drm/dp/i2c: send bare addresses to properly reset i2c connections (v4)

We need bare address packets at the start and end of
each i2c over aux transaction to properly reset the connection
between transactions.  This mirrors what the existing dp i2c
over aux algo currently does.

This fixes EDID fetches on certain monitors especially with
dp bridges.

v2: update as per Ville's comments
    - Set buffer to NULL for zero sized packets
    - abort the entre transaction if one of the messages fails
v3: drop leftover debugging code
v4: integrate Thierry's comments
    - add comments about address only transactions
    - switch back to i and j
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>

drivers/gpu/drm/drm_dp_helper.c

@@ -665,11 +665,26 @@ static int drm_dp_i2c_xfer(struct i2c_adapter *adapter, struct i2c_msg *msgs,
 {
 	struct drm_dp_aux *aux = adapter->algo_data;
 	unsigned int i, j;
-
-	for (i = 0; i < num; i++) {
 	struct drm_dp_aux_msg msg;
-		int err;
+	int err = 0;
+
+	memset(&msg, 0, sizeof(msg));
 
+	for (i = 0; i < num; i++) {
+		msg.address = msgs[i].addr;
+		msg.request = (msgs[i].flags & I2C_M_RD) ?
+			DP_AUX_I2C_READ :
+			DP_AUX_I2C_WRITE;
+		msg.request |= DP_AUX_I2C_MOT;
+		/* Send a bare address packet to start the transaction.
+		 * Zero sized messages specify an address only (bare
+		 * address) transaction.
+		 */
+		msg.buffer = NULL;
+		msg.size = 0;
+		err = drm_dp_i2c_do_msg(aux, &msg);
+		if (err < 0)
+			break;
 		/*
 		 * Many hardware implementations support FIFOs larger than a
 		 * single byte, but it has been empirically determined that
@@ -678,30 +693,28 @@ static int drm_dp_i2c_xfer(struct i2c_adapter *adapter, struct i2c_msg *msgs,
 		 * transferred byte-by-byte.
 		 */
 		for (j = 0; j < msgs[i].len; j++) {
-			memset(&msg, 0, sizeof(msg));
-			msg.address = msgs[i].addr;
-
-			msg.request = (msgs[i].flags & I2C_M_RD) ?
-					DP_AUX_I2C_READ :
-					DP_AUX_I2C_WRITE;
-
-			/*
-			 * All messages except the last one are middle-of-
-			 * transfer messages.
-			 */
-			if ((i < num - 1) || (j < msgs[i].len - 1))
-				msg.request |= DP_AUX_I2C_MOT;
-
 			msg.buffer = msgs[i].buf + j;
 			msg.size = 1;
 
 			err = drm_dp_i2c_do_msg(aux, &msg);
 			if (err < 0)
-				return err;
+				break;
 		}
+		if (err < 0)
+			break;
 	}
+	if (err >= 0)
+		err = num;
+	/* Send a bare address packet to close out the transaction.
+	 * Zero sized messages specify an address only (bare
+	 * address) transaction.
+	 */
+	msg.request &= ~DP_AUX_I2C_MOT;
+	msg.buffer = NULL;
+	msg.size = 0;
+	(void)drm_dp_i2c_do_msg(aux, &msg);
 
-	return num;
+	return err;
 }
 
 static const struct i2c_algorithm drm_dp_i2c_algo = {