Re: [PATCH] drm: pl111: enable render node

[Emil Velikov <emil.l.velikov@gmail.com>]

On Fri, 24 Apr 2020 at 19:54, Peter Collingbourne <pcc@google.com> wrote:
>
> On Fri, Apr 24, 2020 at 4:11 AM Emil Velikov <emil.l.velikov@gmail.com> wrote:
> >
> > On Thu, 23 Apr 2020 at 23:51, Peter Collingbourne <pcc@google.com> wrote:
> > >
> > > The render node is required by Android which does not support the legacy
> > > drmAuth authentication process.
> > >
> > > Signed-off-by: Peter Collingbourne <pcc@google.com>
> > > ---
> > >  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
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