Passing parameters to Linux Device Drivers - builtin, modules

In C Programs, we pass command line parameters through argc/argv. Kernel programming also allows us to pass command line parameters.

The command line parameters provides a single linux driver to do multiple things, for example

• instead of fixing to a single I/O address for read/write, it can provide that as command line argument and allow user to read/write any address. 
• Enable/disable debug logs/printk
• Allow user to set the mode if the driver supports multiple modes

How to pass Parameters to modules?

We can add parameters using module_param macro. Declared in moduleparam.h file

name: name of the variable

type: Type of the Variable. Supported types are charp, bool, invbool, int, long, short, uint, ulong, ushort

perm: Permissions for the sysfs entry.  

E.g. S_IRUGO : Only read by all users

       0 : No sysfs entry

You can also use numeric values like 0644 for permission entry.

Sample Program to demonstrate parameter passing

#include <linux/kernel.h>

#include <linux/module.h>

MODULE_LICENSE("GPL");

char *name = "Embedded";

int loop_count = 1;

module_param(name, charp, S_IRUGO);

module_param(loop_count, int, S_IRUGO);

static int test_arguments_init(void)

{

    int i;

    printk(KERN_INFO"%s: In init\n", __func__);

    printk(KERN_INFO"%s: Loop Count:%d\n", __func__, loop_count);

    for (i = 0; i < loop_count; i++) 

        printk(KERN_INFO"%s: Hi %s\n", __func__, name);

    return 0;

}

static void test_arguments_exit(void)

{

    printk(KERN_INFO"%s: In exit\n", __func__);

}

module_init(test_arguments_init);

module_exit(test_arguments_exit);

How to pass Parameters?

Let's find out what will happen when we pass wrong parameters, if I pass string in loop_count.

It will throw invalid parameter error, and the module is not loaded.

How to find out the values of already loaded modules

$ cat /sys/modules/<module_name>/parameters/<parameter_name>

This is only possible, if the permission field in the module_param macro is not zero.

How can we pass arguments to modules which are builtin?

Module parameters for builtin modules are passed through kernel command line. 

Syntax: <module_name>.<parameter_name>=value

So, if the argument module was builtin, we can append argument.loop_count=5 to the kernel command line for passing 5 as loop_count

How can we pass arguments which are called by modprobe?

modprobe reads /etc/modprobe.conf file for parameters.