Add a module parameter to force the write protection mode of pmbus chips.

2 protections modes are provided to start with:
* 0: Remove the write protection if possible
* 1: Enable full write protection if possible

Of course, if the parameter is not provided, the default write protection
status of the pmbus chips is left untouched.

Suggested-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
---
 Documentation/admin-guide/kernel-parameters.txt |  4 ++
 drivers/hwmon/pmbus/pmbus_core.c                | 74 ++++++++++++++++++-------
 2 files changed, 59 insertions(+), 19 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 09126bb8cc9f..e41d13f46140 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -4672,6 +4672,10 @@
 			Format: { parport<nr> | timid | 0 }
 			See also Documentation/admin-guide/parport.rst.
 
+	pmbus.wp=	[HW] PMBus Chips write protection forced mode
+			Format: { 0 | 1 }
+			See drivers/hwmon/pmbus/pmbus_core.c
+
 	pmtmr=		[X86] Manual setup of pmtmr I/O Port.
 			Override pmtimer IOPort with a hex value.
 			e.g. pmtmr=0x508
diff --git a/drivers/hwmon/pmbus/pmbus_core.c b/drivers/hwmon/pmbus/pmbus_core.c
index 363def768888..ca5803bfd24a 100644
--- a/drivers/hwmon/pmbus/pmbus_core.c
+++ b/drivers/hwmon/pmbus/pmbus_core.c
@@ -31,6 +31,20 @@
 #define PMBUS_ATTR_ALLOC_SIZE	32
 #define PMBUS_NAME_SIZE		24
 
+/*
+ * PMBus write protect forced mode:
+ * PMBus may come up with a variety of write protection configuration.
+ * 'pmbus_wp' may be used if a particular write protection is necessary.
+ * The ability to actually alter the protection may also depend on the chip
+ * so the actual runtime write protection configuration may differ from
+ * the requested one. pmbus_core currently support the following value:
+ * - 0: write protection removed
+ * - 1: write protection fully enabled, including OPERATION and VOUT_COMMAND
+ *      registers. Chips essentially become read-only with this.
+ */
+static int wp = -1;
+module_param(wp, int, 0444);
+
 struct pmbus_sensor {
 	struct pmbus_sensor *next;
 	char name[PMBUS_NAME_SIZE];	/* sysfs sensor name */
@@ -2658,6 +2672,45 @@ static void pmbus_remove_pec(void *dev)
 	device_remove_file(dev, &dev_attr_pec);
 }
 
+static void pmbus_init_wp(struct i2c_client *client, struct pmbus_data *data)
+{
+	int ret;
+
+	switch (wp) {
+	case 0:
+		_pmbus_write_byte_data(client, 0xff,
+				       PMBUS_WRITE_PROTECT, 0);
+		break;
+
+	case 1:
+		_pmbus_write_byte_data(client, 0xff,
+				       PMBUS_WRITE_PROTECT, PB_WP_ALL);
+		break;
+
+	default:
+		/* Ignore the other values */
+		break;
+	}
+
+	ret = _pmbus_read_byte_data(client, 0xff, PMBUS_WRITE_PROTECT);
+
+	switch (ret) {
+	case PB_WP_ALL:
+		data->flags |= PMBUS_OP_PROTECTED;
+		fallthrough;
+	case PB_WP_OP:
+		data->flags |= PMBUS_VOUT_PROTECTED;
+		fallthrough;
+	case PB_WP_VOUT:
+		data->flags |= PMBUS_WRITE_PROTECTED | PMBUS_SKIP_STATUS_CHECK;
+		break;
+
+	default:
+		/* Ignore manufacturer specific and invalid as well as errors */
+		break;
+	}
+}
+
 static int pmbus_init_common(struct i2c_client *client, struct pmbus_data *data,
 			     struct pmbus_driver_info *info)
 {
@@ -2711,25 +2764,8 @@ static int pmbus_init_common(struct i2c_client *client, struct pmbus_data *data,
 	 * faults, and we should not try it. Also, in that case, writes into
 	 * limit registers need to be disabled.
 	 */
-	if (!(data->flags & PMBUS_NO_WRITE_PROTECT)) {
-		ret = _pmbus_read_byte_data(client, 0xff, PMBUS_WRITE_PROTECT);
-
-		switch (ret) {
-		case PB_WP_ALL:
-			data->flags |= PMBUS_OP_PROTECTED;
-			fallthrough;
-		case PB_WP_OP:
-			data->flags |= PMBUS_VOUT_PROTECTED;
-			fallthrough;
-		case PB_WP_VOUT:
-			data->flags |= PMBUS_WRITE_PROTECTED | PMBUS_SKIP_STATUS_CHECK;
-			break;
-
-		default:
-			/* Ignore manufacturer specific and invalid as well as errors */
-			break;
-		}
-	}
+	if (!(data->flags & PMBUS_NO_WRITE_PROTECT))
+		pmbus_init_wp(client, data);
 
 	if (data->info->pages)
 		pmbus_clear_faults(client);

-- 
2.45.2