drm/dp/i2c: Update comments about common i2c over dp assumptions (v3)

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
v3 (chk): rebased on upstream changes

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Thierry Reding <treding@nvidia.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Christian König <christian.koenig@amd.com>

1 files changed, 4 insertions, 0 deletions

diff --git a/include/drm/drm_dp_helper.h b/include/drm/drm_dp_helper.h
index b4f58914bf7d..cfcacec5b89d 100644
--- a/include/drm/drm_dp_helper.h
+++ b/include/drm/drm_dp_helper.h
@@ -456,6 +456,10 @@ 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 {
         const char *name;