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drm/i915: Do a dummy DPCD read before the actual read

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Sometimes we seem to get utter garbage from DPCD reads. The resulting
buffer is filled with the same byte, and the operation completed without
errors. My HP ZR24w monitor seems particularly susceptible to this
problem once it's gone into a sleep mode.

The issue seems to happen only for the first AUX message that wakes the
sink up. But as the first AUX read we often do is the DPCD receiver
cap it does wreak a bit of havoc with subsequent link training etc. when
the receiver cap bw/lane/etc. information is garbage.

A sufficient workaround seems to be to perform a single byte dummy read
before reading the actual data. I suppose that just wakes up the sink
sufficiently and we can just throw away the returned data in case it's
crap. DP_DPCD_REV seems like a sufficiently safe location to read here.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Todd Previte <tprevite@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
This commit is contained in:
Ville Syrjälä 2014-10-16 20:46:09 +03:00 committed by Jani Nikula
parent cac7f24298
commit f6a1906674
1 changed files with 7 additions and 0 deletions
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7
drivers/gpu/drm/i915/intel_dp.c
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@ -2806,6 +2806,13 @@ intel_dp_dpcd_read_wake(struct drm_dp_aux *aux, unsigned int offset,
ssize_t ret;
int i;
/*
* Sometime we just get the same incorrect byte repeated
* over the entire buffer. Doing just one throw away read
* initially seems to "solve" it.
*/
drm_dp_dpcd_read(aux, DP_DPCD_REV, buffer, 1);
for (i = 0; i < 3; i++) {
ret = drm_dp_dpcd_read(aux, offset, buffer, size);
if (ret == size)