drm/sun4i: hdmi: Document PAD_CTRL1 output invert bits

While debugging inverted color from the HDMI output on the A10, I
found that the lowest 3 bits were set. These were cleared on A20
boards that had normal display output. By manually toggling these
bits the mapping of the color components to these bits was found.

While these are not used anywhere, it would be nice to document
them somewhere.
Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171014040252.9621-7-wens@csie.org

drivers/gpu/drm/sun4i/sun4i_hdmi.h

@@ -72,6 +72,11 @@
 #define SUN4I_HDMI_PAD_CTRL1_HALVE_CLK		BIT(6)
 #define SUN4I_HDMI_PAD_CTRL1_REG_AMP(n)		(((n) & 7) << 3)
 
+/* These bits seem to invert the TMDS data channels */
+#define SUN4I_HDMI_PAD_CTRL1_INVERT_R		BIT(2)
+#define SUN4I_HDMI_PAD_CTRL1_INVERT_G		BIT(1)
+#define SUN4I_HDMI_PAD_CTRL1_INVERT_B		BIT(0)
+
 #define SUN4I_HDMI_PLL_CTRL_REG		0x208
 #define SUN4I_HDMI_PLL_CTRL_PLL_EN		BIT(31)
 #define SUN4I_HDMI_PLL_CTRL_BWS			BIT(30)