drm/i915/dp: Dither down to 6bpc if it makes the mode fit

Some active adaptors (VGA usually) only have two lanes at 2.7GHz.
That's a maximum pixel clock of 144MHz at 8bpc, but 192MHz at 6bpc.

Fixes Asus UX31 panel being black at startup due to no valid modes since
dc22ee6fc18ce0f15424e753e8473c306ece95c1.

v2: Rebased to current code, resulting in the fix applying to EDP panels as
    well.  Also changed from spatio-temporal to just spatial dithering on
    pre-ironlake, to be conssitent (and less visual flicker)

Signed-off-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Eric Anholt <eric@anholt.net>
Tested-by: Eric Anholt <eric@anholt.net>
Tested-by: Dirk Hohndel <hohndel@infradead.org>
Signed-off-by: Keith Packard <keithp@keithp.com>

drivers/gpu/drm/i915/intel_display.c

@@ -4670,6 +4670,7 @@ static inline bool intel_panel_use_ssc(struct drm_i915_private *dev_priv)
 /**
  * intel_choose_pipe_bpp_dither - figure out what color depth the pipe should send
  * @crtc: CRTC structure
+ * @mode: requested mode
  *
  * A pipe may be connected to one or more outputs.  Based on the depth of the
  * attached framebuffer, choose a good color depth to use on the pipe.
@@ -4681,13 +4682,15 @@ static inline bool intel_panel_use_ssc(struct drm_i915_private *dev_priv)
  *    HDMI supports only 8bpc or 12bpc, so clamp to 8bpc with dither for 10bpc
  *    Displays may support a restricted set as well, check EDID and clamp as
  *      appropriate.
+ *    DP may want to dither down to 6bpc to fit larger modes
  *
  * RETURNS:
  * Dithering requirement (i.e. false if display bpc and pipe bpc match,
  * true if they don't match).
  */
 static bool intel_choose_pipe_bpp_dither(struct drm_crtc *crtc,
-					 unsigned int *pipe_bpp)
+					 unsigned int *pipe_bpp,
+					 struct drm_display_mode *mode)
 {
 	struct drm_device *dev = crtc->dev;
 	struct drm_i915_private *dev_priv = dev->dev_private;
@@ -4758,6 +4761,11 @@ static bool intel_choose_pipe_bpp_dither(struct drm_crtc *crtc,
 		}
 	}
 
+	if (mode->private_flags & INTEL_MODE_DP_FORCE_6BPC) {
+		DRM_DEBUG_KMS("Dithering DP to 6bpc\n");
+		display_bpc = 6;
+	}
+
 	/*
 	 * We could just drive the pipe at the highest bpc all the time and
 	 * enable dithering as needed, but that costs bandwidth.  So choose
@@ -5019,6 +5027,16 @@ static int i9xx_crtc_mode_set(struct drm_crtc *crtc,
 			pipeconf &= ~PIPECONF_DOUBLE_WIDE;
 	}
 
+	/* default to 8bpc */
+	pipeconf &= ~(PIPECONF_BPP_MASK | PIPECONF_DITHER_EN);
+	if (is_dp) {
+		if (mode->private_flags & INTEL_MODE_DP_FORCE_6BPC) {
+			pipeconf |= PIPECONF_BPP_6 |
+				    PIPECONF_DITHER_EN |
+				    PIPECONF_DITHER_TYPE_SP;
+		}
+	}
+
 	dpll |= DPLL_VCO_ENABLE;
 
 	DRM_DEBUG_KMS("Mode for pipe %c:\n", pipe == 0 ? 'A' : 'B');
@@ -5480,7 +5498,7 @@ static int ironlake_crtc_mode_set(struct drm_crtc *crtc,
 	/* determine panel color depth */
 	temp = I915_READ(PIPECONF(pipe));
 	temp &= ~PIPE_BPC_MASK;
-	dither = intel_choose_pipe_bpp_dither(crtc, &pipe_bpp);
+	dither = intel_choose_pipe_bpp_dither(crtc, &pipe_bpp, mode);
 	switch (pipe_bpp) {
 	case 18:
 		temp |= PIPE_6BPC;

drivers/gpu/drm/i915/intel_dp.c

@@ -208,13 +208,15 @@ intel_dp_link_clock(uint8_t link_bw)
  */
 
 static int
-intel_dp_link_required(struct intel_dp *intel_dp, int pixel_clock)
+intel_dp_link_required(struct intel_dp *intel_dp, int pixel_clock, int check_bpp)
 {
 	struct drm_crtc *crtc = intel_dp->base.base.crtc;
 	struct intel_crtc *intel_crtc = to_intel_crtc(crtc);
 	int bpp = 24;
 
