[PATCH v4] drm/color: Document CTM eqations

[Brian Starkey]

Explicitly state the expected CTM equations in the kerneldoc for the CTM
property, and the form of the matrix on struct drm_color_ctm.

Cc: Ville Syrjälä <ville.syrjala at linux.intel.com>
Cc: Lionel Landwerlin <lionel.g.landwerlin at intel.com>
Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
Signed-off-by: Brian Starkey <brian.starkey at arm.com>
---
 drivers/gpu/drm/drm_color_mgmt.c |   13 +++++++++++++
 include/uapi/drm/drm_mode.h      |   14 +++++++++++++-
 2 files changed, 26 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/drm_color_mgmt.c b/drivers/gpu/drm/drm_color_mgmt.c
index 789b4c65cd69..307f7b0545b6 100644
--- a/drivers/gpu/drm/drm_color_mgmt.c
+++ b/drivers/gpu/drm/drm_color_mgmt.c
@@ -62,6 +62,19 @@
  *	unit/pass-thru matrix should be used. This is generally the driver
  *	boot-up state too.
  *
+ *	The output vector is related to the input vector as below::
+ *
+ *		out[0] = matrix[0] * in[0] + matrix[1] * in[1] + matrix[2] * in[2];
+ *		out[1] = matrix[3] * in[0] + matrix[4] * in[1] + matrix[5] * in[2];
+ *		out[2] = matrix[6] * in[0] + matrix[7] * in[1] + matrix[8] * in[2];
+ *
+ *	The component order in the input/output vectors is assumed to be
+ *	{ R, G, B }.
+ *
+ *	The color-space of the input vector must not be confused with the
+ *	color-space implied by a framebuffer pixel format, which may be the same
+ *	or different.
+ *
  * “GAMMA_LUT”:
  *	Blob property to set the gamma lookup table (LUT) mapping pixel data
  *	after the transformation matrix to data sent to the connector. The
diff --git a/include/uapi/drm/drm_mode.h b/include/uapi/drm/drm_mode.h
index ce7efe2e8a5e..3f179b8867e5 100644
--- a/include/uapi/drm/drm_mode.h
+++ b/include/uapi/drm/drm_mode.h
@@ -525,7 +525,19 @@ struct drm_mode_crtc_lut {
 };
 
 struct drm_color_ctm {
-	/* Conversion matrix in S31.32 format. */
+	/*
+	 * Conversion matrix in row-major form:
+	 *
+	 * | matrix[0] matrix[1] matrix[2] |
+	 * | matrix[3] matrix[4] matrix[5] |
+	 * | matrix[6] matrix[7] matrix[8] |
+	 *
+	 * Each element is in a sign + magnitude format: S31.32, with the sign
+	 * in bit 63, and the magnitude represented as fixed-point 31.32.
+	 *
+	 * i.e. +1.25 = 0x00000001.40000000
+	 *      -1.25 = 0x80000001.40000000
+	 */
 	__s64 matrix[9];
 };
 
-- 
1.7.9.5