMQOS.  This is the maximum 
> delay that can be expected from the moment a wake-up event is signaled 
> and the moment the CPU is back operational.  This is more than just exit 
> latency.  By default this is entry_latency + exit_latency but when there 
> is an abortable PREP phase then it may be shorter than that.
> 
PMQOS angle is right. It is just that the idle code is not
going to do anything with this value. But I see a value adding it
instead of some one doing calculation.

Thanks for clarity Nico !!

regards,
Santosh

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On Wed, 18 Jun 2014, Lorenzo Pieralisi wrote:

> On Fri, Jun 13, 2014 at 06:33:35PM +0100, Nicolas Pitre wrote:
> > On Fri, 13 Jun 2014, Lorenzo Pieralisi wrote:
> > 
> > > On Wed, Jun 11, 2014 at 07:15:16PM +0100, Nicolas Pitre wrote:
> > > > Let's illustrate the different periods on a time line to make it clearer
> > > > (hmmm let's see how this can be managed on a braille display :-O ):
> > > > 
> > > > EXEC:	Normal CPU execution.
> > > > 
> > > > PREP:	Preparation phase before committing the hardware to idle mode
> > > > 	like cache flushing. This is abortable on pending wake-up 
> > > > 	event conditions. The abort latency is assumed to be negligible 
> > > > 	(i.e. less than the ENTRY + EXIT duration). If aborted, we go 
> > > > 	back to EXEC. This phase is optional. If not abortable, this 
> > > > 	should be included in the ENTRY phase instead.
> > > > 
> > > > ENTRY:	The hardware is committed to idle mode. This period must run to
> > > > 	completion up to IDLE before anything else can happen.
> > > > 
> > > > IDLE:	This is the actual power-saving idle period. This may last 
> > > > 	between 0 and infinite time, until a wake-up event occurs.
> > > > 
> > > > EXIT:	Period during which the CPU is brought back to operational
> > > > 	mode (EXEC).
> > > > 
> > > > ...__[EXEC]__|__[PREP]--|__[ENTRY]__|__[IDLE]__|___[EXIT]_--|__[EXEC]__...
> > > >              |          |           |          |            |
> > > > 
> > > >              |<-- entry-latency --->|
> > > > 
> > > >                                                |<- exit-  ->|
> > > >                                                |  latency   |
> > > > 
> > > >              |<-------------- min-residency --------------->|
> > > > 
> > > >                         |<----- worst_wakeup_latency ------>|
> > > > 
> > > > entry-latency: Worst case latency required to enter the idle state.  The 
> > > > exit_latency may be guaranteed only after entry-latency has passed.
> > > > 
> > > > min-residency: Minimum period, including preparation, entry and exit, 
> > > > for a given power mode to be worthwhile energy wise.  It must be at 
> > > > least equal to entry_latency + exit_latency.
> 
> Ok, a minor tweak to the diagram above, min-residency should include
> energy costs related to idle entry and exit, but not the exit-latency
> itself, as long as the energy costs implied by exiting the state are
> factored out in the min-residency-us property.

s/factored out /factored in/

> Hence, to sum it up, I attached below the updated bindings patch:
> 
> I think we are close to an agreement, if anyone disagrees please shout
> as soon as possible so that we can still integrate changes.

Comments:

[...]

> +- state node
> +
> +	Description: must be child of the idle-states node
> +
> +	The state node name shall follow standard device tree naming
> +	rules ([5], 2.2.1 "Node names"), in particular state nodes which
> +	are siblings within a single common parent must be given a unique name.
> +
> +	The idle state entered by executing the wfi instruction (idle_standby
> +	SBSA,[3][4]) is considered standard on all ARM platforms and therefore
> +	must not be listed.
> +
> +	To correctly specify idle states timing and energy related properties,
> +	the following definitions identify the different execution phases
> +	a CPU goes through to enter and exit idle states and the implied
> +	energy metrics:
> +
> +	..__[EXEC]__|__[PREP]__|__[ENTRY]__|__[IDLE]__|__[EXIT]__|__[EXEC]__..
> +		    |          |           |          |          |
> +
> +		    |<------ entry ------->|
> +		    |       latency        |
> +						      |<- exit ->|
> +						      |  latency |
> +		    |<-------- min-residency -------->|
> +			       |<-------  wakeup-latency ------->|
> +
> +	EXEC:	Normal CPU execution.
> +
> +	PREP:	Preparation phase before committing the hardware to idle mode
> +		like cache flushing. This is abortable on pending wake-up
> +		event conditions. The abort latency is assumed to be negligible
> +		(i.e. less than the ENTRY + EXIT duration). If aborted, CPU
> +		goes back to EXEC. This phase is optional. If not abortable,
> +		this should be included in the ENTRY phase instead.
> +
> +	ENTRY:	The hardware is committed to idle mode. This period must run
> +		to completion up to IDLE before anything else can happen.
> +
> +	IDLE:	This is the actual energy-saving idle period. This may last
> +		between 0 and infinite time, until a wake-up event occurs.
> +
> +	EXIT:	Period during which the CPU is brought back to operational
> +		mode (EXEC).
> +
> +	With the definitions provided above, the following list represents
> +	the valid properties for a state node:
[...]

