drm/i915: use pipe_config for lvds dithering

Up to now we've relied on the bios to get this right for us. Let's try
out whether our code has improved a bit, since we should dither
always when the output bpp doesn't match the plane bpp.
- gen5+ should be fine, since we only use the bios hint as an upgrade.
- gen4 changes, since here dithering is still controlled in the lvds
  register.
- gen2/3 has implicit dithering depeding upon whether you use 2 or 3
  lvds pairs (which makes sense, since it only supports 8bpc pipe
  outpu configurations).
- hsw doesn't support lvds.

v2: Remove redudant dither setting.

v3: Completly drop reliance on dev_priv->lvds_dither.

v4: Enable dithering on gen2/3 only when we have a 18bpp panel, since
up-dithering to a 24bpp panel is not supported by the hw. Spotted by
Ville.

v5: Also only enable lvds port dithering on gen4 for 18bpp modes. In
practice this only excludes dithering a 10bpc plane down for a 24bpp
lvds panel. Not something we truly care about. Again noticed by Ville.

v6: Actually git add.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

3 files changed, 22 insertions, 23 deletions

diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
index 26acc421f0c6..38465f0e9710 100644
--- a/drivers/gpu/drm/i915/intel_display.c
+++ b/drivers/gpu/drm/i915/intel_display.c
@@ -5146,8 +5146,7 @@ static int ironlake_get_refclk(struct drm_crtc *crtc)
 }
 
 static void ironlake_set_pipeconf(struct drm_crtc *crtc,
-                                  struct drm_display_mode *adjusted_mode,
-                                  bool dither)
+                                  struct drm_display_mode *adjusted_mode)
 {
         struct drm_i915_private *dev_priv = crtc->dev->dev_private;
         struct intel_crtc *intel_crtc = to_intel_crtc(crtc);
@@ -5176,7 +5175,7 @@ static void ironlake_set_pipeconf(struct drm_crtc *crtc,
         }
 
         val &= ~(PIPECONF_DITHER_EN | PIPECONF_DITHER_TYPE_MASK);
-        if (dither)
+        if (intel_crtc->config.dither)
                 val |= (PIPECONF_DITHER_EN | PIPECONF_DITHER_TYPE_SP);
 
         val &= ~PIPECONF_INTERLACE_MASK;
@@ -5259,8 +5258,7 @@ static void intel_set_pipe_csc(struct drm_crtc *crtc)
 }
 
 static void haswell_set_pipeconf(struct drm_crtc *crtc,
-                                 struct drm_display_mode *adjusted_mode,
-                                 bool dither)
+                                 struct drm_display_mode *adjusted_mode)
 {
         struct drm_i915_private *dev_priv = crtc->dev->dev_private;
         struct intel_crtc *intel_crtc = to_intel_crtc(crtc);
@@ -5270,7 +5268,7 @@ static void haswell_set_pipeconf(struct drm_crtc *crtc,
         val = I915_READ(PIPECONF(cpu_transcoder));
 
         val &= ~(PIPECONF_DITHER_EN | PIPECONF_DITHER_TYPE_MASK);
-        if (dither)
+        if (intel_crtc->config.dither)
                 val |= (PIPECONF_DITHER_EN | PIPECONF_DITHER_TYPE_SP);
 
         val &= ~PIPECONF_INTERLACE_MASK_HSW;
@@ -5631,7 +5629,7 @@ static int ironlake_crtc_mode_set(struct drm_crtc *crtc,
         bool is_lvds = false;
         struct intel_encoder *encoder;
         int ret;
-        bool dither, fdi_config_ok;
+        bool fdi_config_ok;
 
         for_each_encoder_on_crtc(dev, crtc, encoder) {
                 switch (encoder->type) {
@@ -5666,11 +5664,6 @@ static int ironlake_crtc_mode_set(struct drm_crtc *crtc,
         /* Ensure that the cursor is valid for the new mode before changing... */
         intel_crtc_update_cursor(crtc, true);
 
-        /* determine panel color depth */
-        dither = intel_crtc->config.dither;
-        if (is_lvds && dev_priv->lvds_dither)
-                dither = true;
-
         DRM_DEBUG_KMS("Mode for pipe %c:\n", pipe_name(pipe));
         drm_mode_debug_printmodeline(mode);
 
@@ -5737,7 +5730,7 @@ static int ironlake_crtc_mode_set(struct drm_crtc *crtc,
 
         fdi_config_ok = ironlake_check_fdi_lanes(intel_crtc);
 
