drm/i915: properly compute dp dithering for user-created modes

We've only computed whether we need to fall back to 6bpc due to dp
link bandwidth constrains in mode_valid, but not mode_fixup. Under
various circumstances X likes to create new modes which then lack
proper 6bpc flags (if required), resulting in mode_fixup failures and
ultimately black screens.

Chris Wilson pointed out that we still get things wrong for bpp > 24,
but that should be fixed in another patch (and it'll be easier because
this patch consolidates the logic).

The likely culprit for this regression is

commit 3d794f87238f74d80e78a7611c7fbde8a54c85c2
Author: Keith Packard <keithp@keithp.com>
Date:   Wed Jan 25 08:16:25 2012 -0800

    drm/i915: Force explicit bpp selection for intel_dp_link_required

v2: Fix indentation and tune down the too bold claim that this should
fix the world. Both noticed by Chris Wilson.

v3: Try to really git add things.

Reported-and-tested-by: Brice Goglin <Brice.Goglin@ens-lyon.org>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=48170
Cc: stable@kernel.org
Reviewed-by: Adam Jackson <ajax@redhat.com>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
index 110552ff302c1dea71360dfc84da622d1cec1785..4b637919f74f6359b64fc996c04a2f2e6a353def 100644 (file)
--- a/drivers/gpu/drm/i915/intel_dp.c
+++ b/drivers/gpu/drm/i915/intel_dp.c
@@ -219,14 +219,38 @@ intel_dp_max_data_rate(int max_link_clock, int max_lanes)
        return (max_link_clock * max_lanes * 8) / 10;
 }
 
+static bool
+intel_dp_adjust_dithering(struct intel_dp *intel_dp,
+                         struct drm_display_mode *mode,
+                         struct drm_display_mode *adjusted_mode)
+{
+       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;
+
+       mode_rate = intel_dp_link_required(mode->clock, 24);
+       max_rate = intel_dp_max_data_rate(max_link_clock, max_lanes);
+
+       if (mode_rate > max_rate) {
+               mode_rate = intel_dp_link_required(mode->clock, 18);
+               if (mode_rate > max_rate)
+                       return false;
+
+               if (adjusted_mode)
+                       adjusted_mode->private_flags
+                               |= INTEL_MODE_DP_FORCE_6BPC;
+
+               return true;
+       }
+
+       return true;
+}
+
 static int
 intel_dp_mode_valid(struct drm_connector *connector,
                    struct drm_display_mode *mode)
 {
        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)
@@ -236,16 +260,8 @@ intel_dp_mode_valid(struct drm_connector *connector,
                        return MODE_PANEL;
        }
 
-       mode_rate = intel_dp_link_required(mode->clock, 24);
-       max_rate = intel_dp_max_data_rate(max_link_clock, max_lanes);
-
-       if (mode_rate > max_rate) {
-                       mode_rate = intel_dp_link_required(mode->clock, 18);
-                       if (mode_rate > max_rate)
-                               return MODE_CLOCK_HIGH;
-                       else
-                               mode->private_flags |= INTEL_MODE_DP_FORCE_6BPC;
-       }
+       if (!intel_dp_adjust_dithering(intel_dp, mode, NULL))
+               return MODE_CLOCK_HIGH;
 
        if (mode->clock < 10000)
                return MODE_CLOCK_LOW;
@@ -672,7 +688,7 @@ 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 : 24;
+       int bpp;
        static int bws[2] = { DP_LINK_BW_1_62, DP_LINK_BW_2_7 };
 
        if (is_edp(intel_dp) && intel_dp->panel_fixed_mode) {
@@ -686,6 +702,11 @@ intel_dp_mode_fixup(struct drm_encoder *encoder, struct drm_display_mode *mode,
                mode->clock = intel_dp->panel_fixed_mode->clock;
        }
 
+       if (!intel_dp_adjust_dithering(intel_dp, mode, adjusted_mode))
+               return false;
+
+       bpp = adjusted_mode->private_flags & INTEL_MODE_DP_FORCE_6BPC ? 18 : 24;
+
        for (lane_count = 1; lane_count <= max_lane_count; lane_count <<= 1) {
                for (clock = 0; clock <= max_clock; clock++) {
                        int link_avail = intel_dp_max_data_rate(intel_dp_link_clock(bws[clock]), lane_count);