I really think the definitions and timing diagram ought to be 
prominently presented at the beginning of the document in a separate 
section for it rather than being burried in a binding section.  Extra 
discussion points from this thread could go there as well, i.e. the 
reason for each timing parameter, etc.  The latest comment from Santosh 
shows that this is never too clear.

For example, your explanation of what the min residency represents at 
the top of this email should belong to such a section.  Similarly for 
the worst wake-up latency.

Then we could refer to it when defining binding parameters.


Nicolas
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On Wed, 18 Jun 2014, Santosh Shilimkar wrote:

> On Wednesday 18 June 2014 04:51 PM, Nicolas Pitre wrote:
> > On Wed, 18 Jun 2014, Santosh Shilimkar wrote:
> > 
> >> On Wednesday 18 June 2014 01:36 PM, Lorenzo Pieralisi wrote:
> >> [..]
> >>> +	To correctly specify idle states timing and energy related properties,
> >>> +	the following definitions identify the different execution phases
> >>> +	a CPU goes through to enter and exit idle states and the implied
> >>> +	energy metrics:
> >>> +
> >>> +	..__[EXEC]__|__[PREP]__|__[ENTRY]__|__[IDLE]__|__[EXIT]__|__[EXEC]__..
> >>> +		    |          |           |          |          |
> >>> +
> >>> +		    |<------ entry ------->|
> >>> +		    |       latency        |
> >>> +						      |<- exit ->|
> >>> +						      |  latency |
> >>> +		    |<-------- min-residency -------->|
> >>> +			       |<-------  wakeup-latency ------->|
> >>> +
> >> I don't know the wakeup latency makes much sense and also correct.
> >> Hardware wakeup latency is actually exit latency. Is it for failed
> >> or abort-able ilde case ? We are adding this as a new parameter
> >> at least from idle states perspective. I think we should just
> >> avoid it.
> > 
> > I explained the rationale for this parameter in a previous email but 
> > Lorenzo didn't carry it over. To be clearer, this should be "worst case 
> > wake-up latency".  It is of interest for PMQOS.  This is the maximum 
> > delay that can be expected from the moment a wake-up event is signaled 
> > and the moment the CPU is back operational.  This is more than just exit 
> > latency.  By default this is entry_latency + exit_latency but when there 
> > is an abortable PREP phase then it may be shorter than that.
> > 
> PMQOS angle is right. It is just that the idle code is not
> going to do anything with this value. But I see a value adding it
> instead of some one doing calculation.

The idle code should take it into account when a PMQOS restriction is in 
effect i.e. avoid using those modes whose worst case wake-up latency is 
too large.

And cpuidle is being migrated into the scheduler as we speak.  So some 
of the values there, namely entry_latency and exit_latency (taken 
separately for timing purposes) will be directly used by the scheduler 
to decide which CPU to wake up for example.

So there is fundamentally 4 parameters if we want to comprehensively 
support all pertinent use cases.


