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

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

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?

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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