[PATCH v4 RESEND 4/5] drm/dp_helper: Perform throw-away read before actual read in drm_dp_dpcd_read()

[Lyude]

This is part of a patch series to migrate all of the workarounds for
commonly seen behavior from bad sinks in intel_dp_dpcd_read_wake() to drm's
DP helper.

Some sinks will just return garbage for the first aux tranaction they
receive when coming out of sleep mode, so we need to perform an additional
read before the actual read to workaround this.

Signed-off-by: Lyude <cpaul@xxxxxxxxxx>
---
 drivers/gpu/drm/drm_dp_helper.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/gpu/drm/drm_dp_helper.c b/drivers/gpu/drm/drm_dp_helper.c
index 86656ca..702c78a 100644
--- a/drivers/gpu/drm/drm_dp_helper.c
+++ b/drivers/gpu/drm/drm_dp_helper.c
@@ -244,6 +244,13 @@ static int drm_dp_dpcd_access(struct drm_dp_aux *aux, u8 request,
 ssize_t drm_dp_dpcd_read(struct drm_dp_aux *aux, unsigned int offset,
 			 void *buffer, size_t size)
 {
+	/*
+	 * Sometimes we just get the same incorrect byte repeated over the
+	 * entire buffer. Doing one throw away read initially seems to "solve"
+	 * it.
+	 */
+	drm_dp_dpcd_access(aux, DP_AUX_NATIVE_READ, DP_DPCD_REV, buffer, 1);
+
 	return drm_dp_dpcd_access(aux, DP_AUX_NATIVE_READ, offset, buffer,
 				  size);
 }
-- 
2.5.5