mfd: Add simple regmap based I2C driver

There are I2C devices which contain several different functions but
doesn't require any special access functions. For these kind of drivers
an I2C regmap should be enough.

Create an I2C driver which creates an I2C regmap and enumerates its
children. If a device wants to use this as its MFD core driver, it has
to add an individual compatible string. It may provide its own regmap
configuration.

Subdevices can use dev_get_regmap() on the parent to get their regmap
instance.

Signed-off-by: Michael Walle <michael@walle.cc>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

drivers/mfd/Kconfig

@@ -1162,6 +1162,18 @@ config MFD_SI476X_CORE
 	  To compile this driver as a module, choose M here: the
 	  module will be called si476x-core.
 
+config MFD_SIMPLE_MFD_I2C
+	tristate
+	depends on I2C
+	select REGMAP_I2C
+	help
+	  This driver creates a single register map with the intention for it
+	  to be shared by all sub-devices.
+
+	  Once the register map has been successfully initialised, any
+	  sub-devices represented by child nodes in Device Tree will be
+	  subsequently registered.
+
 config MFD_SM501
 	tristate "Silicon Motion SM501"
 	depends on HAS_DMA

drivers/mfd/Makefile

@@ -264,3 +264,4 @@ obj-$(CONFIG_MFD_STMFX) 	+= stmfx.o
 obj-$(CONFIG_MFD_KHADAS_MCU) 	+= khadas-mcu.o
 
 obj-$(CONFIG_SGI_MFD_IOC3)	+= ioc3.o
+obj-$(CONFIG_MFD_SIMPLE_MFD_I2C)	+= simple-mfd-i2c.o

drivers/mfd/simple-mfd-i2c.c

@@ -0,0 +1,56 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Simple MFD - I2C
+ *
+ * This driver creates a single register map with the intention for it to be
+ * shared by all sub-devices.  Children can use their parent's device structure
+ * (dev.parent) in order to reference it.
+ *
+ * Once the register map has been successfully initialised, any sub-devices
+ * represented by child nodes in Device Tree will be subsequently registered.
+ */
+
+#include <linux/i2c.h>
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/of_platform.h>
+#include <linux/regmap.h>
+
+static const struct regmap_config simple_regmap_config = {
+	.reg_bits = 8,
+	.val_bits = 8,
+};
+
+static int simple_mfd_i2c_probe(struct i2c_client *i2c)
+{
+	const struct regmap_config *config;
+	struct regmap *regmap;
+
+	config = device_get_match_data(&i2c->dev);
+	if (!config)
+		config = &simple_regmap_config;
+
+	regmap = devm_regmap_init_i2c(i2c, config);
+	if (IS_ERR(regmap))
+		return PTR_ERR(regmap);
+
+	return devm_of_platform_populate(&i2c->dev);
+}
+
+static const struct of_device_id simple_mfd_i2c_of_match[] = {
+	{}
+};
+MODULE_DEVICE_TABLE(of, simple_mfd_i2c_of_match);
+
+static struct i2c_driver simple_mfd_i2c_driver = {
+	.probe_new = simple_mfd_i2c_probe,
+	.driver = {
+		.name = "simple-mfd-i2c",
+		.of_match_table = simple_mfd_i2c_of_match,
+	},
+};
+module_i2c_driver(simple_mfd_i2c_driver);
+
+MODULE_AUTHOR("Michael Walle <michael@walle.cc>");
+MODULE_DESCRIPTION("Simple MFD - I2C driver");
+MODULE_LICENSE("GPL v2");