From: Nuno Sá <nuno.sa@analog.com>

The LTC4283 is a negative voltage hot swap controller that drives an
external N-channel MOSFET to allow a board to be safely inserted and
removed from a live backplane.

Main usage is as an Hardware Monitoring device. However, it has up to 8
pins that can be configured and used as GPIOs and hence, the device can
also be a GPIO controller (and so being added as MFD device).

Signed-off-by: Nuno Sá <nuno.sa@analog.com>
---
 MAINTAINERS                 |   2 +
 drivers/mfd/Kconfig         |  11 ++++
 drivers/mfd/Makefile        |   1 +
 drivers/mfd/ltc4283.c       | 140 ++++++++++++++++++++++++++++++++++++++++++++
 include/linux/mfd/ltc4283.h |  33 +++++++++++
 5 files changed, 187 insertions(+)

diff --git a/MAINTAINERS b/MAINTAINERS
index 413bb77d5eebe2b51aa9c3af86e7cfd5ab142044..b5f4f1c41c64b738d57c1fb5552a60b4c6b9985c 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -14506,6 +14506,8 @@ L:	linux-hwmon@vger.kernel.org
 L:	linux-gpio@vger.kernel.org
 S:	Supported
 F:	Documentation/devicetree/bindings/mfd/adi,ltc4283.yaml
+F:	drivers/mfd/ltc4283.c
+F:	include/linux/mfd/ltc4283.h
 
 LTC4286 HARDWARE MONITOR DRIVER
 M:	Delphine CC Chiu <Delphine_CC_Chiu@Wiwynn.com>
diff --git a/drivers/mfd/Kconfig b/drivers/mfd/Kconfig
index 425c5fba6cb1e7848dcea05bd77c729a71d48e2c..ec3e02d40fd17a0bba29e3157723055feedebd11 100644
--- a/drivers/mfd/Kconfig
+++ b/drivers/mfd/Kconfig
@@ -900,6 +900,17 @@ config MFD_MAX14577
 	  additional drivers must be enabled in order to use the functionality
 	  of the device.
 
+config MFD_LTC4283
+	tristate "LTC4283 Hot Swap Controller"
+	depends on I2C
+	select MFD_CORE
+	select REGMAP_I2C
+	help
+	  This enables support for the LTC4283 Negative Voltage Hot Swap
+	  Controller. This driver provides common support for accessing the
+	  device; additional drivers must be enabled in order to use the
+	  functionality of the device.
+
 config MFD_MAX77541
 	tristate "Analog Devices MAX77541/77540 PMIC Support"
 	depends on I2C=y
diff --git a/drivers/mfd/Makefile b/drivers/mfd/Makefile
index f7bdedd5a66d16bf8ccee0da1236a441e6f085b0..4a3bcd30ab7e12aed4bc5e48294500c77cb61aa7 100644
--- a/drivers/mfd/Makefile
+++ b/drivers/mfd/Makefile
@@ -23,6 +23,7 @@ obj-$(CONFIG_MFD_EXYNOS_LPASS)	+= exynos-lpass.o
 obj-$(CONFIG_MFD_GATEWORKS_GSC)	+= gateworks-gsc.o
 obj-$(CONFIG_MFD_MACSMC)	+= macsmc.o
 
