On Fri, 24 Apr 2020 at 19:54, Peter Collingbourne <pcc@xxxxxxxxxx> wrote: > > On Fri, Apr 24, 2020 at 4:11 AM Emil Velikov <emil.l.velikov@xxxxxxxxx> wrote: > > > > On Thu, 23 Apr 2020 at 23:51, Peter Collingbourne <pcc@xxxxxxxxxx> wrote: > > > > > > The render node is required by Android which does not support the legacy > > > drmAuth authentication process. > > > > > > Signed-off-by: Peter Collingbourne <pcc@xxxxxxxxxx> > > > --- > > > drivers/gpu/drm/pl111/pl111_drv.c | 2 +- > > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > > The summary talks about drmAuth, yet exposes a render node. Even > > through there's no rendering engine in the HW, as mentioned by Eric. > > > > AFAICT the only way drmAuth is relevant with pl111 is when you want to > > export/import dma bufs. > > Although that is handled in core and the artificial DRM_AUTH > > restriction has been lifted with commit [1]. > > > > -Emil > > > > [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?h=v5.7-rc2&id=30a958526d2cc6df38347336a602479d048d92e7 > > Okay, most likely drmAuth is irrelevant here (I don't know much about > it to be honest; I know that Android uses render nodes, so I figured > that drmAuth must therefore be the thing that it doesn't use). Sorry > for the confusion. Here is a better explanation of why I needed this > change. > > Android has a composer process that opens the primary node and uses > DRM_IOCTL_MODE_ATOMIC to switch between frame buffers, and a renderer > process (surfaceflinger) that opens the render node, prepares frame > buffers and sends them to the composer. One idea for adapting this > architecture to devices without render nodes is to have the renderer > process open the primary node instead. But this runs into a problem: > suppose that the renderer process starts before the composer process. > In this case, the kernel makes the renderer the DRM master, so the > composer can't change the frame buffer. Render nodes don't have this > problem because opening them doesn't affect the master. > > I considered fixing this by having the composer issue > DRM_IOCTL_SET_MASTER, but this requires root privileges. If we require > drivers to provide render nodes and control access to the primary node > while opening up the render node, we can ensure that only authorized > processes can become the master without requiring them to be root. > I think that the crux, is trying to open a _primary_ node for _rendering_ purposes. While that may work - as you outline above - it's usually a bad idea. To step back a bit, in practise we have three SoC combinations: - complete lack of render device - then you default to software rendering [1] - display and render device are one and the same - no change needed, things should just work - display and render devices are separate - surfaceflinger should open the correct _rendering_ device node. [1] Mesa's libEGL (don't recall exact name on Android) can open VGEM render node, to work with the kms-swrast_dri.so software rasteriser module. While I did not try [1] with Android, it was working fine with CrOS. I suggest giving it a try and reporting bugs. Thanks Emil _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
