Most often a gpio_chip is part of an instance-specific structure with state
not exposed by the GPIO interfaces, such as addressing, power management,
-and more. Chips such as codecs will have complex non-GPIO state,
+and more. Chips such as codecs will have complex non-GPIO state.
Any debugfs dump method should normally ignore signals which haven't been
requested as GPIOs. They can use gpiochip_is_requested(), which returns
value instead of just showing a gpio state summary. Plus, it could be
present on production systems without debugging support.
-Given approprate hardware documentation for the system, userspace could
+Given appropriate hardware documentation for the system, userspace could
know for example that GPIO #23 controls the write protect line used to
protect boot loader segments in flash memory. System upgrade procedures
may need to temporarily remove that protection, first importing a GPIO,
is configured as an output, this value may be written;
any nonzero value is treated as high.
-GPIO controllers have paths like /sys/class/gpio/chipchip42/ (for the
+ "edge" ... reads as either "none", "rising", "falling", or
+ "both". Write these strings to select the signal edge(s)
+ that will make poll(2) on the "value" file return.
+
+ This file exists only if the pin can be configured as an
+ interrupt generating input pin.
+
+ "active_low" ... reads as either 0 (false) or 1 (true). Write
+ any nonzero value to invert the value attribute both
+ for reading and writing. Existing and subsequent
+ poll(2) support configuration via the edge attribute
+ for "rising" and "falling" edges will follow this
+ setting.
+
+GPIO controllers have paths like /sys/class/gpio/gpiochip42/ (for the
controller implementing GPIOs starting at #42) and have the following
read-only attributes:
/* reverse gpio_export() */
void gpio_unexport();
+ /* create a sysfs link to an exported GPIO node */
+ int gpio_export_link(struct device *dev, const char *name,
+ unsigned gpio)
+
+ /* change the polarity of a GPIO node in sysfs */
+ int gpio_sysfs_set_active_low(unsigned gpio, int value);
+
After a kernel driver requests a GPIO, it may only be made available in
the sysfs interface by gpio_export(). The driver can control whether the
signal direction may change. This helps drivers prevent userspace code
This explicit exporting can help with debugging (by making some kinds
of experiments easier), or can provide an always-there interface that's
suitable for documenting as part of a board support package.
+
+After the GPIO has been exported, gpio_export_link() allows creating
+symlinks from elsewhere in sysfs to the GPIO sysfs node. Drivers can
+use this to provide the interface under their own device in sysfs with
+a descriptive name.
+
+Drivers can use gpio_sysfs_set_active_low() to hide GPIO line polarity
+differences between boards from user space. This only affects the
+sysfs interface. Polarity change can be done both before and after
+gpio_export(), and previously enabled poll(2) support for either
+rising or falling edge will be reconfigured to follow this setting.