147 lines
5.9 KiB
Plaintext
147 lines
5.9 KiB
Plaintext
GPIO Sysfs Interface for Userspace
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==================================
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Platforms which use the "gpiolib" implementors framework may choose to
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configure a sysfs user interface to GPIOs. This is different from the
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debugfs interface, since it provides control over GPIO direction and
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value instead of just showing a gpio state summary. Plus, it could be
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present on production systems without debugging support.
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Given appropriate hardware documentation for the system, userspace could
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know for example that GPIO #23 controls the write protect line used to
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protect boot loader segments in flash memory. System upgrade procedures
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may need to temporarily remove that protection, first importing a GPIO,
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then changing its output state, then updating the code before re-enabling
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the write protection. In normal use, GPIO #23 would never be touched,
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and the kernel would have no need to know about it.
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Again depending on appropriate hardware documentation, on some systems
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userspace GPIO can be used to determine system configuration data that
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standard kernels won't know about. And for some tasks, simple userspace
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GPIO drivers could be all that the system really needs.
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DO NOT ABUSE SYSFS TO CONTROL HARDWARE THAT HAS PROPER KERNEL DRIVERS.
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PLEASE READ THE DOCUMENT NAMED "drivers-on-gpio.txt" IN THIS DOCUMENTATION
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DIRECTORY TO AVOID REINVENTING KERNEL WHEELS IN USERSPACE. I MEAN IT.
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REALLY.
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Paths in Sysfs
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--------------
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There are three kinds of entries in /sys/class/gpio:
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- Control interfaces used to get userspace control over GPIOs;
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- GPIOs themselves; and
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- GPIO controllers ("gpio_chip" instances).
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That's in addition to standard files including the "device" symlink.
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The control interfaces are write-only:
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/sys/class/gpio/
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"export" ... Userspace may ask the kernel to export control of
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a GPIO to userspace by writing its number to this file.
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Example: "echo 19 > export" will create a "gpio19" node
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for GPIO #19, if that's not requested by kernel code.
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"unexport" ... Reverses the effect of exporting to userspace.
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Example: "echo 19 > unexport" will remove a "gpio19"
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node exported using the "export" file.
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GPIO signals have paths like /sys/class/gpio/gpio42/ (for GPIO #42)
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and have the following read/write attributes:
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/sys/class/gpio/gpioN/
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"direction" ... reads as either "in" or "out". This value may
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normally be written. Writing as "out" defaults to
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initializing the value as low. To ensure glitch free
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operation, values "low" and "high" may be written to
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configure the GPIO as an output with that initial value.
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Note that this attribute *will not exist* if the kernel
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doesn't support changing the direction of a GPIO, or
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it was exported by kernel code that didn't explicitly
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allow userspace to reconfigure this GPIO's direction.
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"value" ... reads as either 0 (low) or 1 (high). If the GPIO
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is configured as an output, this value may be written;
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any nonzero value is treated as high.
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If the pin can be configured as interrupt-generating interrupt
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and if it has been configured to generate interrupts (see the
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description of "edge"), you can poll(2) on that file and
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poll(2) will return whenever the interrupt was triggered. If
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you use poll(2), set the events POLLPRI and POLLERR. If you
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use select(2), set the file descriptor in exceptfds. After
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poll(2) returns, either lseek(2) to the beginning of the sysfs
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file and read the new value or close the file and re-open it
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to read the value.
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"edge" ... reads as either "none", "rising", "falling", or
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"both". Write these strings to select the signal edge(s)
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that will make poll(2) on the "value" file return.
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This file exists only if the pin can be configured as an
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interrupt generating input pin.
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"active_low" ... reads as either 0 (false) or 1 (true). Write
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any nonzero value to invert the value attribute both
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for reading and writing. Existing and subsequent
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poll(2) support configuration via the edge attribute
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for "rising" and "falling" edges will follow this
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setting.
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GPIO controllers have paths like /sys/class/gpio/gpiochip42/ (for the
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controller implementing GPIOs starting at #42) and have the following
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read-only attributes:
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/sys/class/gpio/gpiochipN/
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"base" ... same as N, the first GPIO managed by this chip
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"label" ... provided for diagnostics (not always unique)
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"ngpio" ... how many GPIOs this manages (N to N + ngpio - 1)
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Board documentation should in most cases cover what GPIOs are used for
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what purposes. However, those numbers are not always stable; GPIOs on
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a daughtercard might be different depending on the base board being used,
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or other cards in the stack. In such cases, you may need to use the
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gpiochip nodes (possibly in conjunction with schematics) to determine
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the correct GPIO number to use for a given signal.
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Exporting from Kernel code
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--------------------------
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Kernel code can explicitly manage exports of GPIOs which have already been
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requested using gpio_request():
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/* export the GPIO to userspace */
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int gpiod_export(struct gpio_desc *desc, bool direction_may_change);
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/* reverse gpio_export() */
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void gpiod_unexport(struct gpio_desc *desc);
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/* create a sysfs link to an exported GPIO node */
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int gpiod_export_link(struct device *dev, const char *name,
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struct gpio_desc *desc);
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After a kernel driver requests a GPIO, it may only be made available in
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the sysfs interface by gpiod_export(). The driver can control whether the
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signal direction may change. This helps drivers prevent userspace code
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from accidentally clobbering important system state.
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This explicit exporting can help with debugging (by making some kinds
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of experiments easier), or can provide an always-there interface that's
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suitable for documenting as part of a board support package.
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After the GPIO has been exported, gpiod_export_link() allows creating
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symlinks from elsewhere in sysfs to the GPIO sysfs node. Drivers can
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use this to provide the interface under their own device in sysfs with
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a descriptive name.
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