[v30,01/30] usb: host: xhci: Repurpose event handler for skipping interrupter events

Depending on the interrupter use case, the OS may only be used to handle
the interrupter event ring clean up.  In these scenarios, event TRBs don't
need to be handled by the OS, so introduce an xhci interrupter flag to tag
if the events from an interrupter needs to be handled or not.

Signed-off-by: Wesley Cheng <quic_wcheng@quicinc.com>
---
 drivers/usb/host/xhci-ring.c | 17 +++++++++++++----
 drivers/usb/host/xhci.h      |  1 +
 2 files changed, 14 insertions(+), 4 deletions(-)

On 6.11.2024 21.33, Wesley Cheng wrote:
> Depending on the interrupter use case, the OS may only be used to handle
> the interrupter event ring clean up.  In these scenarios, event TRBs don't
> need to be handled by the OS, so introduce an xhci interrupter flag to tag
> if the events from an interrupter needs to be handled or not.
> 
> Signed-off-by: Wesley Cheng <quic_wcheng@quicinc.com>
> ---
>   drivers/usb/host/xhci-ring.c | 17 +++++++++++++----
>   drivers/usb/host/xhci.h      |  1 +
>   2 files changed, 14 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/usb/host/xhci-ring.c b/drivers/usb/host/xhci-ring.c
> index 9f1e150a1c76..b8f6983b7369 100644
> --- a/drivers/usb/host/xhci-ring.c
> +++ b/drivers/usb/host/xhci-ring.c
> @@ -2931,14 +2931,22 @@ static int handle_tx_event(struct xhci_hcd *xhci,
>   }
>   
>   /*
> - * This function handles one OS-owned event on the event ring. It may drop
> - * xhci->lock between event processing (e.g. to pass up port status changes).
> + * This function handles one OS-owned event on the event ring, or ignores one event
> + * on interrupters which are non-OS owned. It may drop xhci->lock between event
> + * processing (e.g. to pass up port status changes).
>    */
>   static int xhci_handle_event_trb(struct xhci_hcd *xhci, struct xhci_interrupter *ir,
>   				 union xhci_trb *event)
>   {
>   	u32 trb_type;
>   
> +	/*
> +	 * Some interrupters do not need to handle event TRBs, as they may be
> +	 * managed by another entity, but rely on the OS to clean up.
> +	 */
> +	if (ir->skip_events)
> +		return 0;

This works for your special case but is a small step sideways from other possible xhci
secondary interrupter usecases.

We currently support just one event handler function even if we support several secondary
interrupters. Idea was to add support to pass dedicated handlers for each secondary interrupter,
set when the secondary interrupter is requested.

In your case this dedicated handler wouldn't do anything.

This patch again has a different approach, it keeps the default handler, and instead adds
flags to it, preventing it from handling the event trb.

Not sure if we should take the time and implement dedicated handlers now, even if we don't
have any real users yet, or just take this quick change and rework it later when needed.

Thanks
Mathias

Hi Mathias,

On 11/20/2024 3:48 AM, Mathias Nyman wrote:
> On 6.11.2024 21.33, Wesley Cheng wrote:
>> Depending on the interrupter use case, the OS may only be used to handle
>> the interrupter event ring clean up.  In these scenarios, event TRBs don't
>> need to be handled by the OS, so introduce an xhci interrupter flag to tag
>> if the events from an interrupter needs to be handled or not.
>>
>> Signed-off-by: Wesley Cheng <quic_wcheng@quicinc.com>
>> ---
>>   drivers/usb/host/xhci-ring.c | 17 +++++++++++++----
>>   drivers/usb/host/xhci.h      |  1 +
>>   2 files changed, 14 insertions(+), 4 deletions(-)
>>
>> diff --git a/drivers/usb/host/xhci-ring.c b/drivers/usb/host/xhci-ring.c
>> index 9f1e150a1c76..b8f6983b7369 100644
>> --- a/drivers/usb/host/xhci-ring.c
>> +++ b/drivers/usb/host/xhci-ring.c
>> @@ -2931,14 +2931,22 @@ static int handle_tx_event(struct xhci_hcd *xhci,
>>   }
>>     /*
>> - * This function handles one OS-owned event on the event ring. It may drop
>> - * xhci->lock between event processing (e.g. to pass up port status changes).
>> + * This function handles one OS-owned event on the event ring, or ignores one event
>> + * on interrupters which are non-OS owned. It may drop xhci->lock between event
>> + * processing (e.g. to pass up port status changes).
>>    */
>>   static int xhci_handle_event_trb(struct xhci_hcd *xhci, struct xhci_interrupter *ir,
>>                    union xhci_trb *event)
>>   {
>>       u32 trb_type;
>>   +    /*
>> +     * Some interrupters do not need to handle event TRBs, as they may be
>> +     * managed by another entity, but rely on the OS to clean up.
>> +     */
>> +    if (ir->skip_events)
>> +        return 0;
>
> This works for your special case but is a small step sideways from other possible xhci
> secondary interrupter usecases.
>
> We currently support just one event handler function even if we support several secondary
> interrupters. Idea was to add support to pass dedicated handlers for each secondary interrupter,
> set when the secondary interrupter is requested.
>
> In your case this dedicated handler wouldn't do anything.
>
> This patch again has a different approach, it keeps the default handler, and instead adds
> flags to it, preventing it from handling the event trb.
>
> Not sure if we should take the time and implement dedicated handlers now, even if we don't
> have any real users yet, or just take this quick change and rework it later when needed.
>
>
Yes, I think we had a small discussion on this on v20:

https://lore.kernel.org/linux-usb/a88b41f4-7e53-e162-5a6a-2d470e29c0bb@quicinc.com/

Since I didn't have an environment that exercised the path where we'd actually want to handle secondary interrupter events, I wasn't sure if it was valid to add bits and pieces of it to support such use cases w/o proper testing.  I think having this driver (as is) is still a step forward into the right direction, as these APIs are still going to be required if enabling secondary interrupter events in the Linux environment.

Thanks

Wesley Cheng