[Intel-gfx] [PATCH] drm/i915: Retry reading DPCD when bogus values are read

[Takashi Iwai]

I got kernel WARNINGs frequently on Haswell laptops complaining about
invalid max DP link bw.  With drm.debug=0x0e, it turned out that the
obtained DPCD is utterly bogus when it happens:
  [drm:intel_dp_get_dpcd], DPCD: 4d 4d 4d 4d 4d 4d 4d 4d 4d 4d 4d 4d 4d 4d 4d

This seems happening intermittently without plug state changes and
even after the first few reads succeeded.  As a workaround, add a
sanity check for such bogus values and retry reading if it hits.

Signed-off-by: Takashi Iwai <tiwai at suse.de>
---

 drivers/gpu/drm/i915/intel_dp.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
index eb8139da9763..f596353da557 100644
--- a/drivers/gpu/drm/i915/intel_dp.c
+++ b/drivers/gpu/drm/i915/intel_dp.c
@@ -2792,13 +2792,26 @@ intel_dp_get_dpcd(struct intel_dp *intel_dp)
 	struct intel_digital_port *dig_port = dp_to_dig_port(intel_dp);
 	struct drm_device *dev = dig_port->base.base.dev;
 	struct drm_i915_private *dev_priv = dev->dev_private;
-
+	int retry = 0;
 	char dpcd_hex_dump[sizeof(intel_dp->dpcd) * 3];
 
+ again:
 	if (intel_dp_aux_native_read_retry(intel_dp, 0x000, intel_dp->dpcd,
 					   sizeof(intel_dp->dpcd)) == 0)
 		return false; /* aux transfer failed */
 
+	/* sanity check: sometimes DPCD contains a series of same bogus value
+	 * even though the above returned success.  Retry reading in that case.
+	 */
+	if (intel_dp->dpcd[0]) {
+		int i;
+		for (i = 1; i < sizeof(intel_dp->dpcd); i++)
+			if (intel_dp->dpcd[i] != intel_dp->dpcd[0])
+				break;
+		if (i >= sizeof(intel_dp->dpcd) && retry++ < 2)
+			goto again;
+	}
+
 	hex_dump_to_buffer(intel_dp->dpcd, sizeof(intel_dp->dpcd),
 			   32, 1, dpcd_hex_dump, sizeof(dpcd_hex_dump), false);
 	DRM_DEBUG_KMS("DPCD: %s\n", dpcd_hex_dump);
-- 
1.8.4.3