drm/amd/display: document AMDGPU pre-defined transfer functions

Brief documentation about pre-defined transfer function usage on AMD
display driver and standardized EOTFs and inverse EOTFs.

v3:
- Document BT709 OETF (Pekka)
- Fix description of sRGB and pure power funcs (Pekka)

v4:
- Add description of linear and non-linear forms (Harry)
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Co-developed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Melissa Wen <mwen@igalia.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_color.c

@@ -85,6 +85,68 @@ void amdgpu_dm_init_color_mod(void)
 }
 
 #ifdef AMD_PRIVATE_COLOR
+/* Pre-defined Transfer Functions (TF)
+ *
+ * AMD driver supports pre-defined mathematical functions for transferring
+ * between encoded values and optical/linear space. Depending on HW color caps,
+ * ROMs and curves built by the AMD color module support these transforms.
+ *
+ * The driver-specific color implementation exposes properties for pre-blending
+ * degamma TF, shaper TF (before 3D LUT), and blend(dpp.ogam) TF and
+ * post-blending regamma (mpc.ogam) TF. However, only pre-blending degamma
+ * supports ROM curves. AMD color module uses pre-defined coefficients to build
+ * curves for the other blocks. What can be done by each color block is
+ * described by struct dpp_color_capsand struct mpc_color_caps.
+ *
+ * AMD driver-specific color API exposes the following pre-defined transfer
+ * functions:
+ *
+ * - Identity: linear/identity relationship between pixel value and
+ *   luminance value;
+ * - Gamma 2.2, Gamma 2.4, Gamma 2.6: pure power functions;
+ * - sRGB: 2.4: The piece-wise transfer function from IEC 61966-2-1:1999;
+ * - BT.709: has a linear segment in the bottom part and then a power function
+ *   with a 0.45 (~1/2.22) gamma for the rest of the range; standardized by
+ *   ITU-R BT.709-6;
+ * - PQ (Perceptual Quantizer): used for HDR display, allows luminance range
+ *   capability of 0 to 10,000 nits; standardized by SMPTE ST 2084.
+ *
+ * The AMD color model is designed with an assumption that SDR (sRGB, BT.709,
+ * Gamma 2.2, etc.) peak white maps (normalized to 1.0 FP) to 80 nits in the PQ
+ * system. This has the implication that PQ EOTF (non-linear to linear) maps to
+ * [0.0..125.0] where 125.0 = 10,000 nits / 80 nits.
+ *
+ * Non-linear and linear forms are described in the table below:
+ *
+ * ┌───────────┬─────────────────────┬──────────────────────┐
+ * │           │     Non-linear      │   Linear             │
+ * ├───────────┼─────────────────────┼──────────────────────┤
+ * │      sRGB │ UNORM or [0.0, 1.0] │ [0.0, 1.0]           │
+ * ├───────────┼─────────────────────┼──────────────────────┤
+ * │     BT709 │ UNORM or [0.0, 1.0] │ [0.0, 1.0]           │
+ * ├───────────┼─────────────────────┼──────────────────────┤
+ * │ Gamma 2.x │ UNORM or [0.0, 1.0] │ [0.0, 1.0]           │
+ * ├───────────┼─────────────────────┼──────────────────────┤
+ * │        PQ │ UNORM or FP16 CCCS* │ [0.0, 125.0]         │
+ * ├───────────┼─────────────────────┼──────────────────────┤
+ * │  Identity │ UNORM or FP16 CCCS* │ [0.0, 1.0] or CCCS** │
+ * └───────────┴─────────────────────┴──────────────────────┘
+ * * CCCS: Windows canonical composition color space
+ * ** Respectively
+ *
+ * In the driver-specific API, color block names attached to TF properties
+ * suggest the intention regarding non-linear encoding pixel's luminance
+ * values. As some newer encodings don't use gamma curve, we make encoding and
+ * decoding explicit by defining an enum list of transfer functions supported
+ * in terms of EOTF and inverse EOTF, where:
+ *
+ * - EOTF (electro-optical transfer function): is the transfer function to go
+ *   from the encoded value to an optical (linear) value. De-gamma functions
+ *   traditionally do this.
+ * - Inverse EOTF (simply the inverse of the EOTF): is usually intended to go
+ *   from an optical/linear space (which might have been used for blending)
+ *   back to the encoded values. Gamma functions traditionally do this.
+ */
 static const char * const
 amdgpu_transfer_function_names[] = {
 	[AMDGPU_TRANSFER_FUNCTION_DEFAULT]		= "Default",