[PATCH 3.16 117/370] i2c: mux: pca954x: fix i2c mux selection caching

[Ben Hutchings]

3.16.42-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Russell King <rmk+kernel@xxxxxxxxxxxxxxx>

commit 7f638c1cb0a1112dbe0b682a42db30521646686b upstream.

smbus functions return -ve on error, 0 on success.  However,
__i2c_transfer() have a different return signature - -ve on error, or
number of buffers transferred (which may be zero or greater.)

The upshot of this is that the sense of the test is reversed when using
the mux on a bus supporting the master_xfer method: we cache the value
and never retry if we fail to transfer any buffers, but if we succeed,
we clear the cached value.

Fix this by making pca954x_reg_write() return a negative error code for
all failure cases.

Fixes: 463e8f845cbf ("i2c: mux: pca954x: retry updating the mux selection on failure")
Acked-by: Peter Rosin <peda@xxxxxxxxxx>
Signed-off-by: Russell King <rmk+kernel@xxxxxxxxxxxxxxx>
Signed-off-by: Wolfram Sang <wsa@xxxxxxxxxxxxx>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 drivers/i2c/muxes/i2c-mux-pca954x.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

--- a/drivers/i2c/muxes/i2c-mux-pca954x.c
+++ b/drivers/i2c/muxes/i2c-mux-pca954x.c
@@ -133,6 +133,9 @@ static int pca954x_reg_write(struct i2c_
 		buf[0] = val;
 		msg.buf = buf;
 		ret = adap->algo->master_xfer(adap, &msg, 1);
+
+		if (ret >= 0 && ret != 1)
+			ret = -EREMOTEIO;
 	} else {
 		union i2c_smbus_data data;
 		ret = adap->algo->smbus_xfer(adap, client->addr,
@@ -161,7 +164,7 @@ static int pca954x_select_chan(struct i2
 	/* Only select the channel if its different from the last channel */
 	if (data->last_chan != regval) {
 		ret = pca954x_reg_write(adap, client, regval);
-		data->last_chan = ret ? 0 : regval;
+		data->last_chan = ret < 0 ? 0 : regval;
 	}
 
 	return ret;