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

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

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>
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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