Passing parameters to Linux Device Drivers - builtin, modules

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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.



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