[PATCH 3/3] hwmon: (pmbus/fs1412) Add TDK FS1412 driver

[Abdurrahman Hussain]

The TDK FS1412 is a 12A uPOL regulator with PMBus telemetry for input
voltage, output voltage and temperature, plus the matching status
registers. It has over-current and over-temperature protection with
programmable fault limits but no current telemetry: READ_IOUT is not in
its command table. Its MFR_ID reads "KDT", which the driver checks
before binding.

VOUT_MODE reads 0x98: LINEAR mode with exponent -8 and bit 7 set. The
datasheet defines the output voltage limits relative to the output
voltage with a resolution of 1/256, so 0x0133 against a 0.6V command is
a 120% over-voltage limit. The PMBus core takes bits [7:5] as the mode
and rejects the device, so the driver reports VOUT_MODE with bit 7
cleared and converts the four VOUT limits to and from absolute voltages
against VOUT_COMMAND.

OT_WARN_LIMIT is not in the command table either. The register answers,
but the part has no warning threshold and reports OT_WARNING at room
temperature, so the driver hides the register from the core and only
the OT_FAULT_LIMIT based attributes are provided.

READ_VOUT reports the voltage at the feedback pin, so boards that set
the output with a resistor divider scale in2_input in user space.

Signed-off-by: Abdurrahman Hussain <abdurrahman@xxxxxxxxxx>
---
 Documentation/hwmon/fs1412.rst |  87 ++++++++++++++++++++
 Documentation/hwmon/index.rst  |   1 +
 MAINTAINERS                    |   7 ++
 drivers/hwmon/pmbus/Kconfig    |   9 +++
 drivers/hwmon/pmbus/Makefile   |   1 +
 drivers/hwmon/pmbus/fs1412.c   | 179 +++++++++++++++++++++++++++++++++++++++++
 6 files changed, 284 insertions(+)

diff --git a/Documentation/hwmon/fs1412.rst b/Documentation/hwmon/fs1412.rst
new file mode 100644
index 000000000000..b9b12fcbc5d9
--- /dev/null
+++ b/Documentation/hwmon/fs1412.rst
@@ -0,0 +1,87 @@
+.. SPDX-License-Identifier: GPL-2.0-or-later
+
+Kernel driver fs1412
+====================
+
+Supported chips:
+
+  * TDK FS1412
+
+    Prefix: 'fs1412'
+
+    Addresses scanned: -
+
+    Datasheet: https://product.tdk.com/system/files/dam/doc/product/power/switching-power/micro-pol/data_sheet/fs1412_datasheet.pdf
+
+Authors:
+    Abdurrahman Hussain <abdurrahman@xxxxxxxxxx>
+
+
+Description
+-----------
+
+This driver implements support for the TDK FS1412, a 12A uPOL DC/DC
+regulator with PMBus monitoring of its input voltage, output voltage and
+temperature. The part has over-current and over-temperature protection with
+programmable fault limits but no current telemetry, so no current attributes
+are provided.
+
+The device answers on two I2C addresses: a register interface at the I2C
+base address (0x08 by default) and the PMBus interface at the PMBus base
+address (0x70 by default), each plus the offset strapped on the ADDR pin.
+This driver binds to the PMBus address.
+
+The driver is a client driver to the core PMBus driver.
+Please see Documentation/hwmon/pmbus.rst for details on PMBus client drivers.
+
+
+Usage Notes
+-----------
+
+This driver does not auto-detect devices. You will have to instantiate the
+devices explicitly. Please see Documentation/i2c/instantiating-devices.rst for
+details.
+
+READ_VOUT reports the voltage at the feedback (VFB) pin, not the regulated
+output. When the output is set with an external resistor divider, scale
+in2_input and the in2 limits by the divider ratio in user space.
+
+The device stores its output voltage limits relative to VOUT_COMMAND
+(VOUT_MODE bit 7 set). The driver converts them to and from absolute
+voltages, so the in2 limits read and write in the same units as in2_input.
+A written limit is stored as a 1/256 fraction of VOUT_COMMAND, so it may
+read back rounded to that resolution. The over-voltage protection itself
+engages at one of four thresholds the device rounds the fault limit to; see
+the datasheet.
+
+The part reports an over-temperature warning at room temperature and has no
+OT_WARN_LIMIT in its command table, so temp1_max and its alarm are not
+provided; temp1_crit comes from OT_FAULT_LIMIT.
+
+
+Sysfs entries
+-------------
+
+======================= ======================================================
+in1_label		"vin"
+in1_input		Measured input voltage
+in1_min			Minimum input voltage
+in1_min_alarm		Input voltage low alarm
+in1_crit		Critical high input voltage
+in1_crit_alarm		Input voltage critical high alarm
+
+in2_label		"vout1"
+in2_input		Measured voltage at the feedback pin
+in2_min			Minimum output voltage
+in2_min_alarm		Output voltage low alarm
+in2_max			Maximum output voltage
+in2_max_alarm		Output voltage high alarm
+in2_lcrit		Critical low output voltage
+in2_lcrit_alarm		Output voltage critical low alarm
+in2_crit		Critical high output voltage
+in2_crit_alarm		Output voltage critical high alarm
+
+temp1_input		Measured temperature
+temp1_crit		Critical high temperature
+temp1_crit_alarm	Temperature critical high alarm
+======================= ======================================================
diff --git a/Documentation/hwmon/index.rst b/Documentation/hwmon/index.rst
index 9955a525436a..625833374c94 100644
--- a/Documentation/hwmon/index.rst
+++ b/Documentation/hwmon/index.rst
@@ -79,6 +79,7 @@ Hardware Monitoring Kernel Drivers
    f71805f
    f71882fg
    fam15h_power
+   fs1412
    fsp-3y
    ftsteutates
    g760a
diff --git a/MAINTAINERS b/MAINTAINERS
index 65e8a4b5c90b..05461a380bf7 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -26778,6 +26778,13 @@ W:	https://linuxtv.org
 T:	git git://linuxtv.org/media.git
 F:	drivers/media/i2c/tda9840*
 
