[PATCH] gpio: create empty /sys/class/gpio with SYSFS disabled

[Bartosz Golaszewski <brgl@xxxxxxxx>]

From: Bartosz Golaszewski <bartosz.golaszewski@xxxxxxxxxx>

User-space may want to use some kind of a compatibility layer for the
deprecated GPIO sysfs ABI. This would typically involve mounting
a fuse-based filesystem using the GPIO character device to emulate the
sysfs behavior and layout.

With GPIO_SYSFS disabled, the /sys/class/gpio directory doesn't exist
and user-space cannot create it. In order to facilitate moving away from
the sysfs, add a new Kconfig option that indicates to GPIOLIB that is
should create an empty directory where the GPIO class interface would
exist and enable it by default.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@xxxxxxxxxx>
---
As I've mentioned under a different patch, I'm working on a user-space
compatibility layer for the sysfs GPIO interface. FUSE allows us to
emulate almost all its functionalities using libgpiod except for
mounting it under /sys/class/gpio if GPIO_SYSFS is disabled. User-space
cannot create directories in sysfs so if we want to allow users to mount
the FUSE emulator under the old location, we need to create an empty
/sys/class/gpio directory from the kernel and this is what is patch is
for.

 drivers/gpio/Kconfig   | 18 ++++++++++++++++++
 drivers/gpio/gpiolib.c | 12 ++++++++++++
 2 files changed, 30 insertions(+)

diff --git a/drivers/gpio/Kconfig b/drivers/gpio/Kconfig
index bfa6b5a2c537..f2e7163cb46c 100644
--- a/drivers/gpio/Kconfig
+++ b/drivers/gpio/Kconfig
@@ -69,6 +69,24 @@ config GPIO_SYSFS
 	  use the character device /dev/gpiochipN with the appropriate
 	  ioctl() operations instead.
 
+config GPIO_SYSFS_CLASS_DIR_STUB
+	bool "Create empty /sys/class/gpio directory" if EXPERT
+	depends on !GPIO_SYSFS
+	default y
+	help
+	  Say Y here to create an empty /sys/class/gpio directory.
+
+	  User-space may want to use some kind of a compatibility layer for the
+	  deprecated GPIO sysfs ABI. This would typically involve mounting
+	  a fuse-based filesystem using the GPIO character device to emulate
+	  the sysfs behavior and layout.
+
+	  This option makes GPIOLIB create an empty directory at /sys/class/gpio
+	  where user-space can mount the sysfs replacement and avoid having to
+	  change existing programs to adjust to different filesystem paths.
+
+	  If unsure, say Y.
+
 config GPIO_CDEV
 	bool
 	prompt "Character device (/dev/gpiochipN) support" if EXPERT
diff --git a/drivers/gpio/gpiolib.c b/drivers/gpio/gpiolib.c
index 97346b746ef5..31efb580beb8 100644
--- a/drivers/gpio/gpiolib.c
+++ b/drivers/gpio/gpiolib.c
@@ -4873,6 +4873,12 @@ static struct device_driver gpio_stub_drv = {
 	.probe = gpio_stub_drv_probe,
 };
 
+#if IS_ENABLED(CONFIG_GPIO_SYSFS_CLASS_DIR_STUB)
+static const struct class gpio_class_stub = {
+	.name = "gpio",
+};
+#endif /* CONFIG_GPIO_SYSFS_CLASS_DIR_STUB */
+
 static int __init gpiolib_dev_init(void)
 {
 	int ret;
@@ -4899,6 +4905,12 @@ static int __init gpiolib_dev_init(void)
 		return ret;
 	}
 
+#if IS_ENABLED(CONFIG_GPIO_SYSFS_CLASS_DIR_STUB)
+	ret = class_register(&gpio_class_stub);
+	if (ret)
+		pr_err("gpiolib: failed to create the empty gpio class directory\n");
+#endif /* CONFIG_GPIO_SYSFS_CLASS_DIR_STUB */
+
 	gpiolib_initialized = true;
 	gpiochip_setup_devs();
 
-- 
2.43.0