Re: [PATCH v2 3/5] dt-bindings: net: add mac-address-increment option

[Ivan Mikhaylov <fr0st61te@gmail.com>]

On Wed, 2023-05-10 at 16:48 +0200, Krzysztof Kozlowski wrote:
> On 09/05/2023 16:35, Ivan Mikhaylov wrote:
> > Add the mac-address-increment option for specify MAC address taken
> > by
> > any other sources.
> > 
> > Signed-off-by: Paul Fertser <fercerpav@gmail.com>
> > Signed-off-by: Ivan Mikhaylov <fr0st61te@gmail.com>
> > ---
> >  .../devicetree/bindings/net/ethernet-controller.yaml      | 8
> > ++++++++
> >  1 file changed, 8 insertions(+)
> > 
> > diff --git a/Documentation/devicetree/bindings/net/ethernet-
> > controller.yaml b/Documentation/devicetree/bindings/net/ethernet-
> > controller.yaml
> > index 00be387984ac..6900098c5105 100644
> > --- a/Documentation/devicetree/bindings/net/ethernet-
> > controller.yaml
> > +++ b/Documentation/devicetree/bindings/net/ethernet-
> > controller.yaml
> > @@ -34,6 +34,14 @@ properties:
> >      minItems: 6
> >      maxItems: 6
> >  
> > +  mac-address-increment:
> > +    $ref: /schemas/types.yaml#/definitions/int32
> > +    description:
> > +      Specifies the MAC address increment to be added to the MAC
> > address.
> > +      Should be used in cases when there is a need to use MAC
> > address
> > +      different from one obtained by any other level, like u-boot
> > or the
> > +      NC-SI stack.
> 
> We don't store MAC addresses in DT, but provide simple placeholder
> for
> firmware or bootloader. Why shall we store static "increment" part of
> MAC address? Can't the firmware give you proper MAC address?
> 
> Best regards,
> Krzysztof
> 

Krzysztof, maybe that's a point to make commit message with better
explanation from my side. At current time there is at least two cases
where I see it's possible to be used:

1. NC-SI
2. embedded

At NC-SI level there is Get Mac Address command which provides to BMC
mac address from the host which is same as host mac address, it happens
at runtime and overrides old one.

Also, this part was also to be discussed 2 years ago in this thread:
https://lore.kernel.org/all/OF8E108F72.39D22E89-ON00258765.001E46EB-00258765.00251157@ibm.com/

Where Milton provided this information:

DTMF spec DSP0222 NC-SI (network controller sideband interface)
is a method to provide a BMC (Baseboard management controller) shared
access to an external ethernet port for comunication to the management
network in the outside world.  The protocol describes ethernet packets 
that control selective bridging implemented in a host network
controller
to share its phy.  Various NIC OEMs have added a query to find out the 
address the host is using, and some vendors have added code to query
host
nic and set the BMC mac to a fixed offset (current hard coded +1 from
the host value).  If this is compiled in the kernel, the NIC OEM is 
recognised and the BMC doesn't miss the NIC response the address is set
once each time the NCSI stack reinitializes.  This mechanism overrides
any mac-address or local-mac-address or other assignment.

DSP0222
https://www.dmtf.org/documents/pmci/network-controller-sideband-interface-nc-si-specification-110


In embedded case, sometimes you have different multiple ethernet
interfaces which using one mac address which increments or decrements
for particular interface, just for better explanation, there is patch
with explanation which providing them such way of work:
https://github.com/openwrt/openwrt/blob/master/target/linux/generic/pending-5.15/682-of_net-add-mac-address-increment-support.patch

In their rep a lot of dts using such option.

Thanks.