[PATCH 7/9] drm/i915: Fix random aux transactions failures.

[Rodrigo Vivi]

Mainly aux communications on sink_crc
were failing a lot randomly on recent platforms.
The first solution was to try to use intel_dp_dpcd_read_wake, but then
it was suggested to move retries to drm level.

Since drm level was already taking care of retries and didn't want
to through random retries on that level the second solution was to
put the retries at aux_transfer layer what was nacked.

So I realized we had so many retries in different places and
started to organize that a bit. During this organization I noticed
that we weren't handing at all the case were the message size was
zeroed. And this was exactly the case that was affecting sink_crc.

Also we weren't respect BSPec who says this size message = 0 or > 20
are forbidden.

It is a fact that we still have no clue why we are getting this
forbidden value there. But anyway we need to handle that for now
so we return -EBUSY and drm level takes care of the retries that
are already in place.

v2: Print debug messsage when this case is reached as suggested
    by Jani.

Cc: Jani Nikula <jani.nikula at intel.com>
Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi at intel.com>
Tested-by: Daniel Stone <daniels at collabora.com> # SKL
---
 drivers/gpu/drm/i915/intel_dp.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
index 54e85f5..a02bfa1 100644
--- a/drivers/gpu/drm/i915/intel_dp.c
+++ b/drivers/gpu/drm/i915/intel_dp.c
@@ -899,6 +899,19 @@ done:
 	/* Unload any bytes sent back from the other side */
 	recv_bytes = ((status & DP_AUX_CH_CTL_MESSAGE_SIZE_MASK) >>
 		      DP_AUX_CH_CTL_MESSAGE_SIZE_SHIFT);
+
+	/*
+	 * By BSpec: "Message sizes of 0 or >20 are not allowed."
+	 * We have no idea of what happened so we return -EBUSY so
+	 * drm layer takes care for the necessary retries.
+	 */
+	if (recv_bytes == 0 || recv_bytes > 20) {
+		DRM_DEBUG_KMS("Forbidden recv_bytes = %d on aux transaction\n",
+			      recv_bytes);
+		ret = -EBUSY;
+		goto out;
+	}
+
 	if (recv_bytes > recv_size)
 		recv_bytes = recv_size;
 
-- 
2.4.3