+obj-$(CONFIG_MFD_LTC4283)	+= ltc4283.o
 obj-$(CONFIG_MFD_TI_LP873X)	+= lp873x.o
 obj-$(CONFIG_MFD_TI_LP87565)	+= lp87565.o
 obj-$(CONFIG_MFD_TI_AM335X_TSCADC)	+= ti_am335x_tscadc.o
diff --git a/drivers/mfd/ltc4283.c b/drivers/mfd/ltc4283.c
new file mode 100644
index 0000000000000000000000000000000000000000..a45c01278f884630984790c922c057c4f2db9c82
--- /dev/null
+++ b/drivers/mfd/ltc4283.c
@@ -0,0 +1,140 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Analog Devices LTC4283 I2C Negative Voltage Hot Swap Controller
+ *
+ * Copyright 2025 Analog Devices Inc.
+ */
+#include <linux/bitmap.h>
+#include <linux/bitops.h>
+#include <linux/device.h>
+#include <linux/i2c.h>
+#include <linux/mfd/core.h>
+#include <linux/module.h>
+#include <linux/mod_devicetable.h>
+#include <linux/regmap.h>
+#include <linux/property.h>
+
+#include <linux/mfd/ltc4283.h>
+
+static const struct mfd_cell ltc4283_cells[] = {
+	MFD_CELL_OF("ltc4283-hwmon", NULL, NULL, 0, 0, "adi,ltc4283-hwmon"),
+	MFD_CELL_OF("ltc4283-gpio", NULL, NULL, 0, 0, "adi,ltc4283-gpio"),
+};
+
+static bool ltc4283_writable_reg(struct device *dev, unsigned int reg)
+{
+	switch (reg) {
+	case 0x00 ... 0x03:
+		return false;
+	case 0x3c:
+		return false;
+	case 0x86 ... 0x8f:
+		return false;
+	case 0x91 ... 0xa1:
+		return false;
+	case 0xa3:
+		return false;
+	case 0xac:
+		return false;
+	case 0xf1 ... 0xff:
+		return false;
+	default:
+		return true;
+	}
+}
+
+static const struct regmap_config ltc4283_regmap_config = {
+	.reg_bits = 8,
+	.val_bits = 8,
+	.max_register = 0xff,
+	.writeable_reg = ltc4283_writable_reg,
+};
+
+static int ltc4283_get_gpio_pins(struct i2c_client *client, u32 *n_cells)
+{
+	struct device *dev = &client->dev;
+	u32 pins[LTC4283_GPIO_MAX], pin;
+	unsigned long *gpio_mask;
+	int n_pins, ret;
+
+	/*
+	 * The device has up to 8 pins that can be configured either as GPIOS or
+	 * for monitoring purposes. Both gpio and hwmon devices need to have
+	 * this information in order to do proper validations and
+	 * configurations. Hence, this property needs to be in the top level
+	 * device.
+	 */
+	n_pins = device_property_count_u32(dev, "adi,gpio-pins");
+	if (n_pins < 0)
+		return 0;
+	if (n_pins >= LTC4283_GPIO_MAX)
+		return dev_err_probe(dev, -EINVAL, "Too many GPIO pins specified (%d), max is %d\n",
+				     n_pins, LTC4283_GPIO_MAX);
+
+	ret = device_property_read_u32_array(dev, "adi,gpio-pins", pins, n_pins);
+	if (ret)
+		return dev_err_probe(dev, ret, "Failed to read GPIO pins\n");
+
+	gpio_mask = devm_bitmap_zalloc(dev, LTC4283_GPIO_MAX, GFP_KERNEL);
+	if (!gpio_mask)
+		return -ENOMEM;
+
+	for (pin = 0; pin < n_pins; pin++) {
+		if (pins[pin] >= LTC4283_GPIO_MAX)
+			return dev_err_probe(dev, -EINVAL,
+					     "Invalid GPIO pin specified (%u), max is %d\n",
+					     pins[pin], LTC4283_GPIO_MAX);
+
+		__set_bit(pins[pin], gpio_mask);
+	}
+
+	/* Add the GPIO cell */
+	*n_cells += 1;
+	i2c_set_clientdata(client, gpio_mask);
+
+	return 0;
+}
+
+static int ltc4283_probe(struct i2c_client *client)
+{
+	u32 n_cells = ARRAY_SIZE(ltc4283_cells) - 1;
+	struct regmap *regmap;
+	int ret;
+
+	regmap = devm_regmap_init_i2c(client, &ltc4283_regmap_config);
+	if (IS_ERR(regmap))
+		return PTR_ERR(regmap);
+
+	ret = ltc4283_get_gpio_pins(client, &n_cells);
+	if (ret)
+		return ret;
+
+	return devm_mfd_add_devices(&client->dev, PLATFORM_DEVID_AUTO,
+				    ltc4283_cells, n_cells, NULL, 0, NULL);
+}
+
+static const struct of_device_id ltc4283_of_match[] = {
+	{ .compatible = "adi,ltc4283" },
+	{ }
+};
+MODULE_DEVICE_TABLE(of, ltc4283_of_match);
+
+static const struct i2c_device_id ltc4283_i2c_id[] = {
+	{ "ltc4283" },
+	{ }
+};
+MODULE_DEVICE_TABLE(i2c, ltc4283_i2c_id);
+
+static struct i2c_driver ltc4283_driver = {
+	.driver = {
+		.name = "ltc4283",
+		.of_match_table = ltc4283_of_match,
+	},
+	.probe = ltc4283_probe,
+	.id_table = ltc4283_i2c_id,
+};
+module_i2c_driver(ltc4283_driver);
+
+MODULE_AUTHOR("Nuno Sá <nuno.sa@analog.com>");
+MODULE_DESCRIPTION("LTC4283 MFD I2C driver");
+MODULE_LICENSE("GPL");
diff --git a/include/linux/mfd/ltc4283.h b/include/linux/mfd/ltc4283.h
new file mode 100644
index 0000000000000000000000000000000000000000..153594009c07b296ce5743e5e817e96464d81cb3
--- /dev/null
+++ b/include/linux/mfd/ltc4283.h
@@ -0,0 +1,33 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * Analog Devices LTC4283 I2C Negative Voltage Hot Swap Controller
+ *
+ * Copyright 2025 Analog Devices Inc.
+ */
+
+#ifndef __MFD_LTC4283_H_
+#define __MFD_LTC4283_H_
+
+#include <linux/bitops.h>
+#include <linux/bits.h>
+
+/*
+ * We can have up to 8 gpios. 4 PGIOs and 4 ADIOs. PGIOs start at index 4 in the
+ * gpios mask.
+ */
+#define LTC4283_PGIOX_START_NR	4
+
+#define LTC4283_PGIO_CONFIG		0x10
+#define   LTC4283_PGIO_CFG_MASK(pin) \
+	GENMASK(((pin) - LTC4283_PGIOX_START_NR) * 2 + 1, (((pin) - LTC4283_PGIOX_START_NR) * 2))
+#define LTC4283_PGIO_CONFIG_2		0x11
+#define   LTC4283_ADC_MASK		GENMASK(2, 0)
+#define   LTC4283_PGIO_OUT_MASK(pin)	BIT(4 + (pin))
+
+#define LTC4283_GPIO_MAX	8
+
+/* Non-constant mask variant of FIELD_GET() and FIELD_PREP() */
+#define field_get(_mask, _reg)	(((_reg) & (_mask)) >> (ffs(_mask) - 1))
+#define field_prep(_mask, _val)	(((_val) << (ffs(_mask) - 1)) & (_mask))
+
+#endif

