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

[Jeffrey Hugo]

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.

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).

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.

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.

-Jeff