gpio: Add GPIO Aggregator

GPIO controllers are exported to userspace using /dev/gpiochip*
character devices.  Access control to these devices is provided by
standard UNIX file system permissions, on an all-or-nothing basis:
either a GPIO controller is accessible for a user, or it is not.
Currently no mechanism exists to control access to individual GPIOs.

Hence add a GPIO driver to aggregate existing GPIOs, and expose them as
a new gpiochip.

This supports the following use cases:
  - Aggregating GPIOs using Sysfs
    This is useful for implementing access control, and assigning a set
    of GPIOs to a specific user or virtual machine.
  - Generic GPIO Driver
    This is useful for industrial control, where it can provide
    userspace access to a simple GPIO-operated device described in DT,
    cfr. e.g. spidev for SPI-operated devices.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Tested-by: Eugeniu Rosca <erosca@de.adit-jv.com>
Reviewed-by: Eugeniu Rosca <erosca@de.adit-jv.com>
Link: https://lore.kernel.org/r/20200511145257.22970-5-geert+renesas@glider.be
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/gpio/Kconfig

@@ -1541,6 +1541,18 @@ config GPIO_VIPERBOARD
 
 endmenu
 
+config GPIO_AGGREGATOR
+	tristate "GPIO Aggregator"
+	help
+	  Say yes here to enable the GPIO Aggregator, which provides a way to
+	  aggregate existing GPIO lines into a new virtual GPIO chip.
+	  This can serve the following purposes:
+	    - Assign permissions for a collection of GPIO lines to a user,
+	    - Export a collection of GPIO lines to a virtual machine,
+	    - Provide a generic driver for a GPIO-operated device in an
+	      industrial control context, to be operated from userspace using
+	      the GPIO chardev interface.
+
 config GPIO_MOCKUP
 	tristate "GPIO Testing Driver"
 	select IRQ_SIM

drivers/gpio/Makefile

@@ -25,6 +25,7 @@ obj-$(CONFIG_GPIO_74XX_MMIO)		+= gpio-74xx-mmio.o
 obj-$(CONFIG_GPIO_ADNP)			+= gpio-adnp.o
 obj-$(CONFIG_GPIO_ADP5520)		+= gpio-adp5520.o
 obj-$(CONFIG_GPIO_ADP5588)		+= gpio-adp5588.o
+obj-$(CONFIG_GPIO_AGGREGATOR)		+= gpio-aggregator.o
 obj-$(CONFIG_GPIO_ALTERA_A10SR)		+= gpio-altera-a10sr.o
 obj-$(CONFIG_GPIO_ALTERA)  		+= gpio-altera.o
 obj-$(CONFIG_GPIO_AMD8111)		+= gpio-amd8111.o