This is a note to let you know that I've just added the patch titled
drm/i915: Do a dummy DPCD read before the actual read
to the 3.17-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
drm-i915-do-a-dummy-dpcd-read-before-the-actual-read.patch
and it can be found in the queue-3.17 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
>From f6a1906674005377b64ee5431c1418077c1b2425 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Ville=20Syrj=C3=A4l=C3=A4?= <ville.syrjala@xxxxxxxxxxxxxxx>
Date: Thu, 16 Oct 2014 20:46:09 +0300
Subject: 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
From: =?UTF-8?q?Ville=20Syrj=C3=A4l=C3=A4?= <ville.syrjala@xxxxxxxxxxxxxxx>
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 <ville.syrjala@xxxxxxxxxxxxxxx>
Reviewed-by: Todd Previte <tprevite@xxxxxxxxx>
Signed-off-by: Jani Nikula <jani.nikula@xxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
drivers/gpu/drm/i915/intel_dp.c | 7 +++++++
1 file changed, 7 insertions(+)
--- a/drivers/gpu/drm/i915/intel_dp.c
+++ b/drivers/gpu/drm/i915/intel_dp.c
@@ -2364,6 +2364,13 @@ intel_dp_dpcd_read_wake(struct drm_dp_au
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)
Patches currently in stable-queue which might be from ville.syrjala@xxxxxxxxxxxxxxx are
queue-3.17/drm-i915-fix-gmbusfreq-on-vlv-chv.patch
queue-3.17/drm-i915-do-a-dummy-dpcd-read-before-the-actual-read.patch
queue-3.17/drm-i915-properly-reenable-gen8-pipe-irqs.patch
--
To unsubscribe from this list: send the line "unsubscribe stable" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html