Nicolas
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On Wednesday 18 June 2014 05:09 PM, Nicolas Pitre wrote:
> On Wed, 18 Jun 2014, Santosh Shilimkar wrote:
> 
>> On Wednesday 18 June 2014 04:51 PM, Nicolas Pitre wrote:
>>> On Wed, 18 Jun 2014, Santosh Shilimkar wrote:
>>>
>>>> On Wednesday 18 June 2014 01:36 PM, Lorenzo Pieralisi wrote:
>>>> [..]
>>>>> +	To correctly specify idle states timing and energy related properties,
>>>>> +	the following definitions identify the different execution phases
>>>>> +	a CPU goes through to enter and exit idle states and the implied
>>>>> +	energy metrics:
>>>>> +
>>>>> +	..__[EXEC]__|__[PREP]__|__[ENTRY]__|__[IDLE]__|__[EXIT]__|__[EXEC]__..
>>>>> +		    |          |           |          |          |
>>>>> +
>>>>> +		    |<------ entry ------->|
>>>>> +		    |       latency        |
>>>>> +						      |<- exit ->|
>>>>> +						      |  latency |
>>>>> +		    |<-------- min-residency -------->|
>>>>> +			       |<-------  wakeup-latency ------->|
>>>>> +
>>>> I don't know the wakeup latency makes much sense and also correct.
>>>> Hardware wakeup latency is actually exit latency. Is it for failed
>>>> or abort-able ilde case ? We are adding this as a new parameter
>>>> at least from idle states perspective. I think we should just
>>>> avoid it.
>>>
>>> I explained the rationale for this parameter in a previous email but 
>>> Lorenzo didn't carry it over. To be clearer, this should be "worst case 
>>> wake-up latency".  It is of interest for PMQOS.  This is the maximum 
>>> delay that can be expected from the moment a wake-up event is signaled 
>>> and the moment the CPU is back operational.  This is more than just exit 
>>> latency.  By default this is entry_latency + exit_latency but when there 
>>> is an abortable PREP phase then it may be shorter than that.
>>>
>> PMQOS angle is right. It is just that the idle code is not
>> going to do anything with this value. But I see a value adding it
>> instead of some one doing calculation.
> 
> The idle code should take it into account when a PMQOS restriction is in 
> effect i.e. avoid using those modes whose worst case wake-up latency is 
> too large.
> 
> And cpuidle is being migrated into the scheduler as we speak.  So some 
> of the values there, namely entry_latency and exit_latency (taken 
> separately for timing purposes) will be directly used by the scheduler 
> to decide which CPU to wake up for example.
> 
> So there is fundamentally 4 parameters if we want to comprehensively 
> support all pertinent use cases.
> 
Fair enough.

