linux-kernel - [PATCH v2 2/2] gpio: create the /sys/class/gpio mount point with GPIO

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Message-Id: <20241015-gpio-class-mountpoint-v2-2-7709301876ef@linaro.org>
Date: Tue, 15 Oct 2024 10:00:24 +0200
From: Bartosz Golaszewski <brgl@...ev.pl>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
 "Rafael J. Wysocki" <rafael@...nel.org>, 
 Linus Walleij <linus.walleij@...aro.org>, 
 Bartosz Golaszewski <brgl@...ev.pl>, Kent Gibson <warthog618@...il.com>
Cc: linux-kernel@...r.kernel.org, linux-gpio@...r.kernel.org, 
 Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
Subject: [PATCH v2 2/2] gpio: create the /sys/class/gpio mount point with
 GPIO_SYSFS disabled

From: Bartosz Golaszewski <bartosz.golaszewski@...aro.org>

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 always-empty directory where the GPIO class interface
would exist and enable this option by default if GPIO_SYSFS is not
selected.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
---
 drivers/gpio/Kconfig   | 18 ++++++++++++++++++
 drivers/gpio/gpiolib.c |  6 ++++++
 2 files changed, 24 insertions(+)

diff --git a/drivers/gpio/Kconfig b/drivers/gpio/Kconfig
index efddc6455315..1a3535bda779 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_MOUNT_POINT
+	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..1c8bd765d8e1 100644
--- a/drivers/gpio/gpiolib.c
+++ b/drivers/gpio/gpiolib.c
@@ -4899,6 +4899,12 @@ static int __init gpiolib_dev_init(void)
 		return ret;
 	}

+#if IS_ENABLED(CONFIG_GPIO_SYSFS_CLASS_MOUNT_POINT)
+	ret = sysfs_create_mount_point(class_kobj, "gpio");
+	if (ret)
+		pr_err("gpiolib: failed to create the GPIO class mountpoint\n");
+#endif /* CONFIG_GPIO_SYSFS_CLASS_MOUNT_POINT */
+
 	gpiolib_initialized = true;
 	gpiochip_setup_devs();

-- 
2.43.0