[RFC] drm: Allow DRM_IOCTL_MODE_MAP_DUMB for render nodes

[Emil Velikov]

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?

Thanks
Emil