Why there is `gpio_request` instead of `request_region` in raspberry pi driver?

Question

In the book LDD3, if one driver want to control the pins of CPU, it should call request_region() function to declare the usage of the ports.

When I want to implement a simple driver module on my Raspberry Pi, however, that I found in this example the request of ports is implemented by gpio_request() function.

Why and when we need to use gpio_request() instead of request_region()? And, what's the difference purposes for these two functions.

BTW: I searched the LDD3 page by page but I can't find any clues about the GPIO... why there is no any introductions to GPIO? Is it because of the 2.6 kernel version?

Answer 1

In the book LDD3, if one driver want to control the pins of CPU, it should call request_region() function to declare the usage of the ports.

First, the word "port" is ambiguous and requires context. Port can refer to a physical connector (e.g. USB port), or a logical connection (e.g. TCP port).

Your understanding of request_region() is flawed. That routine is for management of I/O address space. Your question is tagged with raspberry-p1 which uses an ARM processor and has no I/O address space to manage. ARM processors use memory-mapped device registers. You would use request_mem_region() in a device driver for the memory addresses of that peripheral's register block.

Each GPIO is controlled by a bit position in one or more control registers. Those registers would be handled by an overall GPIO subsystem. (There's also a lower-layer (closer to the HW) pin-control driver for multiplexed pins, i.e. pins that can be assigned to a peripheral device or used as GPIO.)

The driver for the GPIO (or pin-control) subsystem should perform a request_mem_region() for the memory addresses of the SoC's GPIO control registers. A gpio_request() would be management of an individual pin that is subordinate to management of the registers.

Note that use of request_mem_region() and gpio_request() are not mutually exclusive in a device driver. For instance the driver for a USB controller would request_mem_region() the memory addresses for its control registers. It may also have to gpio_request() for pin(s) that control the power to the USB connector(s) (assuming that's how the power is controlled with logic external to the controller).

why there is no any introductions to GPIO? Is it because of the 2.6 kernel version?

Conventions for using GPIO in Linux appeared in Documentation/gpio.h in 2007 with version 2.6.22. Generic (i.e. standardized rather than platform specific) GPIO support appeared in the Linux kernel several years later with version 2.6.3x(?). Prior to that (and even after) each platform (e.g. SoC manufacturer) had its own set of routines for accessing (and maybe managing) GPIOs.

LDD3 claims to be current as of the 2.6.10 kernel. Also that book may be x86-centric (as Linux has x86 origins), and x86 processors typically do not have GPIOs.