regards,
Santosh

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> -----Original Message-----
> From: Santosh Shilimkar [mailto:santosh.shilimkar@ti.com]
> Sent: 18 June 2014 20:27
> To: Lorenzo Pieralisi; Nicolas Pitre
> Cc: linux-arm-kernel@lists.infradead.org; linux-pm@vger.kernel.org;
> devicetree@vger.kernel.org; Mark Rutland; Sudeep Holla; Catalin
> Marinas; Charles Garcia-Tobin; Rob Herring; grant.likely@linaro.org;
> Peter De Schrijver; Daniel Lezcano; Amit Kucheria; Vincent Guittot;
> Antti Miettinen; Stephen Boyd; Kevin Hilman; Sebastian Capella; Tomasz
> Figa; Mark Brown; Paul Walmsley; Chander Kashyap
> Subject: Re: [PATCH v4 1/6] Documentation: arm: define DT idle states
> bindings
> 
> On Wednesday 18 June 2014 01:36 PM, Lorenzo Pieralisi wrote:
> > On Fri, Jun 13, 2014 at 06:33:35PM +0100, Nicolas Pitre wrote:
> 
> [..]
> > Ok, a minor tweak to the diagram above, min-residency should include
> > energy costs related to idle entry and exit, but not the exit-latency
> > itself, as long as the energy costs implied by exiting the state are
> > factored out in the min-residency-us property.
> >
> > Hence, to sum it up, I attached below the updated bindings patch:
> >
> > I think we are close to an agreement, if anyone disagrees please
> shout
> > as soon as possible so that we can still integrate changes.
> >
> 
> [..]
> 
> >
> > -- >8 --
> > Subject: [PATCH] Documentation: arm: define DT idle states bindings
> >
> > ARM based platforms implement a variety of power management schemes
> that
> > allow processors to enter idle states at run-time.
> > The parameters defining these idle states vary on a per-platform
> basis forcing
> > the OS to hardcode the state parameters in platform specific static
> tables
> > whose size grows as the number of platforms supported in the kernel
> increases
> > and hampers device drivers standardization.
> >
> > Therefore, this patch aims at standardizing idle state device tree
> bindings for
> > ARM platforms. Bindings define idle state parameters inclusive of
> entry methods
> > and state latencies, to allow operating systems to retrieve the
> configuration
> > entries from the device tree and initialize the related power
> management
> > drivers, paving the way for common code in the kernel to deal with
> idle
> > states and removing the need for static data in current and previous
> kernel
> > versions.
> >
> > Reviewed-by: Sebastian Capella <sebcape@gmail.com>
> > Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
> > ---
> Nice work Lorenzo !!
> I have few comments/questions.
> 
> >  Documentation/devicetree/bindings/arm/cpus.txt     |   8 +
> >  .../devicetree/bindings/arm/idle-states.txt        | 561
> +++++++++++++++++++++
> >  2 files changed, 569 insertions(+)
> >  create mode 100644 Documentation/devicetree/bindings/arm/idle-
> states.txt
> >
> > diff --git a/Documentation/devicetree/bindings/arm/cpus.txt
> b/Documentation/devicetree/bindings/arm/cpus.txt
> > index 1fe72a0..a44d4fd 100644
> > --- a/Documentation/devicetree/bindings/arm/cpus.txt
> > +++ b/Documentation/devicetree/bindings/arm/cpus.txt
> > @@ -215,6 +215,12 @@ nodes to be present and contain the properties
> described below.
> >  		Value type: <phandle>
> >  		Definition: Specifies the ACC[2] node associated with this
> CPU.
> >
> > +	- cpu-idle-states
> > +		Usage: Optional
> > +		Value type: <prop-encoded-array>
> > +		Definition:
> > +			# List of phandles to idle state nodes supported
> > +			  by this cpu [3].
> >
> >  Example 1 (dual-cluster big.LITTLE system 32-bit):
> >
> > @@ -411,3 +417,5 @@ cpus {
> >  --
> >  [1] arm/msm/qcom,saw2.txt
> >  [2] arm/msm/qcom,kpss-acc.txt
> > +[3] ARM Linux kernel documentation - idle states bindings
> > +    Documentation/devicetree/bindings/arm/idle-states.txt
> > diff --git a/Documentation/devicetree/bindings/arm/idle-states.txt
> b/Documentation/devicetree/bindings/arm/idle-states.txt
> > new file mode 100644
> > index 0000000..c9e1ec6
> > --- /dev/null
> > +++ b/Documentation/devicetree/bindings/arm/idle-states.txt
> > @@ -0,0 +1,561 @@
> > +==========================================
> > +ARM idle states binding description
> > +==========================================
> > +
> > +==========================================
> > +1 - Introduction
> > +==========================================
> > +
> > +ARM systems contain HW capable of managing power consumption
> dynamically,
> > +where cores can be put in different low-power states (ranging from
> simple
> > +wfi to power gating) according to OSPM policies. The CPU states
> representing
> s/OSPM/OS PM ?
> > +the range of dynamic idle states that a processor can enter at run-
> time, can be
> > +specified through device tree bindings representing the parameters
