From 977d22b6dfd84242fc2a8309f18a75bed2938760 Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?Ville=20Syrj=C3=A4l=C3=A4?= Date: Thu, 16 Oct 2014 20:46:09 +0300 Subject: [PATCH] drm/i915: Do a dummy DPCD read before the actual read MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit commit f6a1906674005377b64ee5431c1418077c1b2425 upstream. 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 Reviewed-by: Todd Previte Signed-off-by: Jani Nikula Signed-off-by: Luis Henriques --- drivers/gpu/drm/i915/intel_dp.c | 7 +++++++ 1 file changed, 7 insertions(+) diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c index fbffcbb9a0f8..ca6a8cf58ab0 100644 --- a/drivers/gpu/drm/i915/intel_dp.c +++ b/drivers/gpu/drm/i915/intel_dp.c @@ -2166,6 +2166,13 @@ 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)