-        ironlake_set_pipeconf(crtc, adjusted_mode, dither);
+        ironlake_set_pipeconf(crtc, adjusted_mode);
 
         /* Set up the display plane register */
         I915_WRITE(DSPCNTR(plane), DISPPLANE_GAMMA_ENABLE);
@@ -5814,7 +5807,6 @@ static int haswell_crtc_mode_set(struct drm_crtc *crtc,
         bool is_cpu_edp = false;
         struct intel_encoder *encoder;
         int ret;
-        bool dither;
 
         for_each_encoder_on_crtc(dev, crtc, encoder) {
                 switch (encoder->type) {
@@ -5850,9 +5842,6 @@ static int haswell_crtc_mode_set(struct drm_crtc *crtc,
         /* Ensure that the cursor is valid for the new mode before changing... */
         intel_crtc_update_cursor(crtc, true);
 
-        /* determine panel color depth */
-        dither = intel_crtc->config.dither;
-
         DRM_DEBUG_KMS("Mode for pipe %c:\n", pipe_name(pipe));
         drm_mode_debug_printmodeline(mode);
 
@@ -5866,7 +5855,7 @@ static int haswell_crtc_mode_set(struct drm_crtc *crtc,
         if (intel_crtc->config.has_pch_encoder)
                 ironlake_fdi_set_m_n(crtc);
 
-        haswell_set_pipeconf(crtc, adjusted_mode, dither);
+        haswell_set_pipeconf(crtc, adjusted_mode);
 
         intel_set_pipe_csc(crtc);
 
diff --git a/drivers/gpu/drm/i915/intel_drv.h b/drivers/gpu/drm/i915/intel_drv.h
index 3595c8204649..a5fe976364e1 100644
--- a/drivers/gpu/drm/i915/intel_drv.h
+++ b/drivers/gpu/drm/i915/intel_drv.h
@@ -213,6 +213,11 @@ struct intel_crtc_config {
         /* DP has a bunch of special case unfortunately, so mark the pipe
          * accordingly. */
         bool has_dp_encoder;
+
+        /*
+         * Enable dithering, used when the selected pipe bpp doesn't match the
+         * plane bpp.
+         */
         bool dither;
 
         /* Controls for the clock computation, to override various stages. */
diff --git a/drivers/gpu/drm/i915/intel_lvds.c b/drivers/gpu/drm/i915/intel_lvds.c
index 563f5052fcfc..84085454b104 100644
--- a/drivers/gpu/drm/i915/intel_lvds.c
+++ b/drivers/gpu/drm/i915/intel_lvds.c
@@ -136,7 +136,10 @@ static void intel_pre_pll_enable_lvds(struct intel_encoder *encoder)
          * special lvds dither control bit on pch-split platforms, dithering is
          * only controlled through the PIPECONF reg. */
         if (INTEL_INFO(dev)->gen == 4) {
-                if (dev_priv->lvds_dither)
+                /* Bspec wording suggests that LVDS port dithering only exists
+                 * for 18bpp panels. */
+                if (intel_crtc->config.dither &&
+                    intel_crtc->config.pipe_bpp == 18)
                         temp |= LVDS_ENABLE_DITHER;
                 else
                         temp &= ~LVDS_ENABLE_DITHER;
@@ -335,7 +338,13 @@ static bool intel_lvds_compute_config(struct intel_encoder *intel_encoder,
                 DRM_DEBUG_KMS("forcing display bpp (was %d) to LVDS (%d)\n",
                               pipe_config->pipe_bpp, lvds_bpp);
                 pipe_config->pipe_bpp = lvds_bpp;
+
+                /* Make sure pre-965 set dither correctly for 18bpp panels. */
+                if (INTEL_INFO(dev)->gen < 4 && lvds_bpp == 18)
+                        pfit_control |= PANEL_8TO6_DITHER_ENABLE;
+
         }
+
         /*
          * We have timings from the BIOS for the panel, put them in
          * to the adjusted mode.  The CRTC will be set up for this mode,
@@ -470,10 +479,6 @@ out:
                 pfit_pgm_ratios = 0;
         }
 
-        /* Make sure pre-965 set dither correctly */
-        if (INTEL_INFO(dev)->gen < 4 && dev_priv->lvds_dither)
-                pfit_control |= PANEL_8TO6_DITHER_ENABLE;
-
         if (pfit_control != lvds_encoder->pfit_control ||
             pfit_pgm_ratios != lvds_encoder->pfit_pgm_ratios) {
                 lvds_encoder->pfit_control = pfit_control;