drm/vc4: Enable limited range RGB output on HDMI with CEA modes.

Fixes broken grayscale ramps on many HDMI monitors, where large areas
at the ends of the ramp would all appear as black or white.

Signed-off-by: Eric Anholt <eric@anholt.net>

drivers/gpu/drm/vc4/vc4_hdmi.c

@@ -298,6 +298,7 @@ static void vc4_hdmi_encoder_mode_set(struct drm_encoder *encoder,
 				      struct drm_display_mode *unadjusted_mode,
 				      struct drm_display_mode *mode)
 {
+	struct vc4_hdmi_encoder *vc4_encoder = to_vc4_hdmi_encoder(encoder);
 	struct drm_device *dev = encoder->dev;
 	struct vc4_dev *vc4 = to_vc4_dev(dev);
 	bool debug_dump_regs = false;
@@ -313,6 +314,7 @@ static void vc4_hdmi_encoder_mode_set(struct drm_encoder *encoder,
 	u32 vertb = (VC4_SET_FIELD(0, VC4_HDMI_VERTB_VSPO) |
 		     VC4_SET_FIELD(mode->vtotal - mode->vsync_end,
 				   VC4_HDMI_VERTB_VBP));
+	u32 csc_ctl;
 
 	if (debug_dump_regs) {
 		DRM_INFO("HDMI regs before:\n");
@@ -351,9 +353,34 @@ static void vc4_hdmi_encoder_mode_set(struct drm_encoder *encoder,
 		 (vsync_pos ? 0 : VC4_HD_VID_CTL_VSYNC_LOW) |
 		 (hsync_pos ? 0 : VC4_HD_VID_CTL_HSYNC_LOW));
 
+	csc_ctl = VC4_SET_FIELD(VC4_HD_CSC_CTL_ORDER_BGR,
+				VC4_HD_CSC_CTL_ORDER);
+
+	if (vc4_encoder->hdmi_monitor && drm_match_cea_mode(mode) > 1) {
+		/* CEA VICs other than #1 requre limited range RGB
+		 * output.  Apply a colorspace conversion to squash
+		 * 0-255 down to 16-235.  The matrix here is:
+		 *
+		 * [ 0      0      0.8594 16]
+		 * [ 0      0.8594 0      16]
+		 * [ 0.8594 0      0      16]
+		 * [ 0      0      0       1]
+		 */
+		csc_ctl |= VC4_HD_CSC_CTL_ENABLE;
+		csc_ctl |= VC4_HD_CSC_CTL_RGB2YCC;
+		csc_ctl |= VC4_SET_FIELD(VC4_HD_CSC_CTL_MODE_CUSTOM,
+					 VC4_HD_CSC_CTL_MODE);
+
+		HD_WRITE(VC4_HD_CSC_12_11, (0x000 << 16) | 0x000);
+		HD_WRITE(VC4_HD_CSC_14_13, (0x100 << 16) | 0x6e0);
+		HD_WRITE(VC4_HD_CSC_22_21, (0x6e0 << 16) | 0x000);
+		HD_WRITE(VC4_HD_CSC_24_23, (0x100 << 16) | 0x000);
+		HD_WRITE(VC4_HD_CSC_32_31, (0x000 << 16) | 0x6e0);
+		HD_WRITE(VC4_HD_CSC_34_33, (0x100 << 16) | 0x000);
+	}
+
 	/* The RGB order applies even when CSC is disabled. */
-	HD_WRITE(VC4_HD_CSC_CTL, VC4_SET_FIELD(VC4_HD_CSC_CTL_ORDER_BGR,
-					       VC4_HD_CSC_CTL_ORDER));
+	HD_WRITE(VC4_HD_CSC_CTL, csc_ctl);
 
 	HDMI_WRITE(VC4_HDMI_FIFO_CTL, VC4_HDMI_FIFO_CTL_MASTER_SLAVE_N);
 

drivers/gpu/drm/vc4/vc4_regs.h

@@ -528,10 +528,17 @@
 # define VC4_HD_CSC_CTL_MODE_SHIFT		2
 # define VC4_HD_CSC_CTL_MODE_RGB_TO_SD_YPRPB	0
 # define VC4_HD_CSC_CTL_MODE_RGB_TO_HD_YPRPB	1
-# define VC4_HD_CSC_CTL_MODE_CUSTOM		2
+# define VC4_HD_CSC_CTL_MODE_CUSTOM		3
 # define VC4_HD_CSC_CTL_RGB2YCC			BIT(1)
 # define VC4_HD_CSC_CTL_ENABLE			BIT(0)
 
+#define VC4_HD_CSC_12_11			0x044
+#define VC4_HD_CSC_14_13			0x048
+#define VC4_HD_CSC_22_21			0x04c
+#define VC4_HD_CSC_24_23			0x050
+#define VC4_HD_CSC_32_31			0x054
+#define VC4_HD_CSC_34_33			0x058
+
 #define VC4_HD_FRAME_COUNT			0x068
 
 /* HVS display list information. */