-- 
2.50.1

On Thu, 14 Aug 2025, Nuno Sá via B4 Relay wrote:

> From: Nuno Sá <nuno.sa@analog.com>
> 
> The LTC4283 is a negative voltage hot swap controller that drives an
> external N-channel MOSFET to allow a board to be safely inserted and
> removed from a live backplane.
> 
> Main usage is as an Hardware Monitoring device. However, it has up to 8
> pins that can be configured and used as GPIOs and hence, the device can
> also be a GPIO controller (and so being added as MFD device).
> 
> Signed-off-by: Nuno Sá <nuno.sa@analog.com>
> ---
>  MAINTAINERS                 |   2 +
>  drivers/mfd/Kconfig         |  11 ++++
>  drivers/mfd/Makefile        |   1 +
>  drivers/mfd/ltc4283.c       | 140 ++++++++++++++++++++++++++++++++++++++++++++
>  include/linux/mfd/ltc4283.h |  33 +++++++++++
>  5 files changed, 187 insertions(+)
> 
> diff --git a/MAINTAINERS b/MAINTAINERS
> index 413bb77d5eebe2b51aa9c3af86e7cfd5ab142044..b5f4f1c41c64b738d57c1fb5552a60b4c6b9985c 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -14506,6 +14506,8 @@ L:	linux-hwmon@vger.kernel.org
>  L:	linux-gpio@vger.kernel.org
>  S:	Supported
>  F:	Documentation/devicetree/bindings/mfd/adi,ltc4283.yaml
> +F:	drivers/mfd/ltc4283.c
> +F:	include/linux/mfd/ltc4283.h
>  
>  LTC4286 HARDWARE MONITOR DRIVER
>  M:	Delphine CC Chiu <Delphine_CC_Chiu@Wiwynn.com>
> diff --git a/drivers/mfd/Kconfig b/drivers/mfd/Kconfig
> index 425c5fba6cb1e7848dcea05bd77c729a71d48e2c..ec3e02d40fd17a0bba29e3157723055feedebd11 100644
> --- a/drivers/mfd/Kconfig
> +++ b/drivers/mfd/Kconfig
> @@ -900,6 +900,17 @@ config MFD_MAX14577
>  	  additional drivers must be enabled in order to use the functionality
>  	  of the device.
>  
> +config MFD_LTC4283
> +	tristate "LTC4283 Hot Swap Controller"
> +	depends on I2C
> +	select MFD_CORE
> +	select REGMAP_I2C
> +	help
> +	  This enables support for the LTC4283 Negative Voltage Hot Swap
> +	  Controller. This driver provides common support for accessing the
> +	  device; additional drivers must be enabled in order to use the
> +	  functionality of the device.
> +
>  config MFD_MAX77541
>  	tristate "Analog Devices MAX77541/77540 PMIC Support"
>  	depends on I2C=y
> diff --git a/drivers/mfd/Makefile b/drivers/mfd/Makefile
> index f7bdedd5a66d16bf8ccee0da1236a441e6f085b0..4a3bcd30ab7e12aed4bc5e48294500c77cb61aa7 100644
> --- a/drivers/mfd/Makefile
> +++ b/drivers/mfd/Makefile
> @@ -23,6 +23,7 @@ obj-$(CONFIG_MFD_EXYNOS_LPASS)	+= exynos-lpass.o
>  obj-$(CONFIG_MFD_GATEWORKS_GSC)	+= gateworks-gsc.o
>  obj-$(CONFIG_MFD_MACSMC)	+= macsmc.o
>  
> +obj-$(CONFIG_MFD_LTC4283)	+= ltc4283.o
>  obj-$(CONFIG_MFD_TI_LP873X)	+= lp873x.o
>  obj-$(CONFIG_MFD_TI_LP87565)	+= lp87565.o
>  obj-$(CONFIG_MFD_TI_AM335X_TSCADC)	+= ti_am335x_tscadc.o
> diff --git a/drivers/mfd/ltc4283.c b/drivers/mfd/ltc4283.c
> new file mode 100644
> index 0000000000000000000000000000000000000000..a45c01278f884630984790c922c057c4f2db9c82
> --- /dev/null
> +++ b/drivers/mfd/ltc4283.c
> @@ -0,0 +1,140 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/*
> + * Analog Devices LTC4283 I2C Negative Voltage Hot Swap Controller
> + *
> + * Copyright 2025 Analog Devices Inc.
> + */
> +#include <linux/bitmap.h>
> +#include <linux/bitops.h>
> +#include <linux/device.h>
> +#include <linux/i2c.h>
> +#include <linux/mfd/core.h>
> +#include <linux/module.h>
> +#include <linux/mod_devicetable.h>
> +#include <linux/regmap.h>
> +#include <linux/property.h>

