drm/i915: Fix random aux transactions failures.

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.
v3: This patch is crucial to make PSR test cases reliably working
    on SKL. So split this patch from the aux re-org series and add
    a FIXME as a promisse to continue that effort besides reminding
    to remove the sleep when that is merged.
v4: Use a bigger usleep range so kernel doesn't need to be interrupted
    on a exact time, as suggested by Paulo.
    But anyway we should discuss the better time
    ranges on the EBUSY handle re-org at drm level since this one here
    is temporary.
v5: s/1000,1500/1000, 1500/ (by Paulo).

Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Tested-by: Daniel Stone <daniels@collabora.com> # SKL
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1449774747-2772-1-git-send-email-rodrigo.vivi@intel.com

drivers/gpu/drm/i915/intel_dp.c

@@ -915,6 +915,27 @@ 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);
+		/*
+		 * FIXME: This patch was created on top of a series that
+		 * organize the retries at drm level. There EBUSY should
+		 * also take care for 1ms wait before retrying.
+		 * That aux retries re-org is still needed and after that is
+		 * merged we remove this sleep from here.
+		 */
+		usleep_range(1000, 1500);
+		ret = -EBUSY;
+		goto out;
+	}
+
 	if (recv_bytes > recv_size)
 		recv_bytes = recv_size;