> required
> > +to enter/exit specific idle states on a given processor.
> > +
> > +According to the Server Base System Architecture document (SBSA,
> [3]), the
> > +power states an ARM CPU can be put into are identified by the
> following list:
> > +
> > +- Running
> > +- Idle_standby
> > +- Idle_retention
> > +- Sleep
> > +- Off
> > +
> > +The power states described in the SBSA document define the basic CPU
> states on
> > +top of which ARM platforms implement power management schemes that
> allow an OS
> > +PM implementation to put the processor in different idle states
> (which include
> > +states listed above; "off" state is not an idle state since it does
> not have
> > +wake-up capabilities, hence it is not considered in this document).
> > +
> > +Idle state parameters (eg entry latency) are platform specific and
> need to be
> > +characterized with bindings that provide the required information to
> OSPM
> Ditto
> > +code so that it can build the required tables and use them at
> runtime.
> > +
> > +The device tree binding definition for ARM idle states is the
> subject of this
> > +document.
> > +
> > +===========================================
> > +2 - idle-states node
> > +===========================================
> > +
> > +ARM processor idle states are defined within the idle-states node,
> which is
> > +a direct child of the cpus node [1] and provides a container where
> the
> > +processor idle states, defined as device tree nodes, are listed.
> > +
> > +- idle-states node
> > +
> > +	Usage: Optional - On ARM systems, is a container of processor idle
> s/is/it is ?
> > +			  states nodes. If the system does not provide CPU
> > +			  power management capabilities or the processor
just
> > +			  supports idle_standby an idle-states node is not
> > +			  required.
> > +
> > +	Description: idle-states node is a container node, where its
> > +		     subnodes describe the CPU idle states.
> > +
> > +	Node name must be "idle-states".
> > +
> > +	The idle-states node's parent node must be the cpus node.
> > +
> > +	The idle-states node's child nodes can be:
> s/idle-states/idle-state
> > +
> > +	- one or more state nodes
> > +
> > +	Any other configuration is considered invalid.
> > +
> > +	An idle-states node defines the following properties:
> > +
> > +	- entry-method
> > +		Usage: Required
> > +		Value type: <stringlist>
> > +		Definition: Describes the method by which a CPU enters the
> > +			    idle states. This property is required and must
be
> > +			    one of:
> > +
> > +			    - "arm,psci"
> > +			      ARM PSCI firmware interface [2].
> > +
> > +			    - "[vendor],[method]"
> > +			      An implementation dependent string with
> > +			      format "vendor,method", where vendor is a
string
> > +			      denoting the name of the manufacturer and
> > +			      method is a string specifying the mechanism
> > +			      used to enter the idle state.
> > +
> > +The nodes describing the idle states (state) can only be defined
> within the
> > +idle-states node, any other configuration is considered invalid and
> therefore
> > +must be ignored.
> > +
> > +===========================================
> > +3 - state node
> > +===========================================
> > +
> > +A state node represents an idle state description and must be
> defined as
> > +follows:
> > +
> > +- state node
> > +
> > +	Description: must be child of the idle-states node
> > +
> > +	The state node name shall follow standard device tree naming
> > +	rules ([5], 2.2.1 "Node names"), in particular state nodes which
> > +	are siblings within a single common parent must be given a unique
> name.
> > +
> > +	The idle state entered by executing the wfi instruction
> (idle_standby
> > +	SBSA,[3][4]) is considered standard on all ARM platforms and
> therefore
> > +	must not be listed.
> > +
> > +	To correctly specify idle states timing and energy related
> properties,
> > +	the following definitions identify the different execution phases
> > +	a CPU goes through to enter and exit idle states and the implied
> > +	energy metrics:
> > +
> > +
> 	..__[EXEC]__|__[PREP]__|__[ENTRY]__|__[IDLE]__|__[EXIT]__|__[EXEC]
> __..
> > +		    |          |           |          |          |
> > +
> > +		    |<------ entry ------->|
> > +		    |       latency        |
> > +						      |<- exit ->|
> > +						      |  latency |
> > +		    |<-------- min-residency -------->|
> > +			       |<-------  wakeup-latency ------->|
> > +
> I don't know the wakeup latency makes much sense and also correct.
> Hardware wakeup latency is actually exit latency. Is it for failed
> or abort-able ilde case ? We are adding this as a new parameter
> at least from idle states perspective. I think we should just
> avoid it.
> 

