hwmon: (pmbus) More stringent checking of VOUT_MODE register

Some PMBus chips do not support the VOUT_MODE register. To make matters worse,
such chips may not return an error when reading the register either, but instead
return 0xff.

Check if the register exists using pmbus_check_byte_register() before reading
its value. In addition, validate the returned value and ignore it if it is 0xff
(which is invalid).

Reported-by: Greg Schnorr <gschnorr@cisco.com>
Tested-by: Greg Schnorr <gschnorr@cisco.com>
Signed-off-by: Guenter Roeck <guenter.roeck@ericsson.com>
Acked-by: Greg Schnorr <gschnorr@cisco.com>

1 files changed, 4 insertions, 3 deletions

diff --git a/drivers/hwmon/pmbus_core.c b/drivers/hwmon/pmbus_core.c
index 0edab1c9553e..b7c64ba31f4c 100644
--- a/drivers/hwmon/pmbus_core.c
+++ b/drivers/hwmon/pmbus_core.c
@@ -1426,10 +1426,11 @@ static void pmbus_find_attributes(struct i2c_client *client,
 static int pmbus_identify_common(struct i2c_client *client,
                                  struct pmbus_data *data)
 {
-        int vout_mode, exponent;
+        int vout_mode = -1, exponent;
 
-        vout_mode = pmbus_read_byte_data(client, 0, PMBUS_VOUT_MODE);
-        if (vout_mode >= 0) {
+        if (pmbus_check_byte_register(client, 0, PMBUS_VOUT_MODE))
+                vout_mode = pmbus_read_byte_data(client, 0, PMBUS_VOUT_MODE);
+        if (vout_mode >= 0 && vout_mode != 0xff) {
                 /*
                  * Not all chips support the VOUT_MODE command,
                  * so a failure to read it is not an error.