Re: [PATCH 1/2] drm/i915: Do a dummy DPCD read before the actual read

Daniel Vetter
2014-10-21T15:57:32Z

On Thu, Oct 16, 2014 at 12:39:29PM -0700, Todd Previte wrote:
>
> On 10/16/2014 10:46 AM, ville.syrjala@linux.intel.com wrote:
> >From: Ville Syrjälä <ville.syrjala@linux.intel.com>
> >
> >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>
> >---
> > 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.
>
> Reviewed-by: Todd Previte <tprevite@gmail.com>

Cc: stable@vger.kernel.org

... one for Jani I guess. But I'm suspicious here too, so maybe we should
extract this read_wake function to the core dp helpers and convince that
some other driver should use it? Adding more people and lists.
-Daniel
--
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch