[net-next,0/3] net: add support for an offset of a nvmem provided MAC address

Boards with multiple ethernet ports might store their MAC addresses not
individually per port but just store one base MAC address. To get the
MAC address of a specific network port we have to add an offset.

This series adds a new device tree property "nvmem-mac-address-offset".

Michael Walle (3):
  dt-bindings: net: add nvmem-mac-address-offset property
  net: add helper eth_addr_add()
  net: implement nvmem-mac-address-offset DT property

 .../bindings/net/ethernet-controller.yaml          |  6 ++++++
 drivers/of/of_net.c                                |  4 ++++
 include/linux/etherdevice.h                        | 14 ++++++++++++++
 net/ethernet/eth.c                                 |  5 +++++
 4 files changed, 29 insertions(+)

On Wed, Apr 14, 2021 at 05:43:49PM +0200, Andrew Lunn wrote:
> On Wed, Apr 14, 2021 at 05:26:55PM +0200, Michael Walle wrote:
> > It is already possible to read the MAC address via a NVMEM provider. But
> > there are boards, esp. with many ports, which only have a base MAC
> > address stored. Thus we need to have a way to provide an offset per
> > network device.
> 
> We need to see what Rob thinks of this. There was recently a patchset
> to support swapping the byte order of the MAC address in a NVMEM. Rob
> said the NVMEM provider should have the property, not the MAC driver.
> This does seems more ethernet specific, so maybe it should be an
> Ethernet property?

There was also this one[1]. I'm not totally opposed, but don't want to 
see a never ending addition of properties to try to describe any 
possible transformation.

Rob

[1] https://patchwork.ozlabs.org/project/devicetree-bindings/patch/20200920095724.8251-4-ansuelsmth@gmail.com/

Am 2021-04-15 23:59, schrieb Rob Herring:
> On Wed, Apr 14, 2021 at 05:43:49PM +0200, Andrew Lunn wrote:
>> On Wed, Apr 14, 2021 at 05:26:55PM +0200, Michael Walle wrote:
>> > It is already possible to read the MAC address via a NVMEM provider. But
>> > there are boards, esp. with many ports, which only have a base MAC
>> > address stored. Thus we need to have a way to provide an offset per
>> > network device.
>> 
>> We need to see what Rob thinks of this. There was recently a patchset
>> to support swapping the byte order of the MAC address in a NVMEM. Rob
>> said the NVMEM provider should have the property, not the MAC driver.
>> This does seems more ethernet specific, so maybe it should be an
>> Ethernet property?
> 
> There was also this one[1]. I'm not totally opposed, but don't want to
> see a never ending addition of properties to try to describe any
> possible transformation.

Agreed, that stuff like ASCII MAC address parsing should be done
elsewhere. But IMHO adding an offset is a pretty common one (as also
pointed out in [1]). And it also need to be a per ethernet device
property.

-michael

[1] 
https://patchwork.ozlabs.org/project/devicetree-bindings/patch/20200920095724.8251-4-ansuelsmth@gmail.com/

On 4/15/2021 2:59 PM, Rob Herring wrote:
> On Wed, Apr 14, 2021 at 05:43:49PM +0200, Andrew Lunn wrote:
>> On Wed, Apr 14, 2021 at 05:26:55PM +0200, Michael Walle wrote:
>>> It is already possible to read the MAC address via a NVMEM provider. But
>>> there are boards, esp. with many ports, which only have a base MAC
>>> address stored. Thus we need to have a way to provide an offset per
>>> network device.
>>
>> We need to see what Rob thinks of this. There was recently a patchset
>> to support swapping the byte order of the MAC address in a NVMEM. Rob
>> said the NVMEM provider should have the property, not the MAC driver.
>> This does seems more ethernet specific, so maybe it should be an
>> Ethernet property?
> 
> There was also this one[1]. I'm not totally opposed, but don't want to 
> see a never ending addition of properties to try to describe any 
> possible transformation.

If only we could load eBPF bytecode embedded into Device Tree ;)

[adding Srinivas Kandagatla and Ansuel Smith]

Am 2021-04-16 00:27, schrieb Michael Walle:
> Am 2021-04-15 23:59, schrieb Rob Herring:

>> On Wed, Apr 14, 2021 at 05:43:49PM +0200, Andrew Lunn wrote:

>>> On Wed, Apr 14, 2021 at 05:26:55PM +0200, Michael Walle wrote:

>>> > It is already possible to read the MAC address via a NVMEM provider. But

>>> > there are boards, esp. with many ports, which only have a base MAC

>>> > address stored. Thus we need to have a way to provide an offset per

>>> > network device.

>>> 

>>> We need to see what Rob thinks of this. There was recently a patchset

>>> to support swapping the byte order of the MAC address in a NVMEM. Rob

>>> said the NVMEM provider should have the property, not the MAC driver.

>>> This does seems more ethernet specific, so maybe it should be an

>>> Ethernet property?

>> 

>> There was also this one[1]. I'm not totally opposed, but don't want to

>> see a never ending addition of properties to try to describe any

>> possible transformation.

> 

> Agreed, that stuff like ASCII MAC address parsing should be done

> elsewhere. But IMHO adding an offset is a pretty common one (as also

> pointed out in [1]). And it also need to be a per ethernet device

> property.


I'm a bit up in the air on this, as I don't know how to proceed here.

To cite Rob from IRC:
   Not really up to me. All the people that care need to come up with
   something flexible enough for common/simple cases and that's not
   going to get extended with every new variation. What I don't want is
   a one-off that's then extended with another one-off.

I already pointed out that this property is per consumer as opposed
to something like endianess swap or parsing a given format. The latter
operates on the nvmem cell.

One random idea is to have a nvmem-cells-transformation (in the lack of
a better name) property for consumers, where you can have some kind of
simple operations like add:
   nvmem-cells-transformation = <NVMEM_ADD 1>
But is that something we really want to have? I'm not sure.

btw. given that there might be other means where a base mac address can
come from in the future, it might make sense to drop the "nvmem-"
prefix and just use "mac-address-offset" (or 
"base-mac-address-offset"?).

> [1] 

> https://patchwork.ozlabs.org/project/devicetree-bindings/patch/20200920095724.8251-4-ansuelsmth@gmail.com/


-michael