drm/dp: add option to disable zero sized address only transactions.

Some older NVIDIA and some newer NVIDIA hardware/firmware seems to
have issues with address only transactions (firmware rejects them).

Add an option to the core drm dp to avoid address only transactions,
This just puts the MOT flag removal on the last message of the transfer
and avoids the start of transfer transaction.

This with the flag set in nouveau, allows eDP probing on GB203 device.

Signed-off-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Ben Skeggs <bskeggs@nvidia.com>
Reviewed-by: Timur Tabi <ttabi@nvidia.com>
Tested-by: Timur Tabi <ttabi@nvidia.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

Author: airlied

drivers/gpu/drm/display/drm_dp_helper.c

@@ -2137,14 +2137,17 @@ static int drm_dp_i2c_xfer(struct i2c_adapter *adapter, struct i2c_msg *msgs,
 
 	for (i = 0; i < num; i++) {
 		msg.address = msgs[i].addr;
-		drm_dp_i2c_msg_set_request(&msg, &msgs[i]);
-		/* 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 (!aux->no_zero_sized) {
+			drm_dp_i2c_msg_set_request(&msg, &msgs[i]);
+			/* 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);
+		}
 
 		/*
 		 * Reset msg.request in case in case it got
@@ -2163,6 +2166,8 @@ static int drm_dp_i2c_xfer(struct i2c_adapter *adapter, struct i2c_msg *msgs,
 			msg.buffer = msgs[i].buf + j;
 			msg.size = min(transfer_size, msgs[i].len - j);
 
+			if (j + msg.size == msgs[i].len && aux->no_zero_sized)
+				msg.request &= ~DP_AUX_I2C_MOT;
 			err = drm_dp_i2c_drain_msg(aux, &msg);
 
 			/*
@@ -2180,15 +2185,17 @@ static int drm_dp_i2c_xfer(struct i2c_adapter *adapter, struct i2c_msg *msgs,
 	}
 	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);
 
+	if (!aux->no_zero_sized) {
+		/* 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 err;
 }
 

include/drm/display/drm_dp_helper.h

@@ -518,6 +518,11 @@ struct drm_dp_aux {
 	 * @powered_down: If true then the remote endpoint is powered down.
 	 */
 	bool powered_down;
+
+	/**
+	 * @no_zero_sized: If the hw can't use zero sized transfers (NVIDIA)
+	 */
+	bool no_zero_sized;
 };
 
 int drm_dp_dpcd_probe(struct drm_dp_aux *aux, unsigned int offset);