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

[Tomeu Vizoso]

On Thu, Apr 25, 2024 at 8:59 PM Jeffrey Hugo <quic_jhugo at quicinc.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 at tomeuvizoso.net>
> > Cc: Oded Gabbay <ogabbay at kernel.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