Hi Santosh, 

To me wake up latency makes up a lot of sense. It is not always the same as
exit latency, it will depend on your system, and just how smart it is. In
some cases the [ENTRY] period may not be negligible in which case exit
latency will be less than the wake up latency. 
In addition, it will generally always be shorter than entry+exit which is
the default value if omitted, this assumes the PREP time is not abortable,
but this is the safer assumption to make.
Wake up latency is really the number that folk have in their head for what
you'd stick into the pm_qos to veto entry into states when you are latency
constrained. 
The one thing that really is an optimisation here is having a separate exit
latency, which is being proposed for use in core selection for the
scheduler.
So if anything was going to be made optional pending new scheduler patches
should that not be entry/exit latency? 
 
Cheers

Charles



> > +	EXEC:	Normal CPU execution.
> > +
> > +	PREP:	Preparation phase before committing the hardware to idle
mode
> > +		like cache flushing. This is abortable on pending wake-up
> > +		event conditions. The abort latency is assumed to be
> negligible
> > +		(i.e. less than the ENTRY + EXIT duration). If aborted, CPU
> > +		goes back to EXEC. This phase is optional. If not abortable,
> > +		this should be included in the ENTRY phase instead.
> > +
> > +	ENTRY:	The hardware is committed to idle mode. This period must
> run
> > +		to completion up to IDLE before anything else can happen.
> > +
> > +	IDLE:	This is the actual energy-saving idle period. This may last
> > +		between 0 and infinite time, until a wake-up event occurs.
> > +
> > +	EXIT:	Period during which the CPU is brought back to operational
> > +		mode (EXEC).
> > +
> > +	With the definitions provided above, the following list represents
> > +	the valid properties for a state node:
> > +
> > +	- compatible
> > +		Usage: Required
> > +		Value type: <stringlist>
> > +		Definition: Must be "arm,idle-state".
> > +
> > +	- logic-state-retained
> > +		Usage: See definition
> > +		Value type: <none>
> > +		Definition: if present logic is retained on state entry,
> > +			    otherwise it is lost.
> > +
> > +	- cache-state-retained
> > +		Usage: See definition
> > +		Value type: <none>
> > +		Definition: if present cache memory is retained on state
> entry,
> > +			    otherwise it is lost.
> > +
> > +	- entry-method-param
> > +		Usage: See definition.
> > +		Value type: <u32>
> > +		Definition: Depends on the idle-states node entry-method
> > +			    property value. Refer to the entry-method
bindings
> > +			    for this property value definition.
> > +
> > +	- entry-latency-us
> > +		Usage: Required
> > +		Value type: <prop-encoded-array>
> > +		Definition: u32 value representing worst case latency in
> > +			    microseconds required to enter the idle state.
> > +			    The exit-latency-us duration may be guaranteed
> > +			    only after entry-latency-us has passed.
> > +
> > +	- exit-latency-us
> > +		Usage: Required
> > +		Value type: <prop-encoded-array>
> > +		Definition: u32 value representing worst case latency
> > +			    in microseconds required to exit the idle state.
> > +
> > +	- min-residency-us
> > +		Usage: Required
> > +		Value type: <prop-encoded-array>
> > +		Definition: u32 value representing minimum residency
duration
> > +			    in microseconds, inclusive of preparation and
> > +			    entry, for this idle state to be considered
> > +			    worthwhile energy wise.
> > +			    The residency time must take into account the
> > +			    energy consumed while entering and exiting the
> > +			    idle state and is therefore expected to be
> > +			    longer than entry-latency-us.
> > +
> > +	- wakeup-latency-us:
> > +		Usage: Optional
> > +		Value type: <prop-encoded-array>
> > +		Definition: u32 value representing maximum delay between the
> > +			    signaling of a wake-up event and the CPU being
> > +			    able to execute normal code again. If omitted,
> > +			    this is assumed to be equal to:
> > +				entry-latency-us + exit-latency-us
> > +
> Rest of the patch looks fine by to me.
> 
> regards,
> Santosh
> 


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Charles,

On Thursday 19 June 2014 03:33 AM, Charles Garcia-Tobin wrote:
> 
> 
>> -----Original Message-----
>> From: Santosh Shilimkar [mailto:santosh.shilimkar@ti.com]
>> Sent: 18 June 2014 20:27
>> To: Lorenzo Pieralisi; Nicolas Pitre

[..]

>>> +===========================================
>>> +3 - state node
>>> +===========================================
>>> +
>>> +A state node represents an idle state description and must be
>> defined as
>>> +follows:
>>> +
>>> +- state node
>>> +
>>> +	Description: must be child of the idle-states node
>>> +
>>> +	The state node name shall follow standard device tree naming
>>> +	rules ([5], 2.2.1 "Node names"), in particular state nodes which
>>> +	are siblings within a single common parent must be given a unique
>> name.
>>> +
>>> +	The idle state entered by executing the wfi instruction
>> (idle_standby
>>> +	SBSA,[3][4]) is considered standard on all ARM platforms and
>> therefore
>>> +	must not be listed.
>>> +
>>> +	To correctly specify idle states timing and energy related
>> properties,
>>> +	the following definitions identify the different execution phases
>>> +	a CPU goes through to enter and exit idle states and the implied
>>> +	energy metrics:
>>> +
>>> +
>> 	..__[EXEC]__|__[PREP]__|__[ENTRY]__|__[IDLE]__|__[EXIT]__|__[EXEC]
>> __..
>>> +		    |          |           |          |          |
>>> +
>>> +		    |<------ entry ------->|
>>> +		    |       latency        |
>>> +						      |<- exit ->|
>>> +						      |  latency |
>>> +		    |<-------- min-residency -------->|
>>> +			       |<-------  wakeup-latency ------->|
>>> +
>> I don't know the wakeup latency makes much sense and also correct.
>> Hardware wakeup latency is actually exit latency. Is it for failed
>> or abort-able ilde case ? We are adding this as a new parameter
>> at least from idle states perspective. I think we should just
>> avoid it.
>>
> 
> Hi Santosh, 
> 
> To me wake up latency makes up a lot of sense. It is not always the same as
> exit latency, it will depend on your system, and just how smart it is. In
> some cases the [ENTRY] period may not be negligible in which case exit
> latency will be less than the wake up latency. 
> In addition, it will generally always be shorter than entry+exit which is
> the default value if omitted, this assumes the PREP time is not abortable,
> but this is the safer assumption to make.
> Wake up latency is really the number that folk have in their head for what
> you'd stick into the pm_qos to veto entry into states when you are latency
> constrained. 
> The one thing that really is an optimisation here is having a separate exit
> latency, which is being proposed for use in core selection for the
> scheduler.
> So if anything was going to be made optional pending new scheduler patches
> should that not be entry/exit latency? 
>  
PM QOS angle Nico pointed out and its clear. The wakeup latency as such is a
worst case wakeup latency from QOS perspective so considering the aborted idle
case it makes sense to have conservative number which includes entry + exit.

