Kernel module development notes

Motivation

Why develop a custom device driver or a kernel mode service?

Manage exclusive hardware access
Reduced event latency - access low-level resources like interrupts
Device accessibility can be controled by file permissions
Access data structures that are not exported out of kernel space
Higher execution privilege, +impact, +security

in short: if you need to support a new device or implement low-latency low-level functionality composed of multiple device drivers

How to get started?

don’t start from scratch
when writing new device drivers - look up examples on how other drivers use peripheral bus framework (spi, i2c, usb, eg. drivers/net/can/spi/mcp251x.c)
when writing combined drivers - start modifying an existing one (eg. drivers/iio/adc/mxs-lradc-adc.c) and add new functionality
misc drivers - create a new misc driver (my-misc-driver.c)

Access hardware from userspace

Userspace application
C Library
System Call Interface (open, read, write, ioctl)
Framework (block, character, subsystems: network, iio)
Driver (mcp25xx) - this is what we program
Bus infrastructure (spi, usb)
Hardware

/dev entry: misc character device, read/write + ioctl typically used by applications to access devices

/sys entry: read/write for simple shell scripts typically used by applications to manage devices

eg. use /dev to access a storage device /sys to tune

Makefile

two build stages
needs kernel built from source or linux-headers package installed
compiler flags with ccflags
out-of-tree build, separete output directory

ifneq ($(KERNELRELEASE),)

my-module-objs := my-plat-drv.o my-other-source.o
obj-m := my_module.o
ccflags-y +=-DDEBUG -DCONFIG_PWM -DCONFIG_HAVE_PWM

else

# Full kernel source
KDIR := $(HOME)/linux
BUILD_DIR ?= $(PWD)/build
BUILD_DIR_MAKEFILE ?= $(PWD)/build/Makefile

# linux-header-* packages, created by make header_install
#KDIR := /lib/modules/$(shell uname -r)/build

default: $(BUILD_DIR_MAKEFILE)
    $(MAKE) -C $(KDIR) M=$(BUILD_DIR) src=$(PWD) modules

$(BUILD_DIR):
    mkdir -p "$@"

$(BUILD_DIR_MAKEFILE): $(BUILD_DIR)
    touch "$@"

all:
    $(MAKE) -C $(KDIR) M=$$PWD

clean:
    $(MAKE) -C $(KDIR) M=$(BUILD_DIR) src=$(PWD) clean
endif

Device tree

documentation
what is device tree? - data structure and language for describing hardware
platform drivers vs dynamically enumerated drivers

Device tree binding

resource mechanism - irq lines, gpios, pwms

apbx@80040000 {
  pwm: pwm@80064000 {
    pinctrl-names = "default";
    pinctrl-0 = <&pwm2_pins_a>;
    status = "okay";
};
  my-module: lradc@80050000 {
    error-gpios = <&gpio1 6 0>;
    status-gpios = <&gpio1 7 0>;
    adc-gpios = <&gpio1 4 0>;
    txen-gpios = <&gpio1 20 0>;
    uart-gpios = <&gpio1 21 0>;
    relayA-gpios = <&gpio2 3 0>;
    relayB-gpios = <&gpio2 4 0>;
    pwms = <&pwm 2 1000000 0>;
    compatible = "my,my-module";
    status = "okay";
};

Aliases

i2c = &i2c

???

Minimal code

// SPDX-License-Identifier: GPL-2.0
#include <linux/init.h>
#include <linux/module.h>

/* Add your code here */
static int __init hello_version_init(void)
{
    pr_info("Hello, World!\n");
    return 0;
}

static void __exit hello_version_exit(void)
{
    pr_info("Goodbye, World!\n");
}

module_init(hello_version_init);
module_exit(hello_version_exit);
//MODULE_INFO(vermagic, "5.10.16-rockchip64 SMP preempt mod_unload aarch64");
MODULE_LICENSE("GPL");

#linux #kernel