drm/i915/dp: BDW cdclk fix for DP audio

According to BSpec, cdclk for BDW has to be not less than 432 MHz with DP
audio enabled, port width x4, and link rate HBR2 (5.4 GHz). With cdclk less
than 432 MHz, enabling audio leads to pipe FIFO underruns and displays
cycling on/off.

From BSpec:
"Display» BDW-SKL» dpr» [Register] DP_TP_CTL [BDW+,EXCLUDE(CHV)]
Workaround : Do not use DisplayPort with CDCLK less than 432 MHz, audio
enabled, port width x4, and link rate HBR2 (5.4 GHz), or else there may
be audio corruption or screen corruption."

Since, some DP configurations (e.g., MST) use port width x4 and HBR2
link rate, let's increase the cdclk to >= 432 MHz to enable audio for those
cases.

v4: Changed commit message
v3: Combine BDW pixel rate adjustments into a function (Jani)
v2: Restrict fix to BDW
    Retain the set cdclk across modesets (Ville)
Cc: stable@vger.kernel.org
Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1478026080-2925-1-git-send-email-dhinakaran.pandiyan@intel.com

drivers/gpu/drm/i915/intel_display.c

@@ -10247,6 +10247,27 @@ static void bxt_modeset_commit_cdclk(struct drm_atomic_state *old_state)
 	bxt_set_cdclk(to_i915(dev), req_cdclk);
 }
 
+static int bdw_adjust_min_pipe_pixel_rate(struct intel_crtc_state *crtc_state,
+					  int pixel_rate)
+{
+	/* pixel rate mustn't exceed 95% of cdclk with IPS on BDW */
+	if (crtc_state->ips_enabled)
+		pixel_rate = DIV_ROUND_UP(pixel_rate * 100, 95);
+
+	/* BSpec says "Do not use DisplayPort with CDCLK less than
+	 * 432 MHz, audio enabled, port width x4, and link rate
+	 * HBR2 (5.4 GHz), or else there may be audio corruption or
+	 * screen corruption."
+	 */
+	if (intel_crtc_has_dp_encoder(crtc_state) &&
+	    crtc_state->has_audio &&
+	    crtc_state->port_clock >= 540000 &&
+	    crtc_state->lane_count == 4)
+		pixel_rate = max(432000, pixel_rate);
+
+	return pixel_rate;
+}
+
 /* compute the max rate for new configuration */
 static int ilk_max_pixel_rate(struct drm_atomic_state *state)
 {
@@ -10272,9 +10293,9 @@ static int ilk_max_pixel_rate(struct drm_atomic_state *state)
 
 		pixel_rate = ilk_pipe_pixel_rate(crtc_state);
 
-		/* pixel rate mustn't exceed 95% of cdclk with IPS on BDW */
-		if (IS_BROADWELL(dev_priv) && crtc_state->ips_enabled)
-			pixel_rate = DIV_ROUND_UP(pixel_rate * 100, 95);
+		if (IS_BROADWELL(dev_priv))
+			pixel_rate = bdw_adjust_min_pipe_pixel_rate(crtc_state,
+								    pixel_rate);
 
 		intel_state->min_pixclk[i] = pixel_rate;
 	}