drm/i915: Try harder to disable trickle feed on VLV
The specs are a bit unclear whether the per-plane trickle feed disable control exists on VLV. There is another trickle feed disable control in the MI_ARB register. After some experimentation it turns out both the DSPCNTR trickle feed bits and the MI_ARB bit can be toggled. However the DSPCNTR bits don't seem to have any effect. The MI_ARB bit, on the other hand, has a noticable effect. I performed an experiment where I reduced the FIFO size via DSPARB and observed the effect of the MI_ARB trickle feed bit on the display. Using a 1920x1080-60 mode, with MI_ARB=0x4 the display started to have problems with DSPARB=0x42424242, whereas with MI_ARB=0x0 the problems didn't start until DSPARB=0x09090909. This seems to confirm that the MI_ARB trickle feed bit actually does work. So replace the use of the DSPCNTR trickle feed bits with MI_ARB on VLV. v2: Amend commit message with results from experimentation Signed-off-by: NVille Syrjälä <ville.syrjala@linux.intel.com> Reviewed-by: NJesse Barnes <jbarnes@virtuousgeek.org> Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>
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