[PATCH 1/2] hwmon: (pmbus/tps53679) Select page 0 for single-page TPS53676

["Pradhan, Sanman" <sanman.pradhan@hpe.com>]

From: Sanman Pradhan <psanman@juniper.net>

tps53676_identify() derives the number of PMBus pages but does not
ensure that page 0 is selected for single-page configurations.
pmbus_set_page() does not update the PAGE register when info->pages is
1, so if boot firmware leaves PAGE set to another value subsequent
register accesses may target the wrong page.

For single-page devices, select page 0 explicitly and verify that the
PAGE register was updated.

Fixes: cb3d37b59012 ("hwmon: (pmbus/tps53679) Add support for TI TPS53676")
Cc: stable@vger.kernel.org
Signed-off-by: Sanman Pradhan <psanman@juniper.net>
---
 drivers/hwmon/pmbus/tps53679.c | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/drivers/hwmon/pmbus/tps53679.c b/drivers/hwmon/pmbus/tps53679.c
index dcd4250b679d..f38c19b8cc43 100644
--- a/drivers/hwmon/pmbus/tps53679.c
+++ b/drivers/hwmon/pmbus/tps53679.c
@@ -248,6 +248,30 @@ static int tps53676_identify(struct i2c_client *client,
 		info->phases[1] = phases_b;
 	}
 
+	/*
+	 * pmbus_set_page() does not update the PAGE register on single-page
+	 * devices, so select page 0 explicitly and verify it in case the
+	 * boot firmware left the device on another page.
+	 */
+	if (info->pages == 1) {
+		ret = i2c_smbus_read_byte_data(client, PMBUS_PAGE);
+		if (ret < 0)
+			return ret;
+		if (ret != 0) {
+			ret = i2c_smbus_write_byte_data(client, PMBUS_PAGE, 0);
+			if (ret < 0)
+				return ret;
+			ret = i2c_smbus_read_byte_data(client, PMBUS_PAGE);
+			if (ret < 0)
+				return ret;
+			if (ret != 0) {
+				dev_err(&client->dev,
+					"failed to select page 0\n");
+				return -EIO;
+			}
+		}
+	}
+
 	return 0;
 }
 
-- 
2.34.1