If you look at current idle governors, only exit latency and target residency
is being used. No matter how we represent it, as long idle governor or idle
C-state selection logic gets that information, things should be fine. So
from that view your point of entry/exit optional makes sense considering
wakeup latency can convey that information indirectly.

Regards,
Santosh
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Hi

Looks we are pretty much agreed on the number now.
In my e-mail though I was questioning what should be optional and what
shouldn't. The current proposal is that wakeup-latency-us is the optional
one, I was thinking that it's make more sense making entry/exit (given the
use is much more specific and yet to be proven) but frankly it is not great
shakes either way, so for me it's fine as it is. The only thing that I think
would be worth clarifying is that the text around wakeup-latency-us, to make
it clear when it makes sense to provide it. So I was thinking something
like:

        - wakeup-latency-us:
                Usage: Optional
                Value type: <prop-encoded-array>
                Definition: u32 value representing maximum delay between the
                            signalling of a wake-up event and the CPU being
                            able to execute normal code again. If omitted,
                            this is assumed to be equal to:
                                entry-latency-us + exit-latency-us
                            It is important to supply this value on systems 
                            where the duration of PREP phase is 
                            non-neglibigle. In such systems 
                            entry-latency-us + exit-latency-us 
                            will exceed wakeup-latency-us by this duration.

The other thing that may be worth adding is some graphs to help explain what
is meant by min-residency. Lorenzo feel free to take this or leave this. But
something like:

The energy consumption of a cpu when it enters a power state can be roughly
characterised by the following graph:

               |
               |
               |
           e   |
           n   |                                      /---
           e   |                               /------
           r   |                        /------
           g   |                  /-----
           y   |           /------
               |       ----
               |      /|
               |     / |
               |    /  |
               |   /   |
               |  /    |
               | /     |                          
               |/      |                          
          -----|-------+----------------------------------
              0|       1                      time


The graph starts with a steep slope and then a shallower one. The first part
denotes the energy costs incurred whilst entering and leaving the power
state. The shallower slope is essentially representing the power consumption
of the state. 
We are defining min-residency for a given state as the period of time after
which choosing that state become the most energy efficient option. A good
way to visualise this, is if we take the same graph above and compare some
states. Due to the limitations of ascii art we are only showing two made up
states C1, and C2:


          |
          |
          |
          |                                                  /-- C1
       e  |                                              /---      
       n  |                                         /----          
       e  |                                     /---               
       r  |                                /----     /----------- C2
       g  |                    /-------/-------------               
       y  |        ------------    /---|
          |       /           /----    |
          |      /        /---         |
          |     /    /----             |
          |    / /---                  |
          |   ---                      |
          |  /                         |
          | /                          |
          |/                           |                  time
       ---/----------------------------+------------------------
          |  better off with C1        | better off with C2
                                       |
                                   min-residency    
                                   for C2               



As you can see, having taken into account entry/exit costs there is period
were C1 is the better choice of state. This is mainly down to the fact that
entry/exit costs are low. However the lower power consumption of C2 means
that after a suitable time, C2 is the better choice. This interval of time
is what we want to call min-residency
                     
