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

[Daniel Stone]

Hi,

On Mon, 20 May 2024 at 08:39, Tomeu Vizoso <tomeu at tomeuvizoso.net> wrote:
> On Fri, May 10, 2024 at 10:34 AM Lucas Stach <l.stach at pengutronix.de> wrote:
> > Am Mittwoch, dem 24.04.2024 um 08:37 +0200 schrieb Tomeu Vizoso:
> > > 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.
> >
> > On the other hand we already have precedence of compute only DRM
> > devices exposing a render node: there are AMD GPUs that don't expose a
> > graphics queue and are thus not able to actually render graphics. Mesa
> > already handles this in part via the PIPE_CAP_GRAPHICS and I think we
> > should simply extend this to not offer a EGL display on screens without
> > that capability.
>
> The problem with this is that the compositors I know don't loop over
> /dev/dri files, trying to create EGL screens and moving to the next
> one until they find one that works.
>
> They take the first render node (unless a specific one has been
> configured), and assumes it will be able to render with it.
>
> To me it seems as if userspace expects that /dev/dri/renderD* devices
> can be used for rendering and by breaking this assumption we would be
> breaking existing software.

Mm, it's sort of backwards from that. Compositors just take a
non-render DRM node for KMS, then ask GBM+EGL to instantiate a GPU
which can work with that. When run in headless mode, we don't take
render nodes directly, but instead just create an EGLDisplay or
VkPhysicalDevice and work backwards to a render node, rather than
selecting a render node and going from there.

So from that PoV I don't think it's really that harmful. The only
complication is in Mesa, where it would see an etnaviv/amdgpu/...
render node and potentially try to use it as a device. As long as Mesa
can correctly skip, there should be no userspace API implications.

That being said, I'm not entirely sure what the _benefit_ would be of
exposing a render node for a device which can't be used by any
'traditional' DRM consumers, i.e. GL/Vulkan/winsys.

Cheers,
Daniel