HID: mcp2221: Don't set bus speed on every transfer

Since the initial commit of this driver the I2C bus speed has been
reconfigured for every single transfer. This is despite the fact that we
never change the speed and it is never "lost" by the chip.
Upon investigation we find that what was really happening was that the
setting of the bus speed had the side effect of cancelling a previous
failed command if there was one, thereby freeing the bus. This is the
part that was actually required to keep the bus operational in the face
of failed commands.

Instead of always setting the speed, we now correctly cancel any failed
commands as they are detected. This means we can just set the bus speed
at probe time and remove the previous speed sets on each transfer.
This has the effect of improving performance and reducing the number of
commands required to complete transfers.

Signed-off-by: Hamish Martin <hamish.martin@alliedtelesis.co.nz>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

drivers/hid/hid-mcp2221.c

@@ -187,6 +187,25 @@ static int mcp_cancel_last_cmd(struct mcp2221 *mcp)
 	return mcp_send_data_req_status(mcp, mcp->txbuf, 8);
 }
 
+/* Check if the last command succeeded or failed and return the result.
+ * If the command did fail, cancel that command which will free the i2c bus.
+ */
+static int mcp_chk_last_cmd_status_free_bus(struct mcp2221 *mcp)
+{
+	int ret;
+
+	ret = mcp_chk_last_cmd_status(mcp);
+	if (ret) {
+		/* The last command was a failure.
+		 * Send a cancel which will also free the bus.
+		 */
+		usleep_range(980, 1000);
+		mcp_cancel_last_cmd(mcp);
+	}
+
+	return ret;
+}
+
 static int mcp_set_i2c_speed(struct mcp2221 *mcp)
 {
 	int ret;
@@ -241,7 +260,7 @@ static int mcp_i2c_write(struct mcp2221 *mcp,
 		usleep_range(980, 1000);
 
 		if (last_status) {
-			ret = mcp_chk_last_cmd_status(mcp);
+			ret = mcp_chk_last_cmd_status_free_bus(mcp);
 			if (ret)
 				return ret;
 		}
@@ -308,7 +327,7 @@ static int mcp_i2c_smbus_read(struct mcp2221 *mcp,
 		if (ret)
 			return ret;
 
-		ret = mcp_chk_last_cmd_status(mcp);
+		ret = mcp_chk_last_cmd_status_free_bus(mcp);
 		if (ret)
 			return ret;
 
@@ -328,11 +347,6 @@ static int mcp_i2c_xfer(struct i2c_adapter *adapter,
 
 	mutex_lock(&mcp->lock);
 
-	/* Setting speed before every transaction is required for mcp2221 */
-	ret = mcp_set_i2c_speed(mcp);
-	if (ret)
-		goto exit;
-
 	if (num == 1) {
 		if (msgs->flags & I2C_M_RD) {
 			ret = mcp_i2c_smbus_read(mcp, msgs, MCP2221_I2C_RD_DATA,
@@ -417,9 +431,7 @@ static int mcp_smbus_write(struct mcp2221 *mcp, u16 addr,
 	if (last_status) {
 		usleep_range(980, 1000);
 
-		ret = mcp_chk_last_cmd_status(mcp);
-		if (ret)
-			return ret;
+		ret = mcp_chk_last_cmd_status_free_bus(mcp);
 	}
 
 	return ret;
@@ -437,10 +449,6 @@ static int mcp_smbus_xfer(struct i2c_adapter *adapter, u16 addr,
 
 	mutex_lock(&mcp->lock);
 
-	ret = mcp_set_i2c_speed(mcp);
-	if (ret)
-		goto exit;
-
 	switch (size) {
 
 	case I2C_SMBUS_QUICK:
@@ -1148,6 +1156,11 @@ static int mcp2221_probe(struct hid_device *hdev,
 	if (i2c_clk_freq < 50)
 		i2c_clk_freq = 50;
 	mcp->cur_i2c_clk_div = (12000000 / (i2c_clk_freq * 1000)) - 3;
+	ret = mcp_set_i2c_speed(mcp);
+	if (ret) {
+		hid_err(hdev, "can't set i2c speed: %d\n", ret);
+		return ret;
+	}
 
 	mcp->adapter.owner = THIS_MODULE;
 	mcp->adapter.class = I2C_CLASS_HWMON;