+TDK FS1412 MICRO POL DRIVER
+M:	Abdurrahman Hussain <abdurrahman@xxxxxxxxxx>
+L:	linux-hwmon@xxxxxxxxxxxxxxx
+S:	Maintained
+F:	Documentation/hwmon/fs1412.rst
+F:	drivers/hwmon/pmbus/fs1412.c
+
 TEA5761 TUNER DRIVER
 M:	Mauro Carvalho Chehab <mchehab@xxxxxxxxxx>
 L:	linux-media@xxxxxxxxxxxxxxx
diff --git a/drivers/hwmon/pmbus/Kconfig b/drivers/hwmon/pmbus/Kconfig
index eb71a0ac044b..030ef72415fe 100644
--- a/drivers/hwmon/pmbus/Kconfig
+++ b/drivers/hwmon/pmbus/Kconfig
@@ -132,6 +132,15 @@ config SENSORS_DELTA_AHE50DC_FAN
 	  This driver can also be built as a module. If so, the module
 	  will be called delta-ahe50dc-fan.
 
+config SENSORS_FS1412
+	tristate "TDK FS1412"
+	help
+	  If you say yes here you get hardware monitoring support for the TDK
+	  FS1412 uPOL regulator.
+
+	  This driver can also be built as a module. If so, the module will
+	  be called fs1412.
+
 config SENSORS_FSP_3Y
 	tristate "FSP/3Y-Power power supplies"
 	help
diff --git a/drivers/hwmon/pmbus/Makefile b/drivers/hwmon/pmbus/Makefile
index e288fe72a437..7bbc0d02bfad 100644
--- a/drivers/hwmon/pmbus/Makefile
+++ b/drivers/hwmon/pmbus/Makefile
@@ -13,6 +13,7 @@ obj-$(CONFIG_SENSORS_APS_379)	+= aps-379.o
 obj-$(CONFIG_SENSORS_BEL_PFE)	+= bel-pfe.o
 obj-$(CONFIG_SENSORS_BPA_RS600)	+= bpa-rs600.o
 obj-$(CONFIG_SENSORS_DELTA_AHE50DC_FAN) += delta-ahe50dc-fan.o
+obj-$(CONFIG_SENSORS_FS1412)	+= fs1412.o
 obj-$(CONFIG_SENSORS_FSP_3Y)	+= fsp-3y.o
 obj-$(CONFIG_SENSORS_HAC300S)	+= hac300s.o
 obj-$(CONFIG_SENSORS_IBM_CFFPS)	+= ibm-cffps.o
diff --git a/drivers/hwmon/pmbus/fs1412.c b/drivers/hwmon/pmbus/fs1412.c
new file mode 100644
index 000000000000..90a72b23967e
--- /dev/null
+++ b/drivers/hwmon/pmbus/fs1412.c
@@ -0,0 +1,179 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Hardware monitoring driver for the TDK FS1412 uPOL regulator
+ *
+ * Copyright 2026 Nexthop Systems.
+ */
+
+#include <linux/bits.h>
+#include <linux/i2c.h>
+#include <linux/math.h>
+#include <linux/minmax.h>
+#include <linux/module.h>
+#include <linux/of.h>
+#include <linux/pmbus.h>
+#include <linux/string.h>
+
+#include "pmbus.h"
+
+#define FS1412_MFR_ID		"KDT"
+#define FS1412_MFR_ID_LEN	(sizeof(FS1412_MFR_ID) - 1)
+
+/* VOUT_MODE: bit 7 selects relative VOUT limits, bits [4:0] the exponent. */
