[Intel-gfx] [PATCH] drm/i915: Add a try limit to avoid infinite loops

[james.ausmus at intel.com]

From: Chris Wolfe <cwolfe at chromium.org>

Unfortunately some combinations of hardware seem to generate successful
communications on the aux channel, which always report deferred. As a
result native_write can wind up in an infinite loop.

This hack adds a large (~10ms) retry limit to avoid a kernel panic,
while hopefully minimizing the impact on working hardware.

Signed-off-by: cwolfe at chromium.org

BUG=chromium-os:34840
TEST=Manually connect DP to VGA adapter to problem system. Added display
     powers up and works normally, rather than black screen and reboot.

Change-Id: Ib1b0001ca8004e65c9c5e353dbdb5e252fd88438
Reviewed-on: https://gerrit.chromium.org/gerrit/34203
Commit-Ready: Chris Wolfe <cwolfe at chromium.org>
Reviewed-by: Chris Wolfe <cwolfe at chromium.org>
Tested-by: Chris Wolfe <cwolfe at chromium.org>
---
 drivers/gpu/drm/i915/intel_dp.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
index e5d16bc..ac7d610 100644
--- a/drivers/gpu/drm/i915/intel_dp.c
+++ b/drivers/gpu/drm/i915/intel_dp.c
@@ -491,6 +491,7 @@ intel_dp_aux_native_write(struct intel_dp *intel_dp,
 	uint8_t	msg[20];
 	int msg_bytes;
 	uint8_t	ack;
+	int try;
 
 	intel_dp_check_edp(intel_dp);
 	if (send_bytes > 16)
@@ -501,7 +502,7 @@ intel_dp_aux_native_write(struct intel_dp *intel_dp,
 	msg[3] = send_bytes - 1;
 	memcpy(&msg[4], send, send_bytes);
 	msg_bytes = send_bytes + 4;
-	for (;;) {
+	for (try = 0; try < 100; try++) {
 		ret = intel_dp_aux_ch(intel_dp, msg, msg_bytes, &ack, 1);
 		if (ret < 0)
 			return ret;
@@ -512,6 +513,10 @@ intel_dp_aux_native_write(struct intel_dp *intel_dp,
 		else
 			return -EIO;
 	}
+	if (try == 100) {
+		DRM_ERROR("too many retries, giving up\n");
+		return -EREMOTEIO;
+	}
 	return send_bytes;
 }
 
-- 
1.8.3.2