Alphabetical.

> +
> +#include <linux/mfd/ltc4283.h>
> +
> +static const struct mfd_cell ltc4283_cells[] = {
> +	MFD_CELL_OF("ltc4283-hwmon", NULL, NULL, 0, 0, "adi,ltc4283-hwmon"),
> +	MFD_CELL_OF("ltc4283-gpio", NULL, NULL, 0, 0, "adi,ltc4283-gpio"),
> +};
> +
> +static bool ltc4283_writable_reg(struct device *dev, unsigned int reg)
> +{
> +	switch (reg) {
> +	case 0x00 ... 0x03:

Define these magic numbers.

> +		return false;
> +	case 0x3c:
> +		return false;
> +	case 0x86 ... 0x8f:
> +		return false;
> +	case 0x91 ... 0xa1:
> +		return false;
> +	case 0xa3:
> +		return false;
> +	case 0xac:
> +		return false;
> +	case 0xf1 ... 0xff:
> +		return false;
> +	default:
> +		return true;
> +	}
> +}
> +
> +static const struct regmap_config ltc4283_regmap_config = {
> +	.reg_bits = 8,
> +	.val_bits = 8,
> +	.max_register = 0xff,
> +	.writeable_reg = ltc4283_writable_reg,
> +};
> +
> +static int ltc4283_get_gpio_pins(struct i2c_client *client, u32 *n_cells)

Why not do this in the GPIO driver?

> +{
> +	struct device *dev = &client->dev;
> +	u32 pins[LTC4283_GPIO_MAX], pin;
> +	unsigned long *gpio_mask;
> +	int n_pins, ret;
> +
> +	/*
> +	 * The device has up to 8 pins that can be configured either as GPIOS or
> +	 * for monitoring purposes. Both gpio and hwmon devices need to have
> +	 * this information in order to do proper validations and
> +	 * configurations. Hence, this property needs to be in the top level
> +	 * device.
> +	 */
> +	n_pins = device_property_count_u32(dev, "adi,gpio-pins");
> +	if (n_pins < 0)
> +		return 0;
> +	if (n_pins >= LTC4283_GPIO_MAX)
> +		return dev_err_probe(dev, -EINVAL, "Too many GPIO pins specified (%d), max is %d\n",
> +				     n_pins, LTC4283_GPIO_MAX);
> +
> +	ret = device_property_read_u32_array(dev, "adi,gpio-pins", pins, n_pins);
> +	if (ret)
> +		return dev_err_probe(dev, ret, "Failed to read GPIO pins\n");
> +
> +	gpio_mask = devm_bitmap_zalloc(dev, LTC4283_GPIO_MAX, GFP_KERNEL);
> +	if (!gpio_mask)
> +		return -ENOMEM;
> +
> +	for (pin = 0; pin < n_pins; pin++) {
> +		if (pins[pin] >= LTC4283_GPIO_MAX)
> +			return dev_err_probe(dev, -EINVAL,
> +					     "Invalid GPIO pin specified (%u), max is %d\n",
> +					     pins[pin], LTC4283_GPIO_MAX);
> +
> +		__set_bit(pins[pin], gpio_mask);
> +	}
> +
> +	/* Add the GPIO cell */
> +	*n_cells += 1;

Just register it anyway and have the GPIO driver error out if it doesn't
have the right cells / properties.

