[PATCH] Enable dithering on intel VCH DVO chips on 18bpp panels

[Thomas Richter <thor@math.tu-berlin.de>]

Hi folks,

this is a patch against drm-intel-nightly that enables an apparently 
undocumented feature of the intel VCH DVO chips. Bit 4 of the VR01 
register controls an automatic dithering for 18bpp outputs which greatly 
improves the visual image quality for 24bpp display buffers.

The bios of my IBM R31 automatically enables this feature, it is 
disabled again by the ivchr driver. The following patch checks whether 
the display is connected with a 18bpp connection and if so, re-enables 
dithering.

This feature has been tested to work fine on 16bpp images and with the 
scaler on and off. There are no ill side effects.

Please let me know whether this patch is acceptable.

Thanks and greetings,

	Thomas


Signed-off-by: Thomas Richter <thor@math.tu-berlin.de>
---
  drivers/gpu/drm/i915/dvo_ivch.c |   21 ++++++++++++++++++---
  1 file changed, 18 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/i915/dvo_ivch.c 
b/drivers/gpu/drm/i915/dvo_ivch.c
index 0f2587f..89b08a8 100644
--- a/drivers/gpu/drm/i915/dvo_ivch.c
+++ b/drivers/gpu/drm/i915/dvo_ivch.c
@@ -23,6 +23,9 @@
   * Authors:
   *    Eric Anholt <eric@anholt.net>
   *
+ * Minor modifications (Dithering enable):
+ *    Thomas Richter <thor@math.tu-berlin.de>
+ *
   */

  #include "dvo.h"
@@ -59,6 +62,8 @@
  # define VR01_DVO_BYPASS_ENABLE                (1 << 1)
  /** Enables the DVO clock */
  # define VR01_DVO_ENABLE               (1 << 0)
+/** Enable dithering for 18bpp panels. Not documented. */
+# define VR01_DITHER_ENABLE             (1 << 4)

  /*
   * LCD Interface Format
@@ -74,6 +79,8 @@
  # define VR10_INTERFACE_2X18           (2 << 2)
  /** Enables 2x24-bit LVDS output */
  # define VR10_INTERFACE_2X24           (3 << 2)
+/** Mask that defines the depth of the pipeline */
+# define VR10_INTERFACE_DEPTH_MASK      (3 << 2)

  /*
   * VR20 LCD Horizontal Display Size
@@ -342,9 +349,15 @@ static void ivch_mode_set(struct intel_dvo_device *dvo,
                           struct drm_display_mode *adjusted_mode)
  {
         uint16_t vr40 = 0;
-       uint16_t vr01;
+       uint16_t vr01 = 0;
+       uint16_t vr10;
+
+       ivch_read(dvo, VR10, &vr10);
+       /* Enable dithering for 18 bpp pipelines */
+       vr10 &= VR10_INTERFACE_DEPTH_MASK;
+       if (vr10 == VR10_INTERFACE_2X18 || vr10 == VR10_INTERFACE_1X18)
+               vr01 = VR01_DITHER_ENABLE;

-       vr01 = 0;
         vr40 = (VR40_STALL_ENABLE | VR40_VERTICAL_INTERP_ENABLE |
                 VR40_HORIZONTAL_INTERP_ENABLE);

@@ -353,7 +366,7 @@ static void ivch_mode_set(struct intel_dvo_device *dvo,
                 uint16_t x_ratio, y_ratio;

                 vr01 |= VR01_PANEL_FIT_ENABLE;
-               vr40 |= VR40_CLOCK_GATING_ENABLE;
+               vr40 |= VR40_CLOCK_GATING_ENABLE | 
VR40_ENHANCED_PANEL_FITTING;
                 x_ratio = (((mode->hdisplay - 1) << 16) /
                            (adjusted_mode->hdisplay - 1)) >> 2;
                 y_ratio = (((mode->vdisplay - 1) << 16) /
@@ -380,6 +393,8 @@ static void ivch_dump_regs(struct intel_dvo_device *dvo)
         DRM_DEBUG_KMS("VR00: 0x%04x\n", val);
         ivch_read(dvo, VR01, &val);
         DRM_DEBUG_KMS("VR01: 0x%04x\n", val);
+       ivch_read(dvo, VR10, &val);
+       DRM_DEBUG_KMS("VR10: 0x%04x\n", val);
         ivch_read(dvo, VR30, &val);
         DRM_DEBUG_KMS("VR30: 0x%04x\n", val);
         ivch_read(dvo, VR40, &val);
--
1.7.10.4
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