gpio: Add virtio-gpio driver

This patch adds a new driver for Virtio based GPIO devices.

This allows a guest VM running Linux to access GPIO lines provided by
the host. It supports all basic operations, except interrupts for the
GPIO lines.

Based on the initial work posted by:
"Enrico Weigelt, metux IT consult" <lkml@metux.net>.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>

MAINTAINERS

@@ -19664,6 +19664,13 @@ F:	Documentation/filesystems/virtiofs.rst
 F:	fs/fuse/virtio_fs.c
 F:	include/uapi/linux/virtio_fs.h
 
+VIRTIO GPIO DRIVER
+M:	Enrico Weigelt, metux IT consult <info@metux.net>
+M:	Viresh Kumar <vireshk@kernel.org>
+S:	Maintained
+F:	drivers/gpio/gpio-virtio.c
+F:	include/uapi/linux/virtio_gpio.h
+
 VIRTIO GPU DRIVER
 M:	David Airlie <airlied@linux.ie>
 M:	Gerd Hoffmann <kraxel@redhat.com>

drivers/gpio/Kconfig

@@ -1669,6 +1669,15 @@ config GPIO_MOCKUP
 	  tools/testing/selftests/gpio/gpio-mockup.sh. Reference the usage in
 	  it.
 
+config GPIO_VIRTIO
+	tristate "VirtIO GPIO support"
+	depends on VIRTIO
+	help
+	  Say Y here to enable guest support for virtio-based GPIO controllers.
+
+	  These virtual GPIOs can be routed to real GPIOs or attached to
+	  simulators on the host (like QEMU).
+
 endmenu
 
 endif

drivers/gpio/Makefile

@@ -165,6 +165,7 @@ obj-$(CONFIG_GPIO_UCB1400)		+= gpio-ucb1400.o
 obj-$(CONFIG_GPIO_UNIPHIER)		+= gpio-uniphier.o
 obj-$(CONFIG_GPIO_VF610)		+= gpio-vf610.o
 obj-$(CONFIG_GPIO_VIPERBOARD)		+= gpio-viperboard.o
+obj-$(CONFIG_GPIO_VIRTIO)		+= gpio-virtio.o
 obj-$(CONFIG_GPIO_VISCONTI)		+= gpio-visconti.o
 obj-$(CONFIG_GPIO_VR41XX)		+= gpio-vr41xx.o
 obj-$(CONFIG_GPIO_VX855)		+= gpio-vx855.o