On 11/22/13 1:36 AM, Takashi Iwai wrote:
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@xxxxxxx>
---
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);
I'm in agreement with Jani's assessment. While it does address the
particular problem at hand, it just looks for duplicate data and issues
a retry if that specific condition is detected. It seems more
appropriate that instead of checking for specific failure modes, we
check to ensure that we have useable values for the basic, critical
fields required for Displayport.
I would suggest that we validate the first 3 bytes of the DPCD. Those
fields are as follows:
0x00 - DPCD revision level
Currently can only be 0x10, 0x11 or 0x12
0x01 - Link rate
Also only 3 values currently - 0x06, 0x0A and 0x14
0x02 - Max lane count
For validation purposes, we just need to look at 4:0 and verify
it's 0x01, 0x02 or 0x04
While bits 6 and 7 are used, they're just feature bits and we
check them later anyways.
This is kind of what I was thinking:
#define MAX_DPCD_REVISION 0x12
u8 lane_count = DPCD[2] & 0x1F;
// Verify DPCD revision is in a valid range. MAX_DPCD_REVISION
should be updated as necessasry
// to reflect the current supported DPCD version.
if (DPCD[0] < 0x10 || DPCD > MAX_DPCD_REVISION)
goto retry;
// These are the only valid values for link rate. More will surely
be added as Displayport moves forward.
if !(DPCD[1] == 0x06 || DPCD[1] == 0x0A || DPCD[1] == 0x14)
goto retry;
// Lane count must be 1, 2 or 4. 3 shalt thou not count, and 5 is
right out...
if !(lane_count == 0x01 || lane_count == 0x02 || lane_count == 0x04)
goto retry;
retry:
retry_count++;
if retry_count > max_retries
goto error
else
goto read_dpcd
-T
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