[PATCHv3 06/30] drm/omap: Add support for render nodes

[Daniel Vetter]

On Thu, Mar 30, 2017 at 08:44:25AM +0200, David Herrmann wrote:
> Hey
> 
> On Wed, Mar 29, 2017 at 11:42 PM, Laurent Pinchart
> <laurent.pinchart at ideasonboard.com> wrote:
> > Hi David,
> >
> > On Wednesday 29 Mar 2017 14:51:48 David Herrmann wrote:
> >> On Wed, Mar 29, 2017 at 2:20 PM, Laurent Pinchart wrote:
> >> > On Wednesday 29 Mar 2017 11:58:23 Tomi Valkeinen wrote:
> >> >> On 29/03/17 11:22, Laurent Pinchart wrote:
> >> >>> On Tuesday 28 Mar 2017 16:07:52 Tomi Valkeinen wrote:
> >> >>>> From: Hemant Hariyani <hemanthariyani at ti.com>
> >> >>>>
> >> >>>> Add support for render nodes in omap driver and allow required
> >> >>>> ioctls to be accessible via render nodes.
> >> >>>
> >> >>> But the OMAP DSS doesn't perform rendering... This seems an abuse of
> >> >>> render nodes, I think the API should instead be implemented by the GPU
> >> >>> driver.
> >> >>
> >> >> I agree that the GPU use case described in the patch sounds a bit odd.
> >> >> Why not allocate from the GPU driver instead. But here a particular
> >> >> issue is that to get TILER buffers we need to ask them from the omapdrm.
> >> >> Probably TILER should not be part of omapdrm, but then, where should it
> >> >> be...
> >> >>
> >> >> We also have writeback in DSS, which can function as a memory to memory
> >> >> or capture device. That's not supported in the mainline, but we have
> >> >> support in the TI kernel.
> >> >>
> >> >> And how about a case where you have the privileged process as KMS
> >> >> master, and another process wants to draw to a buffer with the CPU, and
> >> >> then give the buffer to the privileged process for displaying.
> >> >>
> >> >> So, yes, DSS is not a renderer (well, WB is kind of rendering), but
> >> >> isn't it a valid use case to allocate a buffer from omapdrm?
> >> >
> >> > It could be a valid use case, but it's still an API abuse. It starts
> >> > sounding like a DRM core issue to me. The DRIVER_RENDER flag is not
> >> > documented, so its exact meaning isn't defined. I thought it was supposed
> >> > to flag the device as a renderer (GPU).
> >> >
> >> > commit 1793126fcebd7c18834f95d43b55e387a8803aa8
> >> > Author: David Herrmann <dh.herrmann at gmail.com>
> >> > Date:   Sun Aug 25 18:29:00 2013 +0200
> >> >
> >> >     drm: implement experimental render nodes
> >> >
> >> >     Render nodes provide an API for userspace to use non-privileged GPU
> >> >     commands without any running DRM-Master. It is useful for offscreen
> >> >     rendering, GPGPU clients, and normal render clients which do not
> >> >     perform
> >> >     modesetting.
> >> >
> >> >     [...]
> >> >
> >> > You instead use the flag as a way to enable unprivileged clients to
> >> > allocate buffers. If that's what the flag should mean, I wonder if there
> >> > would be a use case for *not* setting it.
> >>
> >> Correct. You can (and should) enable all your sandboxed commands on
> >> render nodes. That is, if a command only affects the issuing client,
> >> then it is safe for render-nodes. If two clients have a file-context
> >> opened on the render node, they should be unable to affect each other
> >> (minus accounting, resource allocation, etc.).
> >>
> >> The name is historic (I did not come up with it either, but failed at
> >> renaming it..). The DRIVER_RENDER flag only controls whether the
> >> render-node is actually instantiated. I will not object doing that
> >> unconditionally. It will create some useless nodes for legacy drivers,
> >> but we should not care.
> >
> > Couldn't we achieve the same effect without render nodes, by allowing GEM
> > object allocation on the main DRM node by unauthenticated clients ?
> 
> Using a different inode makes sure you can sand-box the node. That is,
> you can now easily mknod a render node in containers and sandboxes
> without risk of exposing any other APIs.
> 
> I guess you could achieve something similar by careful selection of
> which privs to pass to the container. But we preferred a clean cut
> back then.

btw would be lovely if you folks could review the render node docs and
make sure they reflect all the nuances discussed here properly:

https://dri.freedesktop.org/docs/drm/gpu/drm-uapi.html#render-nodes

And if not, please send a patch.

Thanks, Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch