[PATCH v2] drm: writeback: Document behaviour for framebuffer size mismatch

[Derek Foreman <derek.foreman@collabora.com>]

Currently we have not explicitly defined the behaviour when the width
and height of the writeback framebuffer do not match the width and
height of the attached CRTC.

Allow the driver to perform a scale operation to fit the content, so
systems with scaling blocks in the writeback path can easily expose them.
A partially filled buffer should never be returned, so failure remains
an option for all the systems that don't have a scaler, or when scaler
limits are exceeded.

Cropping would make less sense, as the region to crop is underspecified
without some other properties to define an origin.

Signed-off-by: Derek Foreman <derek.foreman@collabora.com>
---
I have a draft merge request against weston for this at:
 https://gitlab.freedesktop.org/wayland/weston/-/merge_requests/2071

Changes in v2:
  - Make it clear that a buffer should not be partially filled

 drivers/gpu/drm/drm_writeback.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/gpu/drm/drm_writeback.c b/drivers/gpu/drm/drm_writeback.c
index 4da5d6094721..b4a002e6d043 100644
--- a/drivers/gpu/drm/drm_writeback.c
+++ b/drivers/gpu/drm/drm_writeback.c
@@ -58,6 +58,12 @@
  *	framebuffer to be written by the writeback connector. This property is
  *	similar to the FB_ID property on planes, but will always read as zero
  *	and is not preserved across commits.
+ *	If the width and height of the framebuffer do not match those of the
+ *	attached CRTC, the driver must either fail or scale (not crop) the
+ *	content to exactly fit the framebuffer.
+ *	If the driver is unable to exactly fill the framebuffer for any reason,
+ *	such as hardware scaler constraints or an odd width for a sub-sampled
+ *	format, the writeback must fail instead of partially filling the buffer.
  *	Userspace must set this property to an output buffer every time it
  *	wishes the buffer to get filled.
  *
-- 
2.53.0