> +	i2c_set_clientdata(client, gpio_mask);
> +
> +	return 0;
> +}
> +
> +static int ltc4283_probe(struct i2c_client *client)
> +{
> +	u32 n_cells = ARRAY_SIZE(ltc4283_cells) - 1;
> +	struct regmap *regmap;
> +	int ret;
> +
> +	regmap = devm_regmap_init_i2c(client, &ltc4283_regmap_config);
> +	if (IS_ERR(regmap))
> +		return PTR_ERR(regmap);
> +
> +	ret = ltc4283_get_gpio_pins(client, &n_cells);
> +	if (ret)
> +		return ret;
> +
> +	return devm_mfd_add_devices(&client->dev, PLATFORM_DEVID_AUTO,
> +				    ltc4283_cells, n_cells, NULL, 0, NULL);
> +}
> +
> +static const struct of_device_id ltc4283_of_match[] = {
> +	{ .compatible = "adi,ltc4283" },
> +	{ }
> +};
> +MODULE_DEVICE_TABLE(of, ltc4283_of_match);
> +
> +static const struct i2c_device_id ltc4283_i2c_id[] = {
> +	{ "ltc4283" },
> +	{ }
> +};
> +MODULE_DEVICE_TABLE(i2c, ltc4283_i2c_id);
> +
> +static struct i2c_driver ltc4283_driver = {
> +	.driver = {
> +		.name = "ltc4283",
> +		.of_match_table = ltc4283_of_match,
> +	},
> +	.probe = ltc4283_probe,
> +	.id_table = ltc4283_i2c_id,
> +};
> +module_i2c_driver(ltc4283_driver);
> +
> +MODULE_AUTHOR("Nuno Sá <nuno.sa@analog.com>");
> +MODULE_DESCRIPTION("LTC4283 MFD I2C driver");

It's not an MFD anything!

And the communication method is meaningless.

You can use "Core driver" instead.

> +MODULE_LICENSE("GPL");
> diff --git a/include/linux/mfd/ltc4283.h b/include/linux/mfd/ltc4283.h
> new file mode 100644
> index 0000000000000000000000000000000000000000..153594009c07b296ce5743e5e817e96464d81cb3
> --- /dev/null
> +++ b/include/linux/mfd/ltc4283.h
> @@ -0,0 +1,33 @@
> +/* SPDX-License-Identifier: GPL-2.0-only */
> +/*
> + * Analog Devices LTC4283 I2C Negative Voltage Hot Swap Controller
> + *
> + * Copyright 2025 Analog Devices Inc.
> + */
> +
> +#ifndef __MFD_LTC4283_H_
> +#define __MFD_LTC4283_H_
> +
> +#include <linux/bitops.h>
> +#include <linux/bits.h>
> +
> +/*
> + * We can have up to 8 gpios. 4 PGIOs and 4 ADIOs. PGIOs start at index 4 in the
> + * gpios mask.
> + */
> +#define LTC4283_PGIOX_START_NR	4
> +
> +#define LTC4283_PGIO_CONFIG		0x10
> +#define   LTC4283_PGIO_CFG_MASK(pin) \
> +	GENMASK(((pin) - LTC4283_PGIOX_START_NR) * 2 + 1, (((pin) - LTC4283_PGIOX_START_NR) * 2))

What do all of these numbers mean?  Can you define them?

> +#define LTC4283_PGIO_CONFIG_2		0x11
> +#define   LTC4283_ADC_MASK		GENMASK(2, 0)
> +#define   LTC4283_PGIO_OUT_MASK(pin)	BIT(4 + (pin))
> +
> +#define LTC4283_GPIO_MAX	8
> +
> +/* Non-constant mask variant of FIELD_GET() and FIELD_PREP() */

Why?

> +#define field_get(_mask, _reg)	(((_reg) & (_mask)) >> (ffs(_mask) - 1))
> +#define field_prep(_mask, _val)	(((_val) << (ffs(_mask) - 1)) & (_mask))
> +
> +#endif
> 
> -- 
> 2.50.1
> 
> 

-- 
Lee Jones [李琼斯]