Cheers

Charles

> -----Original Message-----
> From: Santosh Shilimkar [mailto:santosh.shilimkar@ti.com]
> Sent: 19 June 2014 15:09
> To: Charles Garcia-Tobin; Lorenzo Pieralisi; Nicolas Pitre
> Cc: linux-arm-kernel@lists.infradead.org; linux-pm@vger.kernel.org;
> devicetree@vger.kernel.org; Mark Rutland; Sudeep Holla; Catalin
> Marinas; Rob Herring; grant.likely@linaro.org; Peter De Schrijver;
> Daniel Lezcano; Amit Kucheria; Vincent Guittot; Antti Miettinen;
> Stephen Boyd; Kevin Hilman; Sebastian Capella; Tomasz Figa; Mark Brown;
> Paul Walmsley; Chander Kashyap
> Subject: Re: [PATCH v4 1/6] Documentation: arm: define DT idle states
> bindings
> 
> Charles,
> 
> On Thursday 19 June 2014 03:33 AM, Charles Garcia-Tobin wrote:
> >
> >
> >> -----Original Message-----
> >> From: Santosh Shilimkar [mailto:santosh.shilimkar@ti.com]
> >> Sent: 18 June 2014 20:27
> >> To: Lorenzo Pieralisi; Nicolas Pitre
> 
> [..]
> 
> >>> +===========================================
> >>> +3 - state node
> >>> +===========================================
> >>> +
> >>> +A state node represents an idle state description and must be
> >> defined as
> >>> +follows:
> >>> +
> >>> +- state node
> >>> +
> >>> +	Description: must be child of the idle-states node
> >>> +
> >>> +	The state node name shall follow standard device tree naming
> >>> +	rules ([5], 2.2.1 "Node names"), in particular state nodes which
> >>> +	are siblings within a single common parent must be given a unique
> >> name.
> >>> +
> >>> +	The idle state entered by executing the wfi instruction
> >> (idle_standby
> >>> +	SBSA,[3][4]) is considered standard on all ARM platforms and
> >> therefore
> >>> +	must not be listed.
> >>> +
> >>> +	To correctly specify idle states timing and energy related
> >> properties,
> >>> +	the following definitions identify the different execution phases
> >>> +	a CPU goes through to enter and exit idle states and the implied
> >>> +	energy metrics:
> >>> +
> >>> +
> >> 	..__[EXEC]__|__[PREP]__|__[ENTRY]__|__[IDLE]__|__[EXIT]__|__[EXEC]
> >> __..
> >>> +		    |          |           |          |          |
> >>> +
> >>> +		    |<------ entry ------->|
> >>> +		    |       latency        |
> >>> +						      |<- exit ->|
> >>> +						      |  latency |
> >>> +		    |<-------- min-residency -------->|
> >>> +			       |<-------  wakeup-latency ------->|
> >>> +
> >> I don't know the wakeup latency makes much sense and also correct.
> >> Hardware wakeup latency is actually exit latency. Is it for failed
> >> or abort-able ilde case ? We are adding this as a new parameter
> >> at least from idle states perspective. I think we should just
> >> avoid it.
> >>
> >
> > Hi Santosh,
> >
> > To me wake up latency makes up a lot of sense. It is not always the
> same as
> > exit latency, it will depend on your system, and just how smart it
> is. In
> > some cases the [ENTRY] period may not be negligible in which case
> exit
> > latency will be less than the wake up latency.
> > In addition, it will generally always be shorter than entry+exit
> which is
> > the default value if omitted, this assumes the PREP time is not
> abortable,
> > but this is the safer assumption to make.
> > Wake up latency is really the number that folk have in their head for
> what
> > you'd stick into the pm_qos to veto entry into states when you are
> latency
> > constrained.
> > The one thing that really is an optimisation here is having a
> separate exit
> > latency, which is being proposed for use in core selection for the
> > scheduler.
> > So if anything was going to be made optional pending new scheduler
> patches
> > should that not be entry/exit latency?
> >
> PM QOS angle Nico pointed out and its clear. The wakeup latency as such
> is a
> worst case wakeup latency from QOS perspective so considering the
> aborted idle
> case it makes sense to have conservative number which includes entry +
> exit.
> 
> If you look at current idle governors, only exit latency and target
> residency
> is being used. No matter how we represent it, as long idle governor or
> idle
> C-state selection logic gets that information, things should be fine.
> So
> from that view your point of entry/exit optional makes sense
> considering
> wakeup latency can convey that information indirectly.
> 
> Regards,
> Santosh


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