drm/msm/dpu: document INTF_EDP/INTF_DP difference

Based on the discussions on the mailing list, document enum
dpu_intf_type and it's controversial fields: INTF_DP and INTF_EDP.

INTF_EDP is used for older eDP interface found on msm8x74/msm8x84
INTF_DP is used for both eDP and DP interfaces handled by the msm/dp
driver. The DPU driver does not make a difference between them.
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Patchwork: https://patchwork.freedesktop.org/patch/475555/
Link: https://lore.kernel.org/r/20220222062246.242577-2-dmitry.baryshkov@linaro.orgSigned-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>

drivers/gpu/drm/msm/disp/dpu1/dpu_hw_mdss.h

@@ -217,12 +217,20 @@ enum dpu_intf {
 	INTF_MAX
 };
 
+/*
+ * Historically these values correspond to the values written to the
+ * DISP_INTF_SEL register, which had to programmed manually. On newer MDP
+ * generations this register is NOP, but we keep the values for historical
+ * reasons.
+ */
 enum dpu_intf_type {
 	INTF_NONE = 0x0,
 	INTF_DSI = 0x1,
 	INTF_HDMI = 0x3,
 	INTF_LCDC = 0x5,
+	/* old eDP found on 8x74 and 8x84 */
 	INTF_EDP = 0x9,
+	/* both DP and eDP,  handled by the new DP driver */
 	INTF_DP = 0xa,
 	INTF_TYPE_MAX,