[RFC] drm: Allow DRM_IOCTL_MODE_MAP_DUMB for render nodes

[Emil Velikov]

On 7 August 2018 at 13:28, Daniel Vetter <daniel at ffwll.ch> wrote:
> On Tue, Aug 07, 2018 at 12:01:50PM +0100, Emil Velikov wrote:
>> On 3 August 2018 at 20:50, Sean Paul <seanpaul at chromium.org> wrote:
>> > On Fri, Aug 03, 2018 at 06:03:50PM +0100, Emil Velikov wrote:
>> >> On 3 August 2018 at 16:06, Martin Fuzzey <martin.fuzzey at flowbird.group> wrote:
>> >> > Hi Emil,
>> >> >
>> >> > On 03/08/18 14:35, Emil Velikov wrote:
>> >> >>
>> >> >> Hi Martin,
>> >> >>
>> >> >> On 1 August 2018 at 15:24, Martin Fuzzey <martin.fuzzey at flowbird.group>
>> >> >> wrote:
>> >> >>
>> >> >> Let's start with the not-so obvious question:
>> >> >> Why does one open the imx as render node?
>> >> >>
>> >> >> Of the top of my head:
>> >> >> There is nothing in egl/android that should require an authenticated
>> >> >> device.
>> >> >> Hence, using a card node should be fine - the etnaviv code opens the
>> >> >> render node it needs.
>> >> >
>> >> >
>> >> > Yes, the problem is not in egl/android but in the scanout buffer allocation
>> >> > code.
>> >> >
>> >> > etnaviv opens the render node on the *GPU* (for submitting GPU commands),
>> >> > that part is fine.
>> >> >
>> >> > But scanout buffers need to be allocated from imx-drm not etnaviv.
>> >> >
>> >> > This done by renderonly_create_kms_dumb_buffer_for_resource()
>> >> > [src/gallium/auxiliary/renderonly/renderonly.c]
>> >> > Which uses DRM_IOCTL_MODE_CREATE_DUMB followed by
>> >> > DRM_IOCTL_PRIME_FD_TO_HANDLE
>> >> > on the "kms_fd" (probably poorly named because it's not actually used for
>> >> > modesetting)
>> >> > see imx_drm_screen_create()[ src/gallium/winsys/imx/drm/imx_drm_winsys.c]
>> >> >
>> >> >
>> >> > If the card node is used DRM_IOCTL_MODE_CREATE_DUMB works but
>> >> > DRM_IOCTL_PRIME_FD_TO_HANDLE fails, because the permissions are
>> >> > DRM_AUTH|DRM_UNLOCKED|DRM_RENDER_ALLOW
>> >> >
>> >> Right I missed the DRM_AUTH, in the fd <> handle IOCTLs.
>> >> So in order for things to work, we'd need to either:
>> >>  - allow dumb buffers for render nodes, or
>> >>  - drop the DRM_AUTH for fd <> handle imports
>> >>
>> >> Pointing an alternative solution, for kernel developers to analyse and
>> >> make a decision.
>> >>
>> >> >
>> >> > In android 8.1 the hardware composer runs in a seperate process and it has
>> >> > to use the card node and be drm master (to use the KMS API),
>> >> > therefore, when the surface flinger calls
>> >> > renderonly_create_kms_dumb_buffer_for_resource() it is not authenticated.
>> >> >
>> >> > Making surface flinger use a render node fixes the problem for
>> >> > DRM_IOCTL_PRIME_FD_TO_HANDLE (because that already has DRM_RENDER_ALLOW),
>> >> > but DRM_IOCTL_MODE_CREATE_DUMB now fails without the patch.
>> >> >
>> >> >
>> >> > This probably worked in previous versions of Android where surface flinger
>> >> > and hwc were all in the same process.
>> >> >
>> >> There has been varying hacks for Android through the years. Bringing
>> >> details into the discussion will result in a significant diversion.
>> >> Something we could avoid, for the time being ;-)
>> >
>> > Did someone say diversion?!? The way this was handled prior to using
>> > render/control nodes in drm_hwc/[drm/gbm]_gralloc is that all modesetting was
>> > done via gralloc which was master. The hwc implementation was basically a proxy
>> > backchanneling all of the work to gralloc.
>> >
>> > Anyways, we probably don't want to go back there.
>> >
>> Now that we got the diversion out of the way, any input on my proposal
>> to drop the DRM_AUTH for fd <> imports.
>> Am I missing something pretty obvious that makes the idea a no-go?
>
> Dropping DRM_AUTH is only relevant for the card node. And a card node
> might not be sufficiently isolated against concurrent other clients, which
> is why we don't allow it.
>
Right. I did not spot anything that would make a distinction based on
the card vs render node used.

> What we could do is essentially check whether your driver supports render
> nodes (indicating sufficient amounts of separation), and then allow
> anything for unauthenicated clients if DRM_RENDER_ALLOW is set on the
> ioctl.
>
> But that's just reinventing render nodes on top of legacy card nodes, and
> I'm not clear on what that exactly gains us.
>
As more of a userspace person, it makes sense to keep render nodes for
GPU specifics and card ones - KMS/Display Controller.

> I think the proposal for dumb render nodes (for drivers which only do dumb
> kms buffers and no rendering at all) that's been discusson on irc a bit
> makes a lot more sense.

Ack. Thanks for shedding some light.

-Emil