[PATCH 2/2] hwmon: (pmbus/tps53679) Select page 0 for single-page TPS536C7

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

From: Sanman Pradhan <psanman@juniper.net>

tps536c7_identify() sets info->pages to 1 for a single-channel part and
then accesses page 0 (writing PMBUS_PHASE) without ensuring PAGE is
actually 0. pmbus_set_page() does not update the PAGE register when
info->pages is 1, so if boot firmware left PAGE set to another value the
PHASE writes and subsequent telemetry may target the wrong page.

Select page 0 explicitly and verify it before configuring PHASE.

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 f38c19b8cc43..b68313485511 100644
--- a/drivers/hwmon/pmbus/tps53679.c
+++ b/drivers/hwmon/pmbus/tps53679.c
@@ -299,6 +299,30 @@ static int tps536c7_identify(struct i2c_client *client,
 	 */
 	info->pages = phases_b ? 2 : 1;
 
+	/*
+	 * 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;
+			}
+		}
+	}
+
 	/*
 	 * With info->phases[] left unset the PMBus core never programs the
 	 * PHASE selector, so make sure each page reports the aggregate
-- 
2.34.1