Hi, On Mon, 20 May 2024 at 08:39, Tomeu Vizoso <tomeu@xxxxxxxxxxxxxxx> wrote: > On Fri, May 10, 2024 at 10:34 AM Lucas Stach <l.stach@xxxxxxxxxxxxxx> 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
