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

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>

drm/i915: Do a dummy DPCD read before the actual read
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>
Ville Syrjälä · Jani Nikula
1 parent cac7f24298
Showing 1 changed file with 7 additions and 0 deletions Side-by-side Diff
drivers/gpu/drm/i915/intel_dp.c
@@ -2806,6 +2806,13 @@
 	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)
...	...	@@ -2806,6 +2806,13 @@
2806	2806	ssize_t ret;
2807	2807	int i;
2808	2808
	2809	+ /*
	2810	+ * Sometime we just get the same incorrect byte repeated
	2811	+ * over the entire buffer. Doing just one throw away read
	2812	+ * initially seems to "solve" it.
	2813	+ */
	2814	+ drm_dp_dpcd_read(aux, DP_DPCD_REV, buffer, 1);
	2815	+
2809	2816	for (i = 0; i < 3; i++) {
2810	2817	ret = drm_dp_dpcd_read(aux, offset, buffer, size);
2811	2818	if (ret == size)