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: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/drivers/gpu/drm/i915/intel_pm.c b/drivers/gpu/drm/i915/intel_pm.c
index adc44e4..b27bda0 100644
--- a/drivers/gpu/drm/i915/intel_pm.c
+++ b/drivers/gpu/drm/i915/intel_pm.c
@@ -4873,7 +4873,7 @@
I915_WRITE(GEN7_UCGCTL4, GEN7_L3BANK2X_CLOCK_GATE_DISABLE);
- g4x_disable_trickle_feed(dev);
+ I915_WRITE(MI_ARB_VLV, MI_ARB_DISPLAY_TRICKLE_FEED_DISABLE);
I915_WRITE(CACHE_MODE_1,
_MASKED_BIT_ENABLE(PIXEL_SUBSPAN_COLLECT_OPT_DISABLE));