1. Debugging Uses of sysfs Attributes

1. Online Dynamic Debug Parameters

• Real-time adjustment of driver variables (such as baud rate, mode, strategy, threshold, etc.)
• No need to restart the device or reload the driver

2. Status Monitoring

• Export read-only attributes of hardware/driver internal status: temperature/battery level/register values/internal statistics, etc.
• Facilitates remote collection and monitoring of hardware health

3. Auxiliary Problem Location and Troubleshooting

• Allow temporarily enabling detailed logs, or exporting low-level register information
• Support viewing "last error code", debugging switches, etc.

4. Control Behavior/Function Switching

• Simple echo can temporarily modify behavior configuration, such as switching working modes, resetting statistics, forcing power on/off, etc.

2. Typical Operation Examples

We will explain in combination with the source code of 10_touch_sysfs_file/ from the previous chapter.

1. View Attributes

View value

cat /sys/class/class_test/device_test/myparam
# Output: 42

2. Configure Parameters

Write new value

# Write 88
echo 88 | sudo tee /sys/class/class_test/device_test/myparam

cat /sys/class/class_test/device_test/myparam
# Output: 88

3. Typical Attributes for Enabling/Disabling Debugging Functions

Suppose we designed a switch in the driver:

static int debug_enable = 0; // Debug switch

// Called on read operation
static ssize_t debug_show(struct device *dev, struct device_attribute *attr, char *buf)
{
    // Format debug_enable value as string and write to buf buffer
    return sprintf(buf, "%d\n", debug_enable);
}

// Called on write operation
static ssize_t debug_store(struct device *dev, struct device_attribute *attr, const char *buf, size_t count)
{
    int val;

    // kstrtoint is a kernel helper function: used to parse strings (such as "88\n", "123", "007", etc.) into integer values.
    if (kstrtoint(buf, 10, &val) == 0) {

        // Successfully parsed, update debug_enable value
        debug_enable = !!val; // Convert non-zero values to 1, zero values stay as 0
        printk(KERN_INFO "mychardev: debug_enable set to %d\n", debug_enable); // Print new value to kernel log
    }

    // Return the number of bytes written
    return count;
}

DEVICE_ATTR_RW(debug); // Create read-write device attribute debug

// ...in init: device_create_file(dev, &dev_attr_debug);

debug_enable = !!val; is equivalent to:

if (val)
    debug_enable = 1;
else
    debug_enable = 0;

Explanation

• !val :
  • Logical NOT, returns 1 when val is 0, returns 0 when val is non-zero.
• !!val :
  • Negate once more.
  • Converts any "non-zero" number to 1, keeps 0 as 0.

val	!val	!!val	Assigned to debug_enable
0	1	0	0
1	0	1	1
-3	0	1	1
99	0	1	1

Clean or normalize boolean variable values, avoiding cases where val is a number other than 0/1 (such as 2, -4, 123), so that debug_enable is not a clean 0 or 1. Ensures the switch variable is only 0 or 1.

Used during debugging:

# Enable debugging
echo 1 | sudo tee /sys/class/class_test/device_test/debug

# Disable debugging
echo 0 | sudo tee /sys/class/class_test/device_test/debug

4. Query Multiple States or Configurations

For outputting structured information, common usage:

static ssize_t status_show(struct device *dev, struct device_attribute *attr, char *buf)
{
    return snprintf(buf, PAGE_SIZE,
        "param=%d\nerror=%d\nstatus=%s\n", myparam, last_error, state_str);
}
DEVICE_ATTR_RO(status);

3. Norms and Suggestions

1. Keep Names Simple and Intuitive

Attribute names should be all lowercase, no spaces, with clear meanings such as enable, mode, threshold, debug.

2. Permission Configuration

• RO (read-only): Such as status, statistics, only use show
• RW (read-write): Such as parameters, configurable items, both show and store need to be written
• WO (write-only, rare): Such as sensitive operations (e.g., triggering a reset once)

3. Error and Input Checking

• store callbacks must validate input to avoid chaos caused by writing illegal values
• Such as value range, format checking, return -EINVAL when necessary

4. Synchronization and Safety

• If callbacks involve global variables, hardware operations, pay attention to locking to avoid concurrent access issues

5. Return Value Explanation

• show returns the actual number of bytes written to buf
• store returns count (how many bytes were successfully processed), otherwise the write operation may be abnormal/incomplete