[PATCH 3/4] drm/dp/i2c: Update comments about common i2c over dp assumptions (v2)

[Alex Deucher]

If you are using the common dp over i2c functionality, it is
asumed that the aux transfer function does not modify the any
of the msg structure other than the reply field.  Doing so
breaks the logic in the common code.

v2: update struct drm_dp_aux comments about assumptions

Signed-off-by: Alex Deucher <alexander.deucher at amd.com>
Cc: Ville Syrjälä <ville.syrjala at linux.intel.com>
Cc: Jani Nikula <jani.nikula at intel.com>
Cc: Thierry Reding <treding at nvidia.com>
Reviewed-by: Ville Syrjälä <ville.syrjala at linux.intel.com>
---
 drivers/gpu/drm/drm_dp_helper.c | 4 +++-
 include/drm/drm_dp_helper.h     | 4 ++++
 2 files changed, 7 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/drm_dp_helper.c b/drivers/gpu/drm/drm_dp_helper.c
index 6122a4f..e1c1347 100644
--- a/drivers/gpu/drm/drm_dp_helper.c
+++ b/drivers/gpu/drm/drm_dp_helper.c
@@ -577,7 +577,9 @@ static u32 drm_dp_i2c_functionality(struct i2c_adapter *adapter)
 
 /*
  * Transfer a single I2C-over-AUX message and handle various error conditions,
- * retrying the transaction as appropriate.
+ * retrying the transaction as appropriate.  It is assumed that the
+ * aux->transfer function does not modify anything in the msg other than the
+ * reply field.
  */
 static int drm_dp_i2c_do_msg(struct drm_dp_aux *aux, struct drm_dp_aux_msg *msg)
 {
diff --git a/include/drm/drm_dp_helper.h b/include/drm/drm_dp_helper.h
index b7488c9..10d67018 100644
--- a/include/drm/drm_dp_helper.h
+++ b/include/drm/drm_dp_helper.h
@@ -443,6 +443,10 @@ struct drm_dp_aux_msg {
  * transactions. The drm_dp_aux_register_i2c_bus() function registers an
  * I2C adapter that can be passed to drm_probe_ddc(). Upon removal, drivers
  * should call drm_dp_aux_unregister_i2c_bus() to remove the I2C adapter.
+ *
+ * Note that the aux helper code assumes that the .transfer() function
+ * only modifies the reply field of the drm_dp_aux_msg structure.  The
+ * retry logic and i2c helpers assume this is the case.
  */
 struct drm_dp_aux {
 	struct i2c_adapter ddc;
-- 
1.8.3.1