[PATCH] drm/edid: Fix DDC probe for passive DP dongles

[Todd Previte]

Passive DP->DVI/HDMI dongles show up to the system as HDMI devices, as they
do not have a sink device in them to respond to any AUX traffic. When
probing these dongles over the DDC, sometimes they will NAK the first attempt
even though the transaction is valid and they support the DDC protocol. The
retry loop inside of drm_do_probe_ddc_edid() would normally catch this case
and try the transaction again, resulting in success.

That, however, was thwarted by the fix for fdo.org bug #41059. The patch is:
	commit 9292f37e1f5c79400254dca46f83313488093825
	Author: Eugeni Dodonov <eugeni.dodonov at intel.com>
	Date:   Thu Jan 5 09:34:28 2012 -0200

	drm: give up on edid retries when i2c bus is not responding

This added code to exit immediately if the return code from the
i2c_transfer function was -ENXIO in order to reduce the amount of time spent
in waiting for unresponsive or disconnected devices. For the DP dongles,
this means that the second retry never happens which results in a failed
EDID probe and a black screen.

To work around this problem without undoing the fix for bug #41059, the
number of retries is checked along with the return code. This allows for a
device to NAK once and still continue operations. A second NAK will result
in breaking the loop as it would have before and stopping the DDC probe.

Signed-off-by: Todd Previte <tprevite at gmail.com>
Cc: intel-gfx at lists.freedesktop.org
---
 drivers/gpu/drm/drm_edid.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/drm_edid.c b/drivers/gpu/drm/drm_edid.c
index 7087da3..e8047bd 100644
--- a/drivers/gpu/drm/drm_edid.c
+++ b/drivers/gpu/drm/drm_edid.c
@@ -1238,7 +1238,10 @@ drm_do_probe_ddc_edid(void *data, u8 *buf, unsigned int block, size_t len)
 		 */
 		ret = i2c_transfer(adapter, &msgs[3 - xfers], xfers);
 
-		if (ret == -ENXIO) {
+		/* Passive DP->DVI/HDMI dongles sometimes NAK the first probe
+		 * Try to probe again but if it NAKs, stop trying
+		 */
+		if (ret == -ENXIO && retries < 5) {
 			DRM_DEBUG_KMS("drm: skipping non-existent adapter %s\n",
 					adapter->name);
 			break;
-- 
1.9.1