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>

1 files changed, 35 insertions, 14 deletions

diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
index 110552ff302c..4b637919f74f 100644
--- 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);