On Thursday, November 9th, 2023 at 13:14, Maira Canal <mcanal@xxxxxxxxxx> wrote: > On 11/9/23 04:45, Simon Ser wrote: > > User-space sometimes needs to allocate scanout-capable memory for > > GPU rendering purposes. On a vc4/v3d split render/display SoC, this > > is achieved via DRM dumb buffers: the v3d user-space driver opens > > the primary vc4 node, allocates a DRM dumb buffer there, exports it > > as a DMA-BUF, imports it into the v3d render node, and renders to it. > > > > However, DRM dumb buffers are only meant for CPU rendering, they are > > not intended to be used for GPU rendering. Primary nodes should only > > be used for mode-setting purposes, other programs should not attempt > > to open it. Moreover, opening the primary node is already broken on > > some setups: systemd grants permission to open primary nodes to > > physically logged in users, but this breaks when the user is not > > physically logged in (e.g. headless setup) and when the distribution > > is using a different init (e.g. Alpine Linux uses openrc). > > > > We need an alternate way for v3d to allocate scanout-capable memory. > > For me, it is a bit unclear how we import the vc4 DMA-BUF heap into v3d. > Should we create an IOCTL for it on v3d? Hm, but v3d is the render driver, so it shouldn't have any knowledge of what vc4 -- the display driver -- actually needs to be able to scanout a piece of memory. It's true that other x86 drivers like amdgpu and i915 just have their own IOCTL to allocate scanout-capable memory, however the split render/display SoC situation makes the situation a bit more hairy for vc4/v3d. You can think of the vc4 DMA-BUF heaps as a vc4 alloc IOCTL, except it's using a more standard framework instead of a custom IOCTL. Does this make sense? > Also, if you need some help testing with the RPi 5, you can ping on IRC > and I can try to help by testing. Thank you, appreciated!
