device-drivers

Device Drivers

Purpose

Guide agents through Linux kernel device driver development: the driver model (platform_driver, i2c_driver, spi_driver), character device lifecycle, IRQ handling including threaded IRQs, DMA engine API, regmap for register abstraction, runtime power management, and udev rules for userspace device nodes.

When to Use

• Writing a platform driver for memory-mapped hardware
• Implementing a character device with read/write/ioctl
• Handling hardware interrupts (hard IRQ vs threaded)
• Setting up DMA coherent or streaming mappings
• Abstracting register access with regmap
• Configuring udev rules for /dev node permissions

Workflow

1. Driver model overview

Device tree / ACPI → bus (platform, i2c, spi, pci)
    → struct device → struct device_driver
        → probe() / remove()

// platform_driver.c — minimal platform driver
#include <linux/module.h>
#include <linux/platform_device.h>

static int my_probe(struct platform_device *pdev)
{
    struct resource *res = platform_get_resource(pdev, IORESOURCE_MEM, 0);
    void __iomem *base = devm_ioremap_resource(&pdev->dev, res);
    if (IS_ERR(base))
        return PTR_ERR(base);
    dev_info(&pdev->dev, "probed at %pa\n", &res->start);
    return 0;
}

static void my_remove(struct platform_device *pdev)
{
    dev_info(&pdev->dev, "removed\n");
}

static struct platform_driver my_driver = {
    .probe  = my_probe,
    .remove = my_remove,
    .driver = { .name = "my-device", .owner = THIS_MODULE },
};

module_platform_driver(my_driver);
MODULE_LICENSE("GPL");

2. Character device lifecycle

#include <linux/fs.h>
#include <linux/cdev.h>
#include <linux/uaccess.h>

#define DEVICE_NAME "mydev"
#define MINOR_BASE  0
#define MINOR_COUNT 1

static dev_t dev_num;
static struct cdev my_cdev;
static struct class *dev_class;

static ssize_t my_read(struct file *filp, char __user *buf,
                       size_t count, loff_t *ppos)
{
    char kbuf[64] = "hello from kernel\n";
    size_t len = strlen(kbuf);
    if (*ppos >= len)
        return 0;
    if (count > len - *ppos)
        count = len - *ppos;
    if (copy_to_user(buf, kbuf + *ppos, count))
        return -EFAULT;
    *ppos += count;
    return count;
}

static const struct file_operations my_fops = {
    .owner = THIS_MODULE,
    .read  = my_read,
};

static int __init mydev_init(void)
{
    int ret;
    ret = alloc_chrdev_region(&dev_num, MINOR_BASE, MINOR_COUNT, DEVICE_NAME);
    if (ret)
        return ret;

    cdev_init(&my_cdev, &my_fops);
    my_cdev.owner = THIS_MODULE;
    ret = cdev_add(&my_cdev, dev_num, MINOR_COUNT);
    if (ret)
        goto err_cdev;

    dev_class = class_create(DEVICE_NAME);
    device_create(dev_class, NULL, dev_num, NULL, DEVICE_NAME);
    return 0;

err_cdev:
    unregister_chrdev_region(dev_num, MINOR_COUNT);
    return ret;
}