Re: [PATCH v5] drm/i915/psr: Account for sink CRC raciness on some panels

[Jim Bride <jim.bride@linux.intel.com>]

On Tue, Aug 15, 2017 at 04:41:52PM -0700, Rodrigo Vivi wrote:
> On Tue, Aug 15, 2017 at 9:58 AM, Jim Bride <jim.bride@linux.intel.com> wrote:
> > On Wed, Aug 09, 2017 at 01:40:00PM -0700, Jim Bride wrote:
> >> According to the eDP spec, when the count field in TEST_SINK_MISC
> >> increments then the six bytes of sink CRC information in the DPCD
> >> should be valid.  Unfortunately, this doesn't seem to be the case
> >> on some panels, and as a result we get some incorrect and inconsistent
> >> values from the sink CRC DPCD locations at times.  This problem exhibits
> >> itself more on faster processors (relative failure rates HSW < SKL < KBL.)
> >> In order to try and account for this, we try a lot harder to read the sink
> >> CRC until we get consistent values twice in a row before returning what we
> >> read and delay for a time before trying to read.  We still see some
> >> occasional failures, but reading the sink CRC is much more reliable,
> >> particularly on SKL, with these changes than without.
> >>
> >> v2: * Reduce number of retries when reading the sink CRC (Jani)
> >>     * Refactor to minimize changes to the code (Jani)
> >>     * Rebase
> >> v3: * Rebase
> >> v4: * Switch from do-while to for loop when reading CRC values (Jani)
> >>     * Rebase
> >> v5: * Checkpatch cleanup and commit message tweaks
> >>     * Rebase
> >> Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
> >> Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
> >> Cc: Jani Nikula <jani.nikula@intel.com>
> >
> > I think I addressed all previous review comments for this patch.  Any
> > thoughts?
> 
> I suffered a lot with this unreliable sink crcs in the past. As you I tried many
> different things like this, but they are still unreliable.
> 
> So I believe I'm in favor of one of DK's suggestion:
> " On the other hand, since the only consumers of dp sink crc are tests,
> why can't the kernel just dump what it reads to debugfs and let the test
> deal with erroneous results?
> "
> So let's keep the kernel doing the right thing by the spec and try to
> change test cases to deal with this bad values.

Ok.  I moved the logic into the IGT library's call for reading sink
crcs.

> Or let's find some other way to test this without sink crc... and
> anyways I believe that we should just drop this patch.

This is the longer-term plan.   We need similar tests for PSR 2,
which doesn't support sink crcs, anyhow.

Jim


> >
> > Jim
> >
> >> Signed-off-by: Jim Bride <jim.bride@linux.intel.com>
> >> ---
> >>  drivers/gpu/drm/i915/intel_dp.c | 32 +++++++++++++++++++++++++++++---
> >>  1 file changed, 29 insertions(+), 3 deletions(-)
> >>
> >> diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
> >> index 76c8a0b..b64757c 100644
> >> --- a/drivers/gpu/drm/i915/intel_dp.c
> >> +++ b/drivers/gpu/drm/i915/intel_dp.c
> >> @@ -3906,6 +3906,10 @@ int intel_dp_sink_crc(struct intel_dp *intel_dp, u8 *crc)
> >>       u8 buf;
> >>       int count, ret;
> >>       int attempts = 6;
> >> +     u8 old_crc[6];
> >> +
> >> +     if (crc == NULL)
> >> +             return -ENOMEM;
> >>
> >>       ret = intel_dp_sink_crc_start(intel_dp);
> >>       if (ret)
> >> @@ -3929,11 +3933,33 @@ int intel_dp_sink_crc(struct intel_dp *intel_dp, u8 *crc)
> >>               goto stop;
> >>       }
> >>
> >> -     if (drm_dp_dpcd_read(&intel_dp->aux, DP_TEST_CRC_R_CR, crc, 6) < 0) {
> >> -             ret = -EIO;
> >> -             goto stop;
> >> +     /*
> >> +      * Sometimes it takes a while for the "real" CRC values to land in
> >> +      * the DPCD, so try several times until we get two reads in a row
> >> +      * that are the same.  If we're an eDP panel, delay between reads
> >> +      * for a while since the values take a bit longer to propagate.
> >> +      */
> >> +     for (attempts = 0; attempts < 6; attempts++) {
> >> +             intel_wait_for_vblank(dev_priv, intel_crtc->pipe);
> >> +
> >> +             if (drm_dp_dpcd_read(&intel_dp->aux, DP_TEST_CRC_R_CR,
> >> +                                  crc, 6) < 0) {
> >> +                     ret = -EIO;
> >> +                     break;
> >> +             }
> >> +
> >> +             if (attempts && memcmp(old_crc, crc, 6) == 0)
> >> +                     break;
> >> +             memcpy(old_crc, crc, 6);
> >> +
> >> +             if (is_edp(intel_dp))
> >> +                     usleep_range(20000, 25000);
> >>       }
> >>
> >> +     if (attempts == 6) {
> >> +             DRM_DEBUG_KMS("Failed to get CRC after 6 attempts.\n");
> >> +             ret = -ETIMEDOUT;
> >> +     }
> >>  stop:
> >>       intel_dp_sink_crc_stop(intel_dp);
> >>       return ret;
> >> --
> >> 2.7.4
> >>
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> 
> 
> 
> -- 
> Rodrigo Vivi
> Blog: http://blog.vivi.eng.br
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