On 10/17/2014 1:43 AM, Jani Nikula wrote:
On Thu, 16 Oct 2014, 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.
This makes me suspect our sink dpms and wake handling even more than I
already did. Someone(tm) should dig into the DP and hw specs again with
fresh eyes...
I can go look into this today/next week. This problem sounds vaguely
similar to one I've run across before.
-T
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.
Seems like a pretty harmless thing to do, and we already do loads of
dpcd reads anyway. We should throw this at some related bugs.
So ack.
BR,
Jani.
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)
--
2.0.4
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