-	if (intel_crtc)
+	if (check_bpp)
+		bpp = check_bpp;
+	else if (intel_crtc)
 		bpp = intel_crtc->bpp;
 
 	return (pixel_clock * bpp + 9) / 10;
@@ -233,6 +235,7 @@ intel_dp_mode_valid(struct drm_connector *connector,
 	struct intel_dp *intel_dp = intel_attached_dp(connector);
 	int max_link_clock = intel_dp_link_clock(intel_dp_max_link_bw(intel_dp));
 	int max_lanes = intel_dp_max_lane_count(intel_dp);
+	int max_rate, mode_rate;
 
 	if (is_edp(intel_dp) && intel_dp->panel_fixed_mode) {
 		if (mode->hdisplay > intel_dp->panel_fixed_mode->hdisplay)
@@ -242,9 +245,17 @@ intel_dp_mode_valid(struct drm_connector *connector,
 			return MODE_PANEL;
 	}
 
-	if (intel_dp_link_required(intel_dp, mode->clock)
-	    > intel_dp_max_data_rate(max_link_clock, max_lanes))
-		return MODE_CLOCK_HIGH;
+	mode_rate = intel_dp_link_required(intel_dp, mode->clock, 0);
+	max_rate = intel_dp_max_data_rate(max_link_clock, max_lanes);
+
+	if (mode_rate > max_rate) {
+			mode_rate = intel_dp_link_required(intel_dp,
+							   mode->clock, 18);
+			if (mode_rate > max_rate)
+				return MODE_CLOCK_HIGH;
+			else
+				mode->private_flags |= INTEL_MODE_DP_FORCE_6BPC;
+	}
 
 	if (mode->clock < 10000)
 		return MODE_CLOCK_LOW;
@@ -672,6 +683,7 @@ intel_dp_mode_fixup(struct drm_encoder *encoder, struct drm_display_mode *mode,
 	int lane_count, clock;
 	int max_lane_count = intel_dp_max_lane_count(intel_dp);
 	int max_clock = intel_dp_max_link_bw(intel_dp) == DP_LINK_BW_2_7 ? 1 : 0;
+	int bpp = mode->private_flags & INTEL_MODE_DP_FORCE_6BPC ? 18 : 0;
 	static int bws[2] = { DP_LINK_BW_1_62, DP_LINK_BW_2_7 };
 
 	if (is_edp(intel_dp) && intel_dp->panel_fixed_mode) {
@@ -689,7 +701,7 @@ intel_dp_mode_fixup(struct drm_encoder *encoder, struct drm_display_mode *mode,
 		for (clock = 0; clock <= max_clock; clock++) {
 			int link_avail = intel_dp_max_data_rate(intel_dp_link_clock(bws[clock]), lane_count);
 
-			if (intel_dp_link_required(intel_dp, mode->clock)
+			if (intel_dp_link_required(intel_dp, mode->clock, bpp)
 					<= link_avail) {
 				intel_dp->link_bw = bws[clock];
 				intel_dp->lane_count = lane_count;

drivers/gpu/drm/i915/intel_drv.h

@@ -110,6 +110,7 @@
 /* drm_display_mode->private_flags */
 #define INTEL_MODE_PIXEL_MULTIPLIER_SHIFT (0x0)
 #define INTEL_MODE_PIXEL_MULTIPLIER_MASK (0xf << INTEL_MODE_PIXEL_MULTIPLIER_SHIFT)
+#define INTEL_MODE_DP_FORCE_6BPC (0x10)
 
 static inline void
 intel_mode_set_pixel_multiplier(struct drm_display_mode *mode,