Documentation/amdgpu/display: add DC color caps info

Add details about color correction capabilities and explain a bit about
differences between DC hw generations and also how they are mapped
between DRM and DC interface. Two schemas for DCN 2.0 and 3.0 (converted
to svg from the original png) is included to illustrate it. They were
obtained from a discussion[1] in the amd-gfx mailing list.

[1] https://lore.kernel.org/amd-gfx/20220422142811.dm6vtk6v64jcwydk@mail.igalia.com/

v1:
- remove redundant comments (Harry)
- fix typos (Harry)

v2:
- reword introduction of color section
- add co-dev tag for Harry - who provided most of the info
- fix typos (Tales)
- describe missing struct parameters (Tales and Siqueira)
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>
Reviewed-by: Tales Aparecida <tales.aparecida@gmail.com>
Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

Documentation/gpu/amdgpu/display/display-manager.rst

@@ -49,3 +49,37 @@ Color Management Properties
 
 .. kernel-doc:: drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_color.c
    :internal:
+
+
+DC Color Capabilities between DCN generations
+---------------------------------------------
+
+DRM/KMS framework defines three CRTC color correction properties: degamma,
+color transformation matrix (CTM) and gamma, and two properties for degamma and
+gamma LUT sizes. AMD DC programs some of the color correction features
+pre-blending but DRM/KMS has not per-plane color correction properties.
+
+In general, the DRM CRTC color properties are programmed to DC, as follows:
+CRTC gamma after blending, and CRTC degamma pre-blending. Although CTM is
+programmed after blending, it is mapped to DPP hw blocks (pre-blending). Other
+color caps available in the hw is not currently exposed by DRM interface and
+are bypassed.
+
+.. kernel-doc:: drivers/gpu/drm/amd/display/dc/dc.h
+   :doc: color-management-caps
+
+.. kernel-doc:: drivers/gpu/drm/amd/display/dc/dc.h
+   :internal:
+
+The color pipeline has undergone major changes between DCN hardware
+generations. What's possible to do before and after blending depends on
+hardware capabilities, as illustrated below by the DCN 2.0 and DCN 3.0 families
+schemas.
+
+**DCN 2.0 family color caps and mapping**
+
+.. kernel-figure:: dcn2_cm_drm_current.svg
+
+**DCN 3.0 family color caps and mapping**
+
+.. kernel-figure:: dcn3_cm_drm_current.svg

drivers/gpu/drm/amd/display/dc/dc.h

@@ -118,7 +118,26 @@ struct dc_plane_cap {
 	uint32_t min_height;
 };
 
-// Color management caps (DPP and MPC)
+/**
+ * DOC: color-management-caps
+ *
+ * **Color management caps (DPP and MPC)**
+ *
+ * Modules/color calculates various color operations which are translated to
+ * abstracted HW. DCE 5-12 had almost no important changes, but starting with
+ * DCN1, every new generation comes with fairly major differences in color
+ * pipeline. Therefore, we abstract color pipe capabilities so modules/DM can
+ * decide mapping to HW block based on logical capabilities.
+ */
+
+/**
+ * struct rom_curve_caps - predefined transfer function caps for degamma and regamma
+ * @srgb: RGB color space transfer func
+ * @bt2020: BT.2020 transfer func
+ * @gamma2_2: standard gamma
+ * @pq: perceptual quantizer transfer function
+ * @hlg: hybrid log–gamma transfer function
+ */
 struct rom_curve_caps {
 	uint16_t srgb : 1;
 	uint16_t bt2020 : 1;
@@ -127,36 +146,68 @@ struct rom_curve_caps {
 	uint16_t hlg : 1;
 };
 
+/**
+ * struct dpp_color_caps - color pipeline capabilities for display pipe and
+ * plane blocks
+ *
+ * @dcn_arch: all DCE generations treated the same
+ * @input_lut_shared: shared with DGAM. Input LUT is different than most LUTs,
+ * just plain 256-entry lookup
+ * @icsc: input color space conversion
+ * @dgam_ram: programmable degamma LUT
+ * @post_csc: post color space conversion, before gamut remap
+ * @gamma_corr: degamma correction
+ * @hw_3d_lut: 3D LUT support. It implies a shaper LUT before. It may be shared
+ * with MPC by setting mpc:shared_3d_lut flag
+ * @ogam_ram: programmable out/blend gamma LUT
+ * @ocsc: output color space conversion
+ * @dgam_rom_for_yuv: pre-defined degamma LUT for YUV planes
+ * @dgam_rom_caps: pre-definied curve caps for degamma 1D LUT
+ * @ogam_rom_caps: pre-definied curve caps for regamma 1D LUT
+ *
+ * Note: hdr_mult and gamut remap (CTM) are always available in DPP (in that order)
+ */
 struct dpp_color_caps {
-	uint16_t dcn_arch : 1; // all DCE generations treated the same
-	// input lut is different than most LUTs, just plain 256-entry lookup
-	uint16_t input_lut_shared : 1; // shared with DGAM
+	uint16_t dcn_arch : 1;
+	uint16_t input_lut_shared : 1;
 	uint16_t icsc : 1;
 	uint16_t dgam_ram : 1;
-	uint16_t post_csc : 1; // before gamut remap
+	uint16_t post_csc : 1;
 	uint16_t gamma_corr : 1;
-
-	// hdr_mult and gamut remap always available in DPP (in that order)
-	// 3d lut implies shaper LUT,
-	// it may be shared with MPC - check MPC:shared_3d_lut flag
 	uint16_t hw_3d_lut : 1;
-	uint16_t ogam_ram : 1; // blnd gam
+	uint16_t ogam_ram : 1;
 	uint16_t ocsc : 1;
 	uint16_t dgam_rom_for_yuv : 1;
 	struct rom_curve_caps dgam_rom_caps;
 	struct rom_curve_caps ogam_rom_caps;
 };
 
+/**
+ * struct mpc_color_caps - color pipeline capabilities for multiple pipe and
+ * plane combined blocks
+ *
+ * @gamut_remap: color transformation matrix
+ * @ogam_ram: programmable out gamma LUT
+ * @ocsc: output color space conversion matrix
+ * @num_3dluts: MPC 3D LUT; always assumes a preceding shaper LUT
+ * @shared_3d_lut: shared 3D LUT flag. Can be either DPP or MPC, but single
+ * instance
+ * @ogam_rom_caps: pre-definied curve caps for regamma 1D LUT
+ */
 struct mpc_color_caps {
 	uint16_t gamut_remap : 1;
 	uint16_t ogam_ram : 1;
 	uint16_t ocsc : 1;
-	uint16_t num_3dluts : 3; //3d lut always assumes a preceding shaper LUT
-	uint16_t shared_3d_lut:1; //can be in either DPP or MPC, but single instance
-
+	uint16_t num_3dluts : 3;
+	uint16_t shared_3d_lut:1;
 	struct rom_curve_caps ogam_rom_caps;
 };
 
+/**
+ * struct dc_color_caps - color pipes capabilities for DPP and MPC hw blocks
+ * @dpp: color pipes caps for DPP
+ * @mpc: color pipes caps for MPC
+ */
 struct dc_color_caps {
 	struct dpp_color_caps dpp;
 	struct mpc_color_caps mpc;