[Intel-gfx] [PATCH 08/21] drm/i915/gem: Disallow bonding of virtual engines

[Jason Ekstrand]

On Thu, Apr 29, 2021 at 7:54 AM Tvrtko Ursulin
<tvrtko.ursulin at linux.intel.com> wrote:
>
>
> On 29/04/2021 13:24, Daniel Vetter wrote:
> > On Wed, Apr 28, 2021 at 04:51:19PM +0100, Tvrtko Ursulin wrote:
> >>
> >> On 23/04/2021 23:31, Jason Ekstrand wrote:
> >>> This adds a bunch of complexity which the media driver has never
> >>> actually used.  The media driver does technically bond a balanced engine
> >>> to another engine but the balanced engine only has one engine in the
> >>> sibling set.  This doesn't actually result in a virtual engine.
> >>
> >> For historical reference, this is not because uapi was over-engineered but
> >> because certain SKUs never materialized.
> >
> > Jason said that for SKU with lots of media engines media-driver sets up a
> > set of ctx in userspace with all the pairings (and I guess then load
> > balances in userspace or something like that). Tony Ye also seems to have
> > confirmed that. So I'm not clear on which SKU this is?
>
> Not sure if I should disclose it here. But anyway, platform which is
> currently in upstream and was supposed to be the first to use this uapi
> was supposed to have at least 4 vcs engines initially, or even 8 vcs + 4
> vecs at some point. That was the requirement uapi was designed for. For
> that kind of platform there were supposed to be two virtual engines
> created, with bonding, for instance parent = [vcs0, vcs2], child =
> [vcs1, vcs3]; bonds = [vcs0 - vcs1, vcs2 - vcs3]. With more engines the
> merrier.

I've added the following to the commit message:

    This functionality was originally added to handle cases where we may
    have more than two video engines and media might want to load-balance
    their bonded submits by, for instance, submitting to a balanced vcs0-1
    as the primary and then vcs2-3 as the secondary.  However, no such
    hardware has shipped thus far and, if we ever want to enable such
    use-cases in the future, we'll use the up-and-coming parallel submit API
    which targets GuC submission.

--Jason

> Userspace load balancing, from memory, came into the picture only as a
> consequence of balancing between two types of media pipelines which was
> either working around the rcs contention or lack of sfc, or both. Along
> the lines of - one stage of a media pipeline can be done either as GPGPU
> work, or on the media engine, and so userspace was deciding to spawn "a
> bit of these and a bit of those" to utilise all the GPU blocks. Not
> really about frame split virtual engines and bonding, but completely
> different load balancing, between gpgpu and fixed pipeline.

> > Or maybe the real deal is only future platforms, and there we have GuC
> > scheduler backend.
>
> Yes, because SKUs never materialised.
>
> > Not against adding a bit more context to the commit message, but we need
> > to make sure what we put there is actually correct. Maybe best to ask
> > Tony/Carl as part of getting an ack from them.
>
> I think there is no need - fact uapi was designed for way more engines
> than we got to have is straight forward enough.
>
> Only unasked for flexibility in the uapi was the fact bonding can
> express any dependency and not only N consecutive engines as media fixed
> function needed at the time. I say "at the time" because in fact the
> "consecutive" engines requirement also got more complicated / broken in
> a following gen (via fusing and logical instance remapping), proving the
> point having the uapi disassociated from the hw limitations of the _day_
> was a good call.
>
> Regards,
>
> Tvrtko