Subject: Re: [PATCH net-next 00/19] Mellanox, mlx5 sub function support

From: Jiri Pirko <[email-masked]>
Date: Mon, 11 Nov 2019 14:30:26 +0100

Mon, Nov 11, 2019 at 04:46:01AM CET, [email-masked] wrote:
>On Sun, 10 Nov 2019 10:18:55 +0100, [email-masked] wrote:
>> > What I'm missing is why is it so bad to have a driver register to
>> > multiple subsystems.  
>> 
>> Because these PCI devices seem to do "different" things all in one PCI
>> resource set.  Blame the hardware designers :)
>
>See below, I don't think you can blame the HW designers in this
>particular case :)
>
>> > For the nfp I think the _real_ reason to have a bus was that it
>> > was expected to have some out-of-tree modules bind to it. Something 
>> > I would not encourage :)  
>> 
>> That's not ok, and I agree with you.
>> 
>> But there seems to be some more complex PCI devices that do lots of
>> different things all at once.  Kind of like a PCI device that wants to
>> be both a keyboard and a storage device at the same time (i.e. a button
>> on a disk drive...)
>
>The keyboard which is also a storage device may be a clear cut case
>where multiple devices were integrated into one bus endpoint.

Also, I think that very important differentiator between keyboard/button
and NIC is that keyboard/button is fixed. You have driver bus with 2
devices on constant addresses.

However in case of NIC subfunctions. You have 0 at he beginning and user
instructs to create more (maybe hundreds). Now important questions
appear:

1) How to create devices (what API) - mdev has this figured out
2) How to to do the addressing of the devices. Needs to be
   predictable/defined by the user - mdev has this figured out
3) Udev names of netdevices - udev names that according to the
   bus/address. That is straightforeward with mdev.
   I can't really see how to figure this one in particular with
   per-driver busses :/

>
>The case with these advanced networking adapters is a little different
>in that they are one HW device which has oodles of FW implementing
>clients or acceleration for various networking protocols.
>
>The nice thing about having a fake bus is you can load out-of-tree
>drivers to operate extra protocols quite cleanly.
>
>I'm not saying that's what the code in question is doing, I'm saying 
>I'd personally like to understand the motivation more clearly before
>every networking driver out there starts spawning buses. The only
>argument I've heard so far for the separate devices is reloading subset
>of the drivers, which I'd rate as moderately convincing.