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@xxxxxxxxxxxxxxx>
Cc: Oded Gabbay <ogabbay@xxxxxxxxxx>
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
