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drm/i915: workaround bad DSL readout v3

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On HSW at least (still testing other platforms, but should be harmless
elsewhere), the DSL reg reads back as 0 when read around vblank start
time.  This ends up confusing the atomic start/end checking code, since
it causes the update to appear as if it crossed a frame count boundary.
Avoid the problem by making sure we don't return scanline_offset from
the get_crtc_scanline function.  In moving the code there, I add to add
an additional delay since it could be called and have a legitimate 0
result for some time (depending on the pixel clock).

v2: move hsw dsl read hack to get_crtc_scanline (Ville)
v3: use break instead of goto (Ville)
    update comment with workaround details (Ville)

References: https://bugs.freedesktop.org/show_bug.cgi?id=91579
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
This commit is contained in:
Jesse Barnes 2015-09-22 12:15:54 -07:00 committed by Jani Nikula
parent 95d0be6128
commit 41b578fb0e
1 changed files with 26 additions and 0 deletions
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26
drivers/gpu/drm/i915/i915_irq.c
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@ -639,6 +639,32 @@ static int __intel_get_crtc_scanline(struct intel_crtc *crtc)
else
position = __raw_i915_read32(dev_priv, PIPEDSL(pipe)) & DSL_LINEMASK_GEN3;
/*
* On HSW, the DSL reg (0x70000) appears to return 0 if we
* read it just before the start of vblank. So try it again
* so we don't accidentally end up spanning a vblank frame
* increment, causing the pipe_update_end() code to squak at us.
*
* The nature of this problem means we can't simply check the ISR
* bit and return the vblank start value; nor can we use the scanline
* debug register in the transcoder as it appears to have the same
* problem. We may need to extend this to include other platforms,
* but so far testing only shows the problem on HSW.
*/
if (IS_HASWELL(dev) && !position) {
int i, temp;
for (i = 0; i < 100; i++) {
udelay(1);
temp = __raw_i915_read32(dev_priv, PIPEDSL(pipe)) &
DSL_LINEMASK_GEN3;
if (temp != position) {
position = temp;
break;
}
}
}
/*
* See update_scanline_offset() for the details on the
* scanline_offset adjustment.