gpio Sysfs Interface for Userspace
来源:互联网 发布:果园系统源码 编辑:程序博客网 时间:2024/05/18 18:55
from gpio.txt
Sysfs Interface for Userspace (OPTIONAL)
========================================
Platforms which use the "gpiolib" implementors framework may choose to configure a sysfs user interface to GPIOs. This is different from the debugfs interface, since it provides control over GPIO direction and value instead of just showing a gpio state summary. Plus, it could be present on production systems without debugging support.
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, then changing its output state, then updating the code before re-enabling the write protection. In normal use, GPIO #23 would never be touched, and the kernel would have no need to know about it.
Again depending on appropriate hardware documentation, on some systems userspace GPIO can be used to determine system configuration data that standard kernels won't know about. And for some tasks, simple userspace GPIO drivers could be all that the system really needs.
Paths in Sysfs
-------------- There are three kinds of entry in /sys/class/gpio:
- Control interfaces used to get userspace control over GPIOs;
- GPIOs themselves; and
- GPIO controllers ("gpio_chip" instances).
That's in addition to standard files including the "device" symlink.
The control interfaces are write-only:
/sys/class/gpio/
"export" ... Userspace may ask the kernel to export control of a GPIO to userspace by writing its number to this file.
Example: "echo 19 > export" will create a "gpio19" node
for GPIO #19, if that's not requested by kernel code.
"unexport" ... Reverses the effect of exporting to userspace.
Example: "echo 19 > unexport" will remove a "gpio19"
node exported using the "export" file.
GPIO signals have paths like /sys/class/gpio/gpio42/ (for GPIO #42) and have the following read/write attributes:
/sys/class/gpio/gpioN/
"direction" ... reads as either "in" or "out". This value may normally be written. Writing as "out" defaults to
initializing the value as low. To ensure glitch free
operation, values "low" and "high" may be written to
configure the GPIO as an output with that initial value.
Note that this attribute *will not exist* if the kernel
doesn't support changing the direction of a GPIO, or
it was exported by kernel code that didn't explicitly
allow userspace to reconfigure this GPIO's direction.
"value" ... reads as either 0 (low) or 1 (high). If the GPIO is configured as an output, this value may be written;
any nonzero value is treated as high.
If the pin can be configured as interrupt-generating interrupt
and if it has been configured to generate interrupts (see the description of "edge"), you can poll(2) on that file and
poll(2) will return whenever the interrupt was triggered. If
you use poll(2), set the events POLLPRI and POLLERR. If you
use select(2), set the file descriptor in exceptfds. After
poll(2) returns, either lseek(2) to the beginning of the sysfs
file and read the new value or close the file and re-open it
to read the value.
"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:
/sys/class/gpio/gpiochipN/
"base" ... same as N, the first GPIO managed by this chip
"label" ... provided for diagnostics (not always unique)
"ngpio" ... how many GPIOs this manges (N to N + ngpio - 1)
Board documentation should in most cases cover what GPIOs are used for what purposes. However, those numbers are not always stable; GPIOs on a daughtercard might be different depending on the base board being used, or other cards in the stack. In such cases, you may need to use the gpiochip nodes (possibly in conjunction with schematics) to determine the correct GPIO number to use for a given signal.
- gpio Sysfs Interface for Userspace
- GPIO Sysfs Interface for Userspace
- gpio: sysfs interface
- gpio: sysfs interface
- PCIe userspace tools: lspci, setpci and sysfs
- sysfs方式控制GPIO
- 通过sysfs文件系统操作gpio
- 通过sysfs方式控制GPIO
- linux dts gpio sysfs example
- linux gpio 控制 基于 sysfs
- Userspace tools for MMC/SD devices
- 通过sysfs在用户空间使用GPIO
- 使用文件IO操作GPIO SysFs方式
- 通过sysfs在用户空间使用GPIO
- [Linux应用]通过sysfs在用户空间使用GPIO
- Kernel中GPIO的Sysfs接口的使用
- 树莓派使用文件IO操作GPIO SysFs方式
- [Linux应用]通过sysfs在用户空间使用GPIO
- liferay的theme中VM可以使用的变量一览
- lazyloading for pictures
- 28msec - query data from any source in real time
- 智力题 - 士兵编队与传讯员
- cmake使用方法详解
- gpio Sysfs Interface for Userspace
- 禁止在手机浏览器中缩放
- UVa 624 - CD
- Servlet之过滤器总结--Filter
- Delphi 中 函数参数中的 const 修饰符的本质以及注意事项
- MFC中如何使用_beginthreadex
- 颜色值的问题
- Java SSL/TLS 安全通讯协议介绍
- 一个Demo学完Android中所有的服务