[PATCH 8/8] drm/doc: document the type plane property

[Simon Ser]

On Wednesday, December 16th, 2020 at 10:23 PM, Daniel Vetter <daniel at ffwll.ch> wrote:

> > + * type:
> > + *     Immutable property describing the type of the plane.
> > + *
> > + *     For user-space which has enabled the &DRM_CLIENT_CAP_UNIVERSAL_PLANES
>
> While we're at this: Does the kerneldoc for this cap mention that it's
> implicitly enabled when you're enabling atomic?
>
> Maybe worth repeating here too.

Good point. v2 will do both.

> > + *     capability, the plane type is just a hint and is mostly superseded by
> > + *     atomic test-only commits. The type hint can still be used to come up
> > + *     more easily with a plane configuration accepted by the driver.
> > + *
> > + *     The value of this property can be one of the following:
> > + *
> > + *     "Primary":
> > + *         To light up a CRTC, attaching a primary plane is the most likely to
> > + *         work if it covers the whole CRTC and doesn't have scaling or
> > + *         cropping set up.
> > + *
> > + *         Drivers may support more features for the primary plane, user-space
> > + *         can find out with test-only atomic commits.
>
> We need to mention here that this is the implicit plane used by the
> PAGE_FLIP and SETCRTC ioctl (maybe spell them out in full since these are
> userspace docs).

I intentionally didn't write that down here, because as previously discussed,
user-space has no way to guess the drm_crtc.{primary,cursor} pointers, so
user-space cannot guess which planes will be used for legacy IOCTLs. Adding any
hint that user-space _could_ do it will result in broken user-space.