linux gpio learning

antecedent

Based on linux gpio, files will be generated in the /sys/class/gpiodirectory export, unexport, and of course there are gpiochipxfiles , which gpiochipxare the management of pins. For example, a certain chip may control a certain number of pins, and the number of pins ngpiois .
When executing echo 4 > /sys/class/gpio/export, a directory will /sys/class/gpiobe generated under the gpio4directory , in which there will be value, edge, directionrelated files such as , value is the current value, edge is the pin trigger mode, and direction is the pin input and output mode. But how did this come about echoafter that.

Process scan

• In the initial initialization process there gpiolib_dev_initare exported as core_initcall(gpio/gpiolib.c)
• This function calls bus_registerregister gpio , corresponding to the generated /sys/bus/gpiodirectory
• call againalloc_chrdev_region
• Then set the gpiochipgpiochip_setup_devs
• This processes the total number of gpiochips, and for each call gpiochip_sysfs_register, the file goes gpiolib-sysfs.cto
• call device_create_wtih_groupsto creategpiochip%d
• The parameter corresponding to this function gpio_classis a classclass
• The name of this classdefinition is, and attrgpio is also defined
• attr includes export - export _store,unexport - unexport_store
• The former is the property name, the latter is the function
• That is, the echoaction is a trigger export_storefunction
• This function call gpio_requestprepares pin
• call again gpiod_exportin this function device_create_with_groupsto create gpio%d, among other things

Remark

• Of course, there are still many details that are still unclear, just an overview, which will be added later