Re: Patches for i830 flicker on panning

[Daniel Vetter <daniel@ffwll.ch>]

On Tue, Nov 05, 2013 at 12:20:58AM +0100, Thomas Richter wrote:
> Hi Daniel, dear intel experts,
> 
> the following is a "solution" for the flicker on i830 panning. To
> remind you, when panning is enabled on an i830,
> the display shows an annoying 30Hz flicker on some "forbidden"
> positions. On such positions, half of the frames
> is black, and the other half shows a screen that is "ripped in
> half", with one half scrolled to the wrong
> position, and the other half displayed correctly, with a
> discontinuity in the middle.
> That is, every other frame is black, and the ones that are not black
> are "wrong".
> 
> The patch is, arguably, ugly as hell, and creates a certain
> "yerkiness", though it does its job. One of the side
> effects this flicker causes is complete crashes and lockups with
> video overlays which are avoided by the patch.
> That is, if you play a video with xine, and then pan over a
> forbidden position, the machine crashes and comes
> to a complete stop.
> 
> Sorry if the patch format isn't quite right, and the patch is not
> quite in a final state (not yet fully tested on all possible
> combinations of panning dimensions and display depths), but it's a
> start and "works good enough for me".

Patch is pretty badly whitespace mangled please format it according to
Documentation/SubmittingPatches (git format-patch preferred). Also I think
we should conditionalize this on i830M only, I don't see any other
reports.

One thing to try out still: Does this still work if you tell the ddx to
use a linear frontbuffer?
-Daniel


> 
> For that, patch: i9xx_update_plane(...) in intel_display.c,
> 
>     if (INTEL_INFO(dev)->gen >= 4) {
>         I915_MODIFY_DISPBASE(DSPSURF(plane),
>                      i915_gem_obj_ggtt_offset(obj) +
> intel_crtc->dspaddr_offset);
>         I915_WRITE(DSPTILEOFF(plane), (y << 16) | x);
>         I915_WRITE(DSPLINOFF(plane), linear_offset);
> +    } else if (INTEL_INFO(dev)->gen == 2) {
> +      int xflick = (x >> 4) & 0x03;
> +      if (xflick == 3) {       /* in forbidden column, probably a
> i830 bug */
> +        int xfix  = 16 + (x & ~(0x3f));
> +        int fix_offset = y * fb->pitches[0] + xfix *
> (fb->bits_per_pixel / 8);
> +        I915_WRITE(DSPADDR(plane), i915_gem_obj_ggtt_offset(obj) +
> fix_offset);
> +        POSTING_READ(reg);
> +        intel_wait_for_vblank(dev, intel_crtc->pipe);
> +      }
> +      I915_WRITE(DSPADDR(plane), i915_gem_obj_ggtt_offset(obj) +
> linear_offset);
>     } else
>         I915_WRITE(DSPADDR(plane), i915_gem_obj_ggtt_offset(obj) +
> linear_offset);
>     POSTING_READ(reg);
> 
> The trick is, as you see, first to display the one frame the display
> scrolled off *before* the forbidden column, and
> then scroll to the right position, going to the right. )-:
> 
> *Why* that works is unclear to me. Don't ask, just observation.
> 
> An improvement would be to check whether the x position did change,
> and if it did not, just avoid all the hack to
> allow smooth vertical scrolling. Still to be done. Maybe later the
> week if the above also works on the R31
> (affected by the same problem).
> 
> Greetings,
>     Thomas
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch