2015-04-06 23:11 GMT-03:00 Todd Previte tprevite@gmail.com:
For test 4.2.2.5 to pass per the Link CTS Core 1.2 rev1.1 spec, the source device must attempt at least 7 times to read the EDID when it receives an I2C defer. The normal DRM code makes only 7 retries, regardless of whether or not the response is a native defer or an I2C defer. Test 4.2.2.5 fails since there are native defers interspersed with the I2C defers which results in less than 7 EDID read attempts.
The solution is to decrement the retry counter when an I2C DEFER is returned such that another read attempt will be made. This situation should normally only occur in compliance testing, however, as a worse case real-world scenario, it would result in 13 attempts ( 6 native defers, 7 I2C defers) for a single transaction to complete. The net result is a slightly slower response to an EDID read that shouldn't significantly impact overall performance.
V2:
- Added a check on the number of I2C Defers to limit the number of times that the retries variable will be decremented. This is to address review feedback regarding possible infinite loops from misbehaving sink devices.
Signed-off-by: Todd Previte tprevite@gmail.com Cc: dri-devel@lists.freedesktop.org
drivers/gpu/drm/drm_dp_helper.c | 6 ++++++ 1 file changed, 6 insertions(+)
diff --git a/drivers/gpu/drm/drm_dp_helper.c b/drivers/gpu/drm/drm_dp_helper.c index 79968e3..23025cf 100644 --- a/drivers/gpu/drm/drm_dp_helper.c +++ b/drivers/gpu/drm/drm_dp_helper.c @@ -469,6 +469,12 @@ static int drm_dp_i2c_do_msg(struct drm_dp_aux *aux, struct drm_dp_aux_msg *msg) case DP_AUX_I2C_REPLY_DEFER: DRM_DEBUG_KMS("I2C defer\n"); aux->i2c_defer_count++;
/* DP Compliance Test 4.2.2.5 Requirement:* Must have at least 7 retries for I2C defers on the* transaction to pass this test*/if (aux->i2c_defer_count < 8)
I don't think this is the way to go. During normal (non-compliance-testing) operation we never zero i2c_defer_count, so we can't expect this to work, since we may start drm_dp_i2c_do_msg with a i2c_defer_count value different than zero. Also, during i915.ko DP compliance we only zero i2c_defer_count at the very beginning of each test, not at every aux transaction, and I really think we need a solution that is not specific to compliance testing.
retry--; usleep_range(400, 500); continue;-- 1.9.1
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