On 10/17/2014 2:06 AM, Ville Syrjälä wrote:
On Thu, Oct 16, 2014 at 12:39:29PM -0700, Todd Previte wrote:
On 10/16/2014 10:46 AM, ville.syrjala@xxxxxxxxxxxxxxx wrote:
From: Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx>
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@xxxxxxxxxxxxxxx>
---
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 64c8e04..f07f02c 100644
--- a/drivers/gpu/drm/i915/intel_dp.c
+++ b/drivers/gpu/drm/i915/intel_dp.c
@@ -2870,6 +2870,13 @@ 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)
Seems like a reasonable workaround for this problem, though
investigating the actual root cause might be worthwhile.
Sure. If someone has an AUX analyzer and a HP ZR24w monitor it should
be trivial to look at the traffic and see if there's something bogus in
our AUX communication. Sadly I don't have an AUX analyzer.
I've got the monitor on my desk but no AUX analyzer to use. I'll see if
I can track one down.
-T
Reviewed-by: Todd Previte <tprevite@xxxxxxxxx>
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