[RFC PATCH] gpiolib: Provide and export gpiod_export_name

[Guenter Roeck <linux@xxxxxxxxxxxx>]

gpiod_export_name is similar to gpiod_export, but lets the user
determine the name used to export a gpio pin.

Currently, the pin name is determined by the chip driver with
the 'names' array in the gpio_chip data structure, or it is set
to gpioX, where X is the pin number, if no name is provided by
the chip driver.

It is, however, desirable to be able to provide the pin name when
exporting the pin, for example from platform code. In other words,
it would be useful to move the naming decision from the pin provider
to the pin consumer. The gpio-pca953x driver provides this capability
as part of its platform data. Other drivers could be enhanced in a
similar way; however, this is not always possible or easy to accomplish.
For example, mfd client drivers such as gpio-ich already use platform
data to pass information from the mfd master driver to the client driver.
Overloading this platform data to also provide an array of gpio pin names
would be a challenge if not impossible.

The alternative to use gpiod_export_link is also not always desirable,
since it only creates a named link to a different directory, meaning
the named gpio pin is not available in /sys/class/gpio but only
in some platform specific directory and thus not as generic as possible
and/or useful.

A specific example for a use case is a gpio pin which reports AC power
loss to user space. Depending on the platform and platform variant,
the pin can be provided by various gpio chip drivers and pin numbers.
It would be very desirable to have a well defined location such as
/sys/class/gpio/ac_power_loss for this pin, so user space knows where
to find the attribute without knowledge of the underlying platform
variant or oher hardware details.

Signed-off-by: Guenter Roeck <linux@xxxxxxxxxxxx>
---
Applies to tip of linux-gpio/for-next.

 Documentation/gpio/sysfs.txt  | 12 ++++++++----
 drivers/gpio/gpiolib-sysfs.c  | 23 ++++++++++++++++-------
 include/linux/gpio/consumer.h |  9 +++++++++
 3 files changed, 33 insertions(+), 11 deletions(-)

diff --git a/Documentation/gpio/sysfs.txt b/Documentation/gpio/sysfs.txt
index c2c3a97..8e301b2 100644
--- a/Documentation/gpio/sysfs.txt
+++ b/Documentation/gpio/sysfs.txt
@@ -125,7 +125,11 @@ requested using gpio_request():
 	/* export the GPIO to userspace */
 	int gpiod_export(struct gpio_desc *desc, bool direction_may_change);
 
-	/* reverse gpio_export() */
+	/* export named GPIO to userspace */
+	int gpiod_export_name(struct gpio_desc *desc, const char *ioname,
+			bool direction_may_change);
+
+	/* reverse gpio_export() / gpiod_export_name() */
 	void gpiod_unexport(struct gpio_desc *desc);
 
 	/* create a sysfs link to an exported GPIO node */
@@ -136,9 +140,9 @@ requested using gpio_request():
 	int gpiod_sysfs_set_active_low(struct gpio_desc *desc, int value);
 
 After a kernel driver requests a GPIO, it may only be made available in
-the sysfs interface by gpiod_export(). The driver can control whether the
-signal direction may change. This helps drivers prevent userspace code
-from accidentally clobbering important system state.
+the sysfs interface by gpiod_export() or gpiod_export_name(). The driver
+can control whether the signal direction may change. This helps drivers
+prevent userspace code from accidentally clobbering important system state.
 
 This explicit exporting can help with debugging (by making some kinds
 of experiments easier), or can provide an always-there interface that's
diff --git a/drivers/gpio/gpiolib-sysfs.c b/drivers/gpio/gpiolib-sysfs.c
index be45a92..7d36230 100644
--- a/drivers/gpio/gpiolib-sysfs.c
+++ b/drivers/gpio/gpiolib-sysfs.c
@@ -523,13 +523,12 @@ static struct class gpio_class = {
  *
  * Returns zero on success, else an error.
  */
-int gpiod_export(struct gpio_desc *desc, bool direction_may_change)
+int gpiod_export_name(struct gpio_desc *desc, const char *ioname,
+		      bool direction_may_change)
 {
 	unsigned long		flags;
 	int			status;
-	const char		*ioname = NULL;
 	struct device		*dev;
-	int			offset;
 
 	/* can't export until sysfs is available ... */
 	if (!gpio_class.p) {
@@ -560,10 +559,6 @@ int gpiod_export(struct gpio_desc *desc, bool direction_may_change)
 		direction_may_change = false;
 	spin_unlock_irqrestore(&gpio_lock, flags);
 
-	offset = gpio_chip_hwgpio(desc);
-	if (desc->chip->names && desc->chip->names[offset])
-		ioname = desc->chip->names[offset];
-
 	dev = device_create(&gpio_class, desc->chip->dev, MKDEV(0, 0),
 			    desc, ioname ? ioname : "gpio%u",
 			    desc_to_gpio(desc));
@@ -600,6 +595,20 @@ fail_unlock:
 	gpiod_dbg(desc, "%s: status %d\n", __func__, status);
 	return status;
 }
+EXPORT_SYMBOL_GPL(gpiod_export_name);
+
+int gpiod_export(struct gpio_desc *desc, bool direction_may_change)
+{
+	const char *ioname = NULL;
+
+	if (desc) {
+		int offset = gpio_chip_hwgpio(desc);
+
+		if (desc->chip->names && desc->chip->names[offset])
+			ioname = desc->chip->names[offset];
+	}
+	return gpiod_export_name(desc, ioname, direction_may_change);
+}
 EXPORT_SYMBOL_GPL(gpiod_export);
 
 static int match_export(struct device *dev, const void *data)
diff --git a/include/linux/gpio/consumer.h b/include/linux/gpio/consumer.h
index 05e53cc..986da3e 100644
--- a/include/linux/gpio/consumer.h
+++ b/include/linux/gpio/consumer.h
@@ -260,6 +260,8 @@ static inline int desc_to_gpio(const struct gpio_desc *desc)
 #if IS_ENABLED(CONFIG_GPIOLIB) && IS_ENABLED(CONFIG_GPIO_SYSFS)
 
 int gpiod_export(struct gpio_desc *desc, bool direction_may_change);
+int gpiod_export_name(struct gpio_desc *desc, const char *ioname,
+		      bool direction_may_change);
 int gpiod_export_link(struct device *dev, const char *name,
 		      struct gpio_desc *desc);
 int gpiod_sysfs_set_active_low(struct gpio_desc *desc, int value);
@@ -273,6 +275,13 @@ static inline int gpiod_export(struct gpio_desc *desc,
 	return -ENOSYS;
 }
 
+static inline int gpiod_export_name(struct gpio_desc *desc,
+				    const char *ioname,
+				    bool direction_may_change)
+{
+	return -ENOSYS;
+}
+
 static inline int gpiod_export_link(struct device *dev, const char *name,
 				    struct gpio_desc *desc)
 {
-- 
1.9.1

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