Subject: Re: [RFC PATCH v3 net-next] Documentation: devlink: Add devlink-sd

From: Jiri Pirko <[email-masked]>
Date: Mon, 19 Feb 2024 09:59:03 +0100

Fri, Feb 16, 2024 at 10:47:50PM CET, [email-masked] wrote:
>
>
>On 2/15/2024 6:07 PM, Jakub Kicinski wrote:
>> On Thu, 15 Feb 2024 09:41:31 -0800 Jacob Keller wrote:
>>> I don't know offhand if we have a device which can share pools
>>> specifically, but we do have multi-PF devices which have a lot of shared
>>> resources. However, due to the multi-PF PCIe design. I looked into ways
>>> to get a single devlink across the devices.. but ultimately got stymied
>>> and gave up.
>>>
>>> This left us with accepting the limitation that each PF gets its own
>>> devlink and can't really communicate with other PFs.
>>>
>>> The existing solution has just been to partition the shared resources
>>> evenly across PFs, typically via firmware. No flexibility.
>>>
>>> I do think the best solution here would be to figure out a generic way
>>> to tie multiple functions into a single devlink representing the device.
>>> Then each function gets the set of devlink_port objects associated to
>>> it. I'm not entirely sure how that would work. We could hack something
>>> together with auxbus.. but thats pretty ugly. Some sort of orchestration
>>> in the PCI layer that could identify when a device wants to have some
>>> sort of "parent" driver which loads once and has ties to each of the
>>> function drivers would be ideal.
>>>
>>> Then this parent driver could register devlink, and each function driver
>>> could connect to it and allocate ports and function-specific resources.
>>>
>>> Alternatively a design which loads a single driver that maintains
>>> references to each function could work but that requires a significant
>>> change to the entire driver design and is unlikely to be done for
>>> existing drivers...
>> 
>> I think the complexity mostly stems from having to answer what the
>> "right behavior" is. At least that's what I concluded when thinking
>> about it back at Netronome :)  If you do a strict hierarchy where
>> one PF is preassigned the role of the leader, and just fail if anything
>> unexpected happens - it should be doable. We already kinda have the
>> model where devlink is the "first layer of probing" and "reload_up()"
>> is the second.
>> 
>
>You can of course just assign it such that one PF "owns" things, but
>that seems a bit confusing if there isn't a clear mechanism for users to
>understand which PF is the owner. I guess they can check
>devlink/netlink/whatever and see the resources there. It also still
>doesn't provide a communication mechanism to actually pass sub-ownership
>across the PFs, unless your device firmware can do that for you.
>
>The other option commonly used is partitioning so you just pre-determine
>how to slice the resources up per PF. This isn't flexible, but it is simple.

I will cook up a rfc for the devlink instance to represent the parent of
the PFs. I think we have everything we need in place already. Will send
that soon.