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

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Date: Thu, 9 May 2024 17:41:18 +0300
From: Oded Gabbay <ogabbay@...nel.org>
To: Tomeu Vizoso <tomeu@...euvizoso.net>
Cc: Jeffrey Hugo <quic_jhugo@...cinc.com>, linux-kernel@...r.kernel.org,
	Lucas Stach <l.stach@...gutronix.de>,
	Russell King <linux+etnaviv@...linux.org.uk>,
	Christian Gmeiner <christian.gmeiner@...il.com>,
	David Airlie <airlied@...il.com>, Daniel Vetter <daniel@...ll.ch>,
	etnaviv@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH] drm/etnaviv: Create an accel device node if compute-only

On Thu, May 09, 2024 at 03:53:01PM +0200, Tomeu Vizoso wrote:
> Oded, Dave,
> 
> Do you have an opinion on this?
> 
> Thanks,
> 
> Tomeu
Hi Tomeu,

Sorry for not replying earlier, I was down with Covid (again...).

To your question, I don't have an objection to what you are
suggesting. My personal view of accel is that it is an integral part of 
DRM and therefore, if there is an *existing* drm driver that wants to 
create an accel node, I'm not against it. 

There is the question of why you want to expose an accel node, and
here I would like to hear Dave's and Sima's opinion on your suggested
solution as it may affect the direction of other drm drivers.

Thanks,
Oded.

p.s.
Please only use bottom-posting when replying, thanks :)

> 
> On Fri, Apr 26, 2024 at 8:10 AM Tomeu Vizoso <tomeu@...euvizoso.net> wrote:
> >
> > On Thu, Apr 25, 2024 at 8:59 PM Jeffrey Hugo <quic_jhugo@...cinc.com> wrote:
> > >
> > > On 4/24/2024 12:37 AM, Tomeu Vizoso wrote:
> > > > If we expose a render node for NPUs without rendering capabilities, the
> > > > userspace stack will offer it to compositors and applications for
> > > > rendering, which of course won't work.
> > > >
> > > > Userspace is probably right in not questioning whether a render node
> > > > might not be capable of supporting rendering, so change it in the kernel
> > > > instead by exposing a /dev/accel node.
> > > >
> > > > Before we bring the device up we don't know whether it is capable of
> > > > rendering or not (depends on the features of its blocks), so first try
> > > > to probe a rendering node, and if we find out that there is no rendering
> > > > hardware, abort and retry with an accel node.
> > > >
> > > > Signed-off-by: Tomeu Vizoso <tomeu@...euvizoso.net>
> > > > Cc: Oded Gabbay <ogabbay@...nel.org>
> > >
> > > I hope Oded chimes in as Accel maintainer.  I think Airlie/Vetter had
> > > also previously mentioned they'd have opinions on what is Accel vs DRM.
> > >
> > > This gets a nack from me in its current state.  This is not a strong
> > > nack, and I don't want to discourage you.  I think there is a path forward.
> > >
> > > The Accel subsystem documentation says that accel drivers will reside in
> > > drivers/accel/ but this does not.
> >
> > Indeed, there is that code organization aspect.
> >
> > > Also, the commit text for "accel: add dedicated minor for accelerator
> > > devices" mentions -
> > >
> > > "for drivers that
> > > declare they handle compute accelerator, using a new driver feature
> > > flag called DRIVER_COMPUTE_ACCEL. It is important to note that this
> > > driver feature is mutually exclusive with DRIVER_RENDER. Devices that
> > > want to expose both graphics and compute device char files should be
> > > handled by two drivers that are connected using the auxiliary bus
> > > framework."
> > >
> > > I don't see any of that happening here (two drivers connected by aux
> > > bus, one in drivers/accel).
> >
> > Well, the text refers to devices, not drivers. The case we are talking
> > about is a driver that wants to sometimes expose an accel node, and
> > sometimes a render node, depending on the hardware it is dealing with.
> > So there would either be a device exposing a single render node, or a
> > device exposing a single accel node.
> >
> > Though by using the auxiliary bus we could in theory solve the code
> > organization problem mentioned above, I'm not quite seeing how to do
> > this in a clean way. The driver in /drivers/gpu/drm would have to be a
> > DRM driver that doesn't register a DRM device, but registers a device
> > in the auxiliary bus for the driver in /drivers/accel to bind to? Or
> > are you seeing some possibility that would fit better in the current
> > DRM framework?
> >
> > > I think this is the first case we've had of a combo DRM/Accel usecase,
> > > and so there isn't an existing example to refer you to on how to
> > > structure things.  I think you are going to be the first example where
> > > we figure all of this out.
> >
> > Yep, I will be grateful for any ideas on how to structure this.
> >
> > > On a more implementation note, ioctls for Accel devices should not be
> > > marked DRM_RENDER_ALLOW.  Seems like your attempt to reuse as much of
> > > the code as possible trips over this.
> >
> > Indeed, thanks.
> >
> > Cheers,
> >
> > Tomeu
> >
> > > -Jeff