Driver porting: miscellaneous changes

No more kdev_t

If your driver uses the i_rdev field of the inode structure, there are a couple of new macros you can use:

    unsigned int iminor(struct inode *inode);
    unsigned int imajor(struct inode *inode);

Consider using these while fixing your code; the next time the type of i_rdev changes, you will be happier.

malloc.h

Designated initializers

    static struct some_structure = {
	field1:  value,
	field2:  value
    };

    static struct some_structure = {
	.field1 = value,
	.field2 = value,
	...
    };

The min() and max() macros

    #define max(a,b) ((a) > (b) ? (a) : (b))

Linus initially added his own definition of min() and max() which added a third argument - an explicit type. That change upset people badly enough that some put substantial amounts of time into developing two-argument versions that are type and side-effect safe. The result now lives in <linux/kernel.h>, and should be used in preference to any locally-defined version.

Dependent read barriers

On most architectures, reads from memory will not be reordered across a data dependency. Consider this code fragment:

    int *a, *b, x;

    a = b;
    x = *a;

Here, you would expect that *a would yield the value pointed to by b; it would be surprising to have the read of *a reordered in front of the assignment to a. Some architectures can do just that sort of reordering, however. One could ensure that such reordering does not happen by inserting a rmb() (read memory barrier) between the two assignment. But that would reduce performance needlessly on many systems. So a new barrier, read_barrier_depends(), has been added in this case. It should only be used in situations where data dependencies exist. On architectures where data dependencies will force ordering, read_barrier_depends() will do nothing. On other architectures, it expands into a regular read barrier. See this patch posting for more information.

User-mode helpers

The prototype of call_usermodehelper() has changed:

    int call_usermodehelper(char *path, char **argv, char **envp, 
                            int wait);

The new wait flag controls whether call_usermodehelper() returns before the user-mode process exits. If wait is set to a non-zero value, the function will wait for the process to finish its work, and the return value will be what the process itself returned. Otherwise the return is immediate, and the return value only indicates whether the user-mode process was successfully started or not.

Note that the older exec_usermodehelper() function has been removed from the 2.5 kernel.

New request_module() prototype

As of 2.5.70, the prototype of request_module() has changed. This function now takes a printf-style argument list. As a result, code which used to look like:

    char module_name[32];

    sprintf(module_name, "foo-device-%d", number);
    request_module(module_name);

can now be rewritten as:

    request_module("foo-device-%d", number);

Most in-kernel code has already been changed to do things the new way.

devfs

Note also that devfs has been officially marked as "deprecated."