+#define FS1412_VOUT_MODE_RELATIVE	BIT(7)
+#define FS1412_VOUT_MODE_EXP_MASK	0x1f
+#define FS1412_VOUT_EXP_SHIFT		8	/* exponent -8: 1/256 V per LSB */
+
+/*
+ * VOUT_MODE reads 0x98: LINEAR mode with exponent -8 and bit 7 set, which
+ * makes the VOUT limit commands relative to VOUT_COMMAND. The PMBus core
+ * treats bits [7:5] as the mode, so report the mode and exponent with bit
+ * 7 cleared and convert the relative limits in the word accessors below.
+ */
+static int fs1412_read_byte_data(struct i2c_client *client, int page, int reg)
+{
+	int ret;
+
+	if (reg != PMBUS_VOUT_MODE)
+		return -ENODATA;
+
+	ret = pmbus_read_byte_data(client, page, reg);
+	if (ret < 0 || ret == 0xff)
+		return ret;
+
+	return ret & ~FS1412_VOUT_MODE_RELATIVE;
+}
+
+static bool fs1412_vout_limit(int reg)
+{
+	switch (reg) {
+	case PMBUS_VOUT_OV_FAULT_LIMIT:
+	case PMBUS_VOUT_OV_WARN_LIMIT:
+	case PMBUS_VOUT_UV_WARN_LIMIT:
+	case PMBUS_VOUT_UV_FAULT_LIMIT:
+		return true;
+	default:
+		return false;
+	}
+}
+
+/*
+ * The VOUT limits hold the limit as a fraction of VOUT_COMMAND in the same
+ * 1/256 units, so 0x0133 on a 0.6 V command is a 120% limit. Report them
+ * as absolute voltages: limit = fraction * VOUT_COMMAND.
+ */
+static int fs1412_read_word_data(struct i2c_client *client, int page,
+				 int phase, int reg)
+{
+	int rel, cmd;
+
+	/*
+	 * OT_WARN_LIMIT is not in the command table. The register answers,
+	 * but the part has no warning threshold and reports OT_WARNING at
+	 * room temperature, so keep the core from exposing either.
+	 */
+	if (reg == PMBUS_OT_WARN_LIMIT)
+		return -ENXIO;
+	if (!fs1412_vout_limit(reg))
+		return -ENODATA;
+
+	rel = pmbus_read_word_data(client, page, phase, reg);
+	if (rel < 0)
+		return rel;
+
+	cmd = pmbus_read_word_data(client, page, 0xff, PMBUS_VOUT_COMMAND);
+	if (cmd < 0)
+		return cmd;
+
+	return min_t(u32, DIV_ROUND_CLOSEST((u32)rel * cmd,
+					     1 << FS1412_VOUT_EXP_SHIFT), 0xffff);
+}
+
+static int fs1412_write_word_data(struct i2c_client *client, int page,
+				  int reg, u16 word)
+{
+	int cmd;
+
+	if (reg == PMBUS_OT_WARN_LIMIT)
+		return -ENXIO;
+	if (!fs1412_vout_limit(reg))
+		return -ENODATA;
+
+	cmd = pmbus_read_word_data(client, page, 0xff, PMBUS_VOUT_COMMAND);
