drm/gamma: Clarify gamma lut uapi

Interpreting it as a 0.16 fixed point means we can't accurately
represent 1.0. Which is one of the values we really should be able to
represent.

Since most (all?) luts have lower precision this will only affect
rounding of 0xffff.

Cc: Uma Shankar <[email protected]>
Cc: Ville Syrjälä <[email protected]>
Cc: Shashank Sharma <[email protected]>
Cc: "Kumar, Kiran S" <[email protected]>
Cc: Kausal Malladi <[email protected]>
Cc: Lionel Landwerlin <[email protected]>
Cc: Matt Roper <[email protected]>
Cc: Rob Bradford <[email protected]>
Cc: Daniel Stone <[email protected]>
Cc: Stefan Schake <[email protected]>
Cc: Eric Anholt <[email protected]>
Cc: Maarten Lankhorst <[email protected]>
Cc: Harry Wentland <[email protected]>
Cc: Leo Li <[email protected]>
Cc: [email protected]
Cc: James (Qian) Wang <[email protected]>
Cc: Liviu Dudau <[email protected]>
Cc: Mali DP Maintainers <[email protected]>
Cc: CK Hu <[email protected]>
Cc: Philipp Zabel <[email protected]>
Cc: Yannick Fertre <[email protected]>
Cc: Philippe Cornu <[email protected]>
Cc: Benjamin Gaignard <[email protected]>
Cc: Vincent Abriou <[email protected]>
Cc: Tomi Valkeinen <[email protected]>
Cc: Boris Brezillon <[email protected]>
Signed-off-by: Daniel Vetter <[email protected]>
Acked-by: Liviu Dudau <[email protected]>
Acked-by: Lionel Landwerlin <[email protected]>
Reviewed-by: Ville Syrjälä <[email protected]>
Reviewed-by: Matt Roper <[email protected]>
Reviewed-by: Philippe Cornu <[email protected]>
Reviewed-by: Maarten Lankhorst <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

1 files changed, 2 insertions, 1 deletions

diff --git a/include/uapi/drm/drm_mode.h b/include/uapi/drm/drm_mode.h
index 09d72966899a..83cd1636b9be 100644
--- a/include/uapi/drm/drm_mode.h
+++ b/include/uapi/drm/drm_mode.h
@@ -621,7 +621,8 @@ struct drm_color_ctm {
 
 struct drm_color_lut {
 	/*
-	 * Data is U0.16 fixed point format.
+	 * Values are mapped linearly to 0.0 - 1.0 range, with 0x0 == 0.0 and
+	 * 0xffff == 1.0.
 	 */
 	__u16 red;
 	__u16 green;