[PATCH] drm: drivers may provide multiple primary planes per CRTC

[Daniel Vetter]

On Thu, Aug 06, 2020 at 10:33:31AM +0000, Simon Ser wrote:
> Some drivers may expose primary planes compatible with multiple CRTCs.
> Make this clear in the docs: the current wording may be misunderstood as
> "exactly one primary plane per CRTC".
> 
> Signed-off-by: Simon Ser <contact at emersion.fr>
> Cc: Daniel Vetter <daniel at ffwll.ch>
> ---
>  drivers/gpu/drm/drm_plane.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_plane.c b/drivers/gpu/drm/drm_plane.c
> index b7b90b3a2e38..108a922e8c23 100644
> --- a/drivers/gpu/drm/drm_plane.c
> +++ b/drivers/gpu/drm/drm_plane.c
> @@ -49,8 +49,8 @@
>   * &struct drm_plane (possibly as part of a larger structure) and registers it
>   * with a call to drm_universal_plane_init().
>   *
> - * Cursor and overlay planes are optional. All drivers should provide one
> - * primary plane per CRTC to avoid surprising userspace too much. See enum
> + * Cursor and overlay planes are optional. All drivers should provide at least
> + * one primary plane per CRTC to avoid surprising userspace too much. See enum

I think that's even more confusing, since this reads like there could be
multiple primary planes for a specific CRTC. That's not the case, there'
only one pointer going from drm_crtc->primary to a drm_plane in the
kernel.

The problem is that userspace doesn't have a drm_property to read this
pointer, and needs to guess.

I thought the rule is:

Nth primary plane (or cursor) is the primary plane for the Nth crtc.
Enumaration with increasing drm kms object ids.

And I guess we should explain that on some hw any plane (including primary
ones, since that's only a sw construct) can be freely assinged to crtc.

Yes it's probably the most gloriously bonkers uapi we've come up with.
Might be so bad that a libdrm helper to look up the primary plane for a
crtc (or it's cursor plane if it exists) would be in order :-)

Cheers, Daniel


>   * drm_plane_type for a more in-depth discussion of these special uapi-relevant
>   * plane types. Special planes are associated with their CRTC by calling
>   * drm_crtc_init_with_planes().
> -- 
> 2.28.0
> 
> 

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