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

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Message-ID: <8de01e81-43dc-71af-f56f-4fba957b0b0b@linaro.org>
Date:   Fri, 12 May 2023 08:22:56 +0200
From:   Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
To:     Ivan Mikhaylov <fr0st61te@...il.com>,
        Samuel Mendoza-Jonas <sam@...dozajonas.com>,
        "David S . Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>
Cc:     netdev@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, openbmc@...ts.ozlabs.org,
        Paul Fertser <fercerpav@...il.com>
Subject: Re: [PATCH v2 3/5] dt-bindings: net: add mac-address-increment option

On 11/05/2023 01:31, Ivan Mikhaylov wrote:
> 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@...il.com>
>>> Signed-off-by: Ivan Mikhaylov <fr0st61te@...il.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/

Which was not sent to Rob though...


> 
> 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.

None of these explain why this is property of the hardware. I understand
that this is something you want Linux to do, but DT is not for that
purpose. Do not encode system policies into DT and what above commit
says is a policy.

Best regards,
Krzysztof