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. V3: - Fixed the limit value to 7 instead of 8 to get the correct retry count. - Combined the increment of the defer count into the if-statement V4: - Removed i915 tag from subject as the patch is not i915-specific

Signed-off-by: Todd Previte tprevite@gmail.com Cc: dri-devel@lists.freedesktop.org --- drivers/gpu/drm/drm_dp_helper.c | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/drm_dp_helper.c b/drivers/gpu/drm/drm_dp_helper.c index 79968e3..9ecfd27 100644 --- a/drivers/gpu/drm/drm_dp_helper.c +++ b/drivers/gpu/drm/drm_dp_helper.c @@ -468,7 +468,12 @@ 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 < 7) + retry = 0; usleep_range(400, 500); continue;