Linux Device Driver Training in Hyderabad

What Is sysfs?

sysfs is a virtual filesystem, mounted at /sys, that exposes the internal kernel object hierarchy (kobjects) to user space. Every device, driver, bus, and class registered with the Linux driver model gets a directory under /sys, and each attribute of that object becomes a plain text file inside that directory. Unlike procfs, which grew organically and allowed arbitrarily formatted output, sysfs enforces a strict convention: one value per file, and ideally the file should be readable with a single cat and writable with a single echo.

Defining a Device Attribute

The macro DEVICE_ATTR (and its variants DEVICE_ATTR_RW, DEVICE_ATTR_RO, DEVICE_ATTR_WO) wires a sysfs file to a pair of show/store callback functions. This is the standard, modern pattern used across the kernel tree today.

#include <linux/device.h>
#include <linux/sysfs.h>

static int brightness_value;

static ssize_t brightness_show(struct device *dev,
                                struct device_attribute *attr, char *buf)
{
    return sysfs_emit(buf, "%d\n", brightness_value);
}

static ssize_t brightness_store(struct device *dev,
                                 struct device_attribute *attr,
                                 const char *buf, size_t count)
{
    int val;

    if (kstrtoint(buf, 10, &val))
        return -EINVAL;

    brightness_value = val;
    return count;
}

static DEVICE_ATTR_RW(brightness);

static struct attribute *mydriver_attrs[] = {
    &dev_attr_brightness.attr,
    NULL,
};

ATTRIBUTE_GROUPS(mydriver);

Notice the use of sysfs_emit() instead of sprintf() for the show function. This is the current recommended helper; it guarantees the output never overruns the fixed PAGE_SIZE buffer sysfs hands you, something older tutorials using raw sprintf did not protect against.

Attaching the Attribute Group to a Device

Once you have an attribute group, the cleanest way to attach it in a modern driver is through the dev_groups field of the device’s class, so the sysfs files are created and removed automatically as the device is added and removed:

static struct class mydriver_class = {
    .name = "mydriver",
    .dev_groups = mydriver_groups,
};

This avoids the older pattern of manually calling sysfs_create_group() in probe and sysfs_remove_group() in remove, though that manual approach is still valid and you will see it in plenty of in-tree drivers.

Common Mistakes

• Packing multiple unrelated values into a single sysfs file, which breaks the one-value-per-file convention and makes scripting painful.
• Using sprintf() in a show callback instead of sysfs_emit(), risking a buffer overrun on unusually long output.
• Forgetting that store callbacks receive a buffer that is not guaranteed to be null-terminated in every kernel version, which is why kstrtoint() style parsers are preferred over raw string functions.
• Granting write permission on attributes that can destabilize hardware without any validation of the incoming value.

sysfs vs procfs at a Glance