On Mon, May 20, 2024 at 1:19 PM Daniel Stone <daniel@xxxxxxxxxxxxx> wrote: > > 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. What I don't understand yet from Lucas proposal is how this isn't going to break existing userspace. I mean, even if we find a good way of having userspace skip non-rendering render nodes, what about existing userspace that isn't able to do that? Any updates to newer kernels are going to break them. Regards, Tomeu
