renderonly: write down usage rules
The renderonly helpers are extremely easy to mis-use. Write down the expectations.
I've seen many mistakes in the past, including:
- Forgetting to create the scanout resource on import 1 2, causing bugs such as 3.
- Assuming the scanout resource always exists 4.
- Returning a GEM handle valid for the driver's internal DRM FD, but invalid for the caller's DRM FD 5.
- Not implementing resource_get_param, breaking stride/offset/modifier queries when no scanout resource is available 6 7.
Signed-off-by: Simon Ser contact@emersion.fr
cc @daniels @anarsoul @alyssa @enunes @yuq825 @lynxeye @austriancoder @jpewhacker @itoral @chema @tomeu @robclark @anholt @roman.stratiienko
(Sorry for the huge cc list, I just want to make sure everyone using renderonly agrees about this.)