Hi Lucas, Do you have any idea on how not to break userspace if we expose a render node? Cheers, Tomeu On Wed, Jun 12, 2024 at 4:26 PM Tomeu Vizoso <tomeu@xxxxxxxxxxxxxxx> wrote: > > 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
