[RFC] drm: Allow DRM_IOCTL_MODE_MAP_DUMB for render nodes

[Martin Fuzzey]

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


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.

Regards,

Martin