[PATCH] drm/gamma: Clarify gamma lut uapi

[Daniel Vetter]

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 <uma.shankar at intel.com>
Cc: Ville Syrjälä <ville.syrjala at linux.intel.com>
Cc: Shashank Sharma <shashank.sharma at intel.com>
Cc: "Kumar, Kiran S" <kiran.s.kumar at intel.com>
Cc: Kausal Malladi <kausalmalladi at gmail.com>
Cc: Lionel Landwerlin <lionel.g.landwerlin at intel.com>
Cc: Matt Roper <matthew.d.roper at intel.com>
Cc: Rob Bradford <robert.bradford at intel.com>
Cc: Daniel Stone <daniels at collabora.com>
Cc: Stefan Schake <stschake at gmail.com>
Cc: Eric Anholt <eric at anholt.net>
Cc: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
Cc: Harry Wentland <harry.wentland at amd.com>
Cc: Leo Li <sunpeng.li at amd.com>
Cc: amd-gfx at lists.freedesktop.org
Cc: James (Qian) Wang <james.qian.wang at arm.com>
Cc: Liviu Dudau <liviu.dudau at arm.com>
Cc: Mali DP Maintainers <malidp at foss.arm.com>
Cc: CK Hu <ck.hu at mediatek.com>
Cc: Philipp Zabel <p.zabel at pengutronix.de>
Cc: Yannick Fertre <yannick.fertre at st.com>
Cc: Philippe Cornu <philippe.cornu at st.com>
Cc: Benjamin Gaignard <benjamin.gaignard at linaro.org>
Cc: Vincent Abriou <vincent.abriou at st.com>
Cc: Tomi Valkeinen <tomi.valkeinen at ti.com>
Cc: Boris Brezillon <bbrezillon at kernel.org>
Signed-off-by: Daniel Vetter <daniel.vetter at ffwll.ch>Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>
---
 include/uapi/drm/drm_mode.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

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;
-- 
2.20.1