+	if (cmd < 0)
+		return cmd;
+	if (!cmd)
+		return -EINVAL;
+
+	word = clamp_val(DIV_ROUND_CLOSEST(word << FS1412_VOUT_EXP_SHIFT, cmd),
+			 0, 0xffff);
+
+	return pmbus_write_word_data(client, page, reg, word);
+}
+
+static struct pmbus_driver_info fs1412_info = {
+	.pages = 1,
+	.format[PSC_VOLTAGE_IN] = linear,
+	.format[PSC_VOLTAGE_OUT] = linear,
+	.format[PSC_TEMPERATURE] = linear,
+	.func[0] = PMBUS_HAVE_VIN | PMBUS_HAVE_STATUS_INPUT |
+		   PMBUS_HAVE_VOUT | PMBUS_HAVE_STATUS_VOUT |
+		   PMBUS_HAVE_TEMP | PMBUS_HAVE_STATUS_TEMP,
+	.read_byte_data = fs1412_read_byte_data,
+	.read_word_data = fs1412_read_word_data,
+	.write_word_data = fs1412_write_word_data,
+};
+
+static int fs1412_probe(struct i2c_client *client)
+{
+	char buf[I2C_SMBUS_BLOCK_MAX + 1] = { 0 };
+	struct device *dev = &client->dev;
+	int ret;
+
+	ret = i2c_smbus_read_block_data(client, PMBUS_MFR_ID, buf);
+	if (ret < 0)
+		return dev_err_probe(dev, ret, "Failed to read PMBUS_MFR_ID\n");
+
+	if (ret != FS1412_MFR_ID_LEN ||
+	    strncmp(buf, FS1412_MFR_ID, FS1412_MFR_ID_LEN))
+		return dev_err_probe(dev, -ENODEV,
+				     "Unsupported Manufacturer ID '%s'\n", buf);
+
+	/* The relative limit conversion assumes the 1/256 V exponent. */
+	ret = i2c_smbus_read_byte_data(client, PMBUS_VOUT_MODE);
+	if (ret < 0)
+		return dev_err_probe(dev, ret, "Failed to read VOUT_MODE\n");
+	if ((ret & FS1412_VOUT_MODE_EXP_MASK) != 0x18)
+		return dev_err_probe(dev, -ENODEV,
+				     "Unsupported VOUT_MODE 0x%02x\n", ret);
+
+	return pmbus_do_probe(client, &fs1412_info);
+}
+
+static const struct i2c_device_id fs1412_id[] = {
+	{ .name = "fs1412" },
+	{ }
+};
+MODULE_DEVICE_TABLE(i2c, fs1412_id);
+
+static const struct of_device_id fs1412_of_match[] = {
+	{ .compatible = "tdk,fs1412" },
+	{ }
+};
+MODULE_DEVICE_TABLE(of, fs1412_of_match);
+
+static struct i2c_driver fs1412_driver = {
+	.driver = {
+		.name = "fs1412",
+		.of_match_table = fs1412_of_match,
+	},
+	.probe = fs1412_probe,
+	.id_table = fs1412_id,
+};
+
+module_i2c_driver(fs1412_driver);
+
+MODULE_AUTHOR("Abdurrahman Hussain <abdurrahman@xxxxxxxxxx>");
+MODULE_DESCRIPTION("PMBus driver for the TDK FS1412 uPOL regulator");
+MODULE_LICENSE("GPL");
+MODULE_IMPORT_NS("PMBUS");

-- 
2.54.0