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

[Jacob Keller <jacob.e.keller@intel.com>]

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.

Well, a lot comes from the choices made at hardware design to have a
multi-function device but then still have pieces shared across PFs, with
no clear plan for how to actually orchestrate that sharing...

> 
> Have you had a chance to take a closer look at mlx5 "socket direct"
> (rename pending) implementation?
> 

Hm. No I hadn't seen that yet.

> BTW Jiri, weren't you expecting that to use component drivers or some
> such?