[PATCH 2/4] drm/dp/i2c: send bare addresses to properly reset i2c connections (v2)

[Alex Deucher]

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

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>
---
 drivers/gpu/drm/drm_dp_helper.c | 54 +++++++++++++++++++++++------------------
 1 file changed, 31 insertions(+), 23 deletions(-)

diff --git a/drivers/gpu/drm/drm_dp_helper.c b/drivers/gpu/drm/drm_dp_helper.c
index 74724aa..125f84d 100644
--- a/drivers/gpu/drm/drm_dp_helper.c
+++ b/drivers/gpu/drm/drm_dp_helper.c
@@ -664,12 +664,25 @@ static int drm_dp_i2c_xfer(struct i2c_adapter *adapter, struct i2c_msg *msgs,
 			   int num)
 {
 	struct drm_dp_aux *aux = adapter->algo_data;
-	unsigned int i, j;
+	unsigned int m, b;
+	struct drm_dp_aux_msg msg;
+	int err = 0;
 
-	for (i = 0; i < num; i++) {
-		struct drm_dp_aux_msg msg;
-		int err;
+	memset(&msg, 0, sizeof(msg));
 
+	for (m = 0; m < num; m++) {
+		msg.address = msgs[m].addr;
+		msg.request = (msgs[m].flags & I2C_M_RD) ?
+			DP_AUX_I2C_READ :
+			DP_AUX_I2C_WRITE;
+		msg.request |= DP_AUX_I2C_MOT;
+		msg.buffer = NULL;
+		msg.size = 0;
+		err = drm_dp_i2c_do_msg(aux, &msg);
+		if (err < 0) {
+			printk("error %d in bare address write\n", err);
+			break;
+		}
 		/*
 		 * Many hardware implementations support FIFOs larger than a
 		 * single byte, but it has been empirically determined that
@@ -677,31 +690,26 @@ static int drm_dp_i2c_xfer(struct i2c_adapter *adapter, struct i2c_msg *msgs,
 		 * decreased performance. Therefore each message is simply
 		 * 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;
+		for (b = 0; b < msgs[m].len; b++) {
+			msg.buffer = msgs[m].buf + b;
 			msg.size = 1;
 
 			err = drm_dp_i2c_do_msg(aux, &msg);
 			if (err < 0)
-				return err;
+				break;
 		}
+		if (err < 0)
+			break;
 	}
-
-	return num;
+	if (err >= 0)
+		err = num;
+	/* send a bare address packet to close out the connection */
+	msg.request &= ~DP_AUX_I2C_MOT;
+	msg.buffer = NULL;
+	msg.size = 0;
+	(void)drm_dp_i2c_do_msg(aux, &msg);
+
+	return err;
 }
 
 static const struct i2c_algorithm drm_dp_i2c_algo = {
-- 
1.8.3.1