Re: [PATCH v3 2/4] drm/i915/psr: Account for sink CRC raciness on some panels

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

On Fri, Jul 14, 2017 at 12:46:08PM +0300, Jani Nikula wrote:
> On Tue, 11 Jul 2017, Jim Bride <jim.bride@linux.intel.com> 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 and KBL, 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
> >
> > Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
> > Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
> > Cc: Jani Nikula <jani.nikula@intel.com>
> > Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
> > Signed-off-by: Jim Bride <jim.bride@linux.intel.com>
> > ---
> >  drivers/gpu/drm/i915/intel_dp.c | 40 ++++++++++++++++++++++++++++++++++++----
> >  1 file changed, 36 insertions(+), 4 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
> > index 2d42d09..69c8130c 100644
> > --- a/drivers/gpu/drm/i915/intel_dp.c
> > +++ b/drivers/gpu/drm/i915/intel_dp.c
> > @@ -3906,6 +3906,14 @@ 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) {
> 
> As DK said, please drop the check.
> 
> > +		memset(crc, 0, 6);
> > +		memset(old_crc, 0xff, 6);
> 
> Both unnecessary, see below.
> 
> > +	} else {
> > +		return -ENOMEM;
> > +	}
> >  
> >  	ret = intel_dp_sink_crc_start(intel_dp);
> >  	if (ret)
> > @@ -3929,11 +3937,35 @@ 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;
> > -	}
> > +	attempts = 6;
> > +
> > +	/*
> > +	 * 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.
> > +	 */
> > +	do {
> 
> Never use a do-while when a for loop will do. for (i = 0; i < 6; i++)
> gets interpreted in the spine, no need for further processing.

I had used do-while because it was used earlier to read the test
count (code I didn't touch.)  In any event, I can convert to a
for loop easy enough.

> > +		intel_wait_for_vblank(dev_priv, intel_crtc->pipe);
> > +		if (is_edp(intel_dp))
> > +			usleep_range(20000, 25000);
> 
> Is the intention to do these *between* reads? If yes, then move this
> *after* the memcmp to only do this between reads.

Moved.

> > +
> > +		if (drm_dp_dpcd_read(&intel_dp->aux, DP_TEST_CRC_R_CR,
> > +				     crc, 6) < 0) {
> > +			ret = -EIO;
> > +			goto stop;
> 
> break;

Changed.

> > +		}
> > +
> > +		if (memcmp(old_crc, crc, 6) == 0) {
> > +			ret = 0;
> > +			goto stop;
> > +		} else {
> > +			memcpy(old_crc, crc, 6);
> > +		}
> 
> After you've switched this to the for loop, you can do:
> 
> 	if (i && memcmp(old_crc, crc, sizeof(old_crc)) == 0)
>         	break;
> 	memcpy(old_crc, crc, sizeof(old_crc));

Changed.  I'll submit a new version of the patch after I retest.

Jim

> > +	} while (--attempts);
> >  
> 
> 	if (i == 6) {
> 
> > +	DRM_DEBUG_KMS("Failed to get CRC after 6 attempts.\n");
> > +	ret = -ETIMEDOUT;
> 
> 	}
> 
> >  stop:
> >  	intel_dp_sink_crc_stop(intel_dp);
> >  	return ret;
> 
> -- 
> Jani Nikula, Intel Open Source Technology Center
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