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 <redacted>
---
Changes since v7:
 - none

Changes since v6:
 - added SIMPLE_MFD_I2C help text
 - revised description in module header

Changes since v5:
 - removed "select MFD_CORE" in Kconfig
 - removed help text in Kconfig, we assume that the users of this
   driver will have a "select MFD_SIMPLE_MFD_I2C". Instead added
   a small description to the driver itself.
 - removed "struct simple_mfd_i2c_config" and use regmap_config
   directly
 - changed builtin_i2c_driver() to module_i2c_driver(), added
   MODULE_ boilerplate
 - cleaned up the included files

Changes since v4:
 - new patch. Lee, please bear with me. I didn't want to delay the
   new version (where a lot of remarks on the other patches were
   addressed) even more, just because we haven't figured out how
   to deal with the MFD part. So for now, I've included this one.

 drivers/mfd/Kconfig          | 12 ++++++++
 drivers/mfd/Makefile         |  1 +
 drivers/mfd/simple-mfd-i2c.c | 56 ++++++++++++++++++++++++++++++++++++
 3 files changed, 69 insertions(+)
 create mode 100644 drivers/mfd/simple-mfd-i2c.c

diff --git a/drivers/mfd/Kconfig b/drivers/mfd/Kconfig
index 33df0837ab41..6e1a38944d28 100644
--- a/drivers/mfd/Kconfig
+++ b/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

diff --git a/drivers/mfd/Makefile b/drivers/mfd/Makefile
index a60e5f835283..78d24a3e7c9e 100644
--- a/drivers/mfd/Makefile
+++ b/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

diff --git a/drivers/mfd/simple-mfd-i2c.c b/drivers/mfd/simple-mfd-i2c.c
new file mode 100644
index 000000000000..09b40ebfdf07
--- /dev/null
+++ b/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 reference it.