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

[Dhinakaran Pandiyan <dhinakaran.pandiyan@gmail.com>]

On Tuesday, July 11, 2017 3:19:54 PM PDT 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 and KBL, with these changes than without.

What's the goal here? Is this retry loop being added to deal with an IGT 
failure? A bit of context as to how this is related to PSR will be helpful.

> 
> 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) {

Why is this needed?

> +		memset(crc, 0, 6);
> +		memset(old_crc, 0xff, 6);

I don't know much about crc values, is 0xff a known invalid value?

> +	} 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 {
> +		intel_wait_for_vblank(dev_priv, intel_crtc->pipe);

Why wait for a vblank if the idea is to check for consistent crc values for 
the same frame (I'm assuming that is the idea) ? 

> +		if (is_edp(intel_dp))
> +			usleep_range(20000, 25000);
> +
> +		if (drm_dp_dpcd_read(&intel_dp->aux, DP_TEST_CRC_R_CR,
> +				     crc, 6) < 0) {
> +			ret = -EIO;
> +			goto stop;
> +		}
> +
> +		if (memcmp(old_crc, crc, 6) == 0) {
> +			ret = 0;
> +			goto stop;
> +		} else {
> +			memcpy(old_crc, crc, 6);
> +		}
> +	} while (--attempts);
> 
> +	DRM_DEBUG_KMS("Failed to get CRC after 6 attempts.\n");
> +	ret = -ETIMEDOUT;
>  stop:
>  	intel_dp_sink_crc_stop(intel_dp);
>  	return ret;


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