linux-kernel - Re: [PATCH] drm/etnaviv: Create an accel device node if compute-only

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Date: Fri, 28 Jun 2024 19:47:10 +0200
From: Daniel Vetter <daniel@...ll.ch>
To: Daniel Stone <daniel@...ishbar.org>
Cc: Lucas Stach <l.stach@...gutronix.de>,
	Tomeu Vizoso <tomeu@...euvizoso.net>, linux-kernel@...r.kernel.org,
	Oded Gabbay <ogabbay@...nel.org>,
	Russell King <linux+etnaviv@...linux.org.uk>,
	Christian Gmeiner <christian.gmeiner@...il.com>,
	David Airlie <airlied@...il.com>, etnaviv@...ts.freedesktop.org,
	dri-devel@...ts.freedesktop.org,
	Daniel Stone <daniels@...labora.com>,
	Daniel Vetter <daniel@...ll.ch>
Subject: Re: [PATCH] drm/etnaviv: Create an accel device node if compute-only

On Wed, Jun 26, 2024 at 08:26:04PM +0100, Daniel Stone wrote:
> On Wed, 26 Jun 2024 at 18:52, Daniel Vetter <daniel@...ll.ch> wrote:
> > On Wed, Jun 26, 2024 at 11:39:01AM +0100, Daniel Stone wrote:
> > > On Wed, 26 Jun 2024 at 09:28, Lucas Stach <l.stach@...gutronix.de> wrote:
> > > > So we are kind of stuck here between breaking one or the other use-
> > > > case. I'm leaning heavily into the direction of just fixing Mesa, so we
> > > > can specify the type of screen we need at creation time to avoid the
> > > > renderonly issue, porting this change as far back as reasonably
> > > > possible and file old userspace into shit-happens.
> > >
> > > Yeah, honestly this sounds like the best solution to me too.
> >
> > Yeah mesa sounds kinda broken here ...
> >
> > What might work in the kernel is if you publish a fake 3d engine that's
> > too new for broken mesa, if that's enough to make it fail to bind? And if
> > mesa still happily binds against that, then yeah it's probably too broken
> > and we need etnaviv-v2 (as a drm driver uapi name, I think that's what
> > mesa filters?) for anything new (including the NN-only ones).
> >
> > I would still try to avoid that, but just in case someone screams about
> > regressions.
> 
> It's not just etnaviv, it's literally every Mesa driver which works
> with decoupled render/display. So that would be etnaviv-v2,
> panfrost-v2, panthor-v2, v3d-v2, powervr-v2, ... albeit those don't
> tend to have multiple instances.

So essentially mesa just burns&crashes when old mesa runs on a newer
kernel with support for a chip that mesa doesn't know about?

> Anyway, I'm still leaning towards the answer being: this is not an
> etnaviv regression caused by NPU, it's a longstanding generic Mesa
> issue for which the answer is to fix the known fragility.

If the above is correct, then yes I think we should just fix mesa. Feels
like the breakage is too obviously there, and that's all we'll do unless
the screaming gets too loud.
-Sima
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch