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

[Krzysztof Kozlowski]

On 29/05/2023 22:59, Paul Fertser wrote:
> Hello Krzysztof,
> 
> Let me try to clarify a bit on the particular usecase and answer your
> questions.
> 
> Let's consider a server motherboard manufactured and sold by a single
> company. This motherboard includes I210 (Ethernet Controlleer) chip
> along with the other necessary parts right there, soldered to the PCB,
> non-replaceable. This I210 is connected to the host CPU with a PCIe
> lane and acts as a regular network adapter. In addition to that this
> chip is connected using NC-SI (management channel) to the BMC SoC
> (also permanently soldered to the board).
> 
> There is a separate EEPROM connected directly to I210 which hosts its
> firmware and many operational parameters, including the MAC
> address. This EEPROM is not anyhow accessible by the BMC (the host can
> read/write it using special protocol over PCIe). Intel expects the
> board manufacturer to embed a MAC address from the manufacturer's
> range in the EEPROM configuration. But in many cases it's desirable to
> use a separate MAC address for the BMC (then I210 acts as if it has an
> integrated switch), so the board manufacturer can, by its internal
> policy, allocate two consecutive MAC addresses to each motherboard.
> 
> The only way BMC can learn the MAC address used by I210 is by a
> special vendor-specific NC-SI command, and it can provide just a
> single address, the one used by the host. NC-SI is using Ethernet
> frames with a special type, so to execute this command the network
> driver needs to be (at least partially) functional. I do not really
> imagine nvmem getting support to read it this way.
> 
> On Wed, May 17, 2023 at 09:26:35PM +0200, Krzysztof Kozlowski wrote:
>> I would like to remind this question.
>> "why different boards with same device should have different offset/value?"
> 
> In the usecase we're aiming for the DT is describing a specific board
> from manufacturer that guarantees the offset to be correct, as none of
> the parts are replaceable and the MAC address is flashed into the
> I210 EEPROM during manufacturing.
> 
>> Let me extend this question with one more:
>> "Why for all your boards of one type, so using the same DTS, would you
>> use one value of incrementing MAC address?"
> 
> Here we assume that for all the boards supported by a particular DT
> the board manufacturer guarantees the MAC address offset by internal
> production policy, by allocating the addresses from the manufacturer's
> pool.

OK, embed such information in the commit or property description.

> 
>> But you hard-code the number, just in BMC DTS. How does it differ from
>> BMC hard-coding it differently?
>>
>> You encode policy - or software decisions - into Devicetree.
> 
> But MAC address of an Ethernet equipment is an inherent part of the
> hardware. It's just that we can't store it in an nvmem-addressable
> cell in this case, unfortunately.
> 
>> Why devices with same board cannot use different values? One board "1"
>> and second "2" for MAC increments? I am sure that one customer could
>> have it different.
> 
> You assume that the customers might be allocating their own MAC
> addresses for the network interface of a motherboard, that might be
> true if the customer gets such a board from an ODM. But such a
> customer not willing to follow the MAC address offsets policy is not
> much different from a customer who e.g. modifies flash partitions or
> storage format making the nvmem references invalid, and so requiring a
> separate DT.
> 
>> If you want to convince us, please illustrate it in a real world
>> upstreamed DTS (or explain why it cannot). Otherwise I don't see
>> justification as it is not a hardware property.
> 
> Can you please tell how you would imagine a responsible vendor tackle
> the usecase I outlined?

I would imagine him to upstream the DTS. I asked yo illustrate it. There
is still no DTS user for it so I have doubts it is used as intended.

> Guess it's not by a startup script that would
> be getting a MAC address from an interface, applying the offset, and
> then change it on the same interface?
> 
> Thank you for the review and discussion.
> 

Best regards,
Krzysztof