[PATCH v4 RESEND 2/4] 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 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
v3: * Rebase
v4: * Switch from do-while to for loop when reading CRC values (Jani)
    * 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 | 33 ++++++++++++++++++++++++++++++---
 1 file changed, 30 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
index 2d42d09..c90ca1c 100644
--- a/drivers/gpu/drm/i915/intel_dp.c
+++ b/drivers/gpu/drm/i915/intel_dp.c
@@ -3906,6 +3906,11 @@ 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 +3934,33 @@ 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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