[PATCH] drm/i915: Avoid flicker with horizontal panning on 830GM

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

Dear intel-gfx developers,

When panning is enabled on the 830GM, horizontal panning creates a lot 
of flickering on specific pixel positions.
After testing, I found that the reason for this is that panning works by 
altering the frame origin pointer, which,
however, has certain alignment restrictions. If the pointer is not 
aligned correctly, the screen starts to flicker
as, probably, DMA fails.

The following patch against drm/i915/intel_display.c fixes the issue by 
ensuring correct alignment. As result,
horizontal panning works correctly, but is a bit "jumpy". Unclear 
whether the problem affects any other
chipset revisions, thus the patch is currently only enabled for rev.2.

Greetings,
     Thomas


diff --git a/drivers/gpu/drm/i915/intel_display.c 
b/drivers/gpu/drm/i915/intel_display.c
index bcb62fe..8304e30 100644
--- a/drivers/gpu/drm/i915/intel_display.c
+++ b/drivers/gpu/drm/i915/intel_display.c
@@ -1987,8 +1987,13 @@ static int i9xx_update_plane(struct drm_crtc 
*crtc, struct drm_framebuffer *fb,

         I915_WRITE(reg, dspcntr);

-       linear_offset = y * fb->pitches[0] + x * (fb->bits_per_pixel / 8);
-
+       if (INTEL_INFO(dev)->gen > 2) {
+         linear_offset = y * fb->pitches[0] + x * (fb->bits_per_pixel / 8);
+       } else {
+         /* align the linear offset to 64 pixel boundaries */
+         linear_offset = y * fb->pitches[0] + (x & -32) * 
(fb->bits_per_pixel / 8);
+       }
+
         if (INTEL_INFO(dev)->gen >= 4) {
                 intel_crtc->dspaddr_offset =
                         intel_gen4_compute_page_offset(&x, &y, 
obj->tiling_mode,