32-bit dev_t progress

Andries Brouwer released a new set of patches this week which brings the long-planned expansion of dev_t closer to reality. These patches rework the character device infrastructure to make it safe for much larger numbers of devices. For now, at least, it is not even necessary to change any char drivers to work properly with the new code.

The first patch clears out the char device code within the filesystem area. This code included a whole structure for tracking devices, managing reference counts, etc. That structure was only used in one place, however, and Andries decided that, rather than fix it up to work with larger device numbers, he would just hack it out altogether. The rest of the kernel will not really notice its absence, for now.

The core of the work is in the second patch. Here, the longstanding static chrdevs array is removed. A static array of devices works reasonably well when there is a maximum of 255 of them; it's rather less convenient when there can be thousands of device numbers. In its place is a simple hash table with linked lists of registered char drivers.

There is a new way of registering a char driver:

    int register_chrdev_region(unsigned int major, 
                               unsigned int baseminor,
                               int minorct, 
                               const char *name,
                               struct file_operations *fops);

The new baseminor and minorct arguments describe the range of minor numbers that the driver is prepared to deal with. Char drivers should eventually be converted to the new interface, but there is no great hurry; the register_chrdev() interface is still supported as:

    int register_chrdev(unsigned int major, const char *name,
                        struct file_operations *fops)
    {
       	    return register_chrdev_region(major, 0, 256, name, fops);
    }

So unchanged char drivers will still work, and will not be confronted with minor numbers greater than 255.

For now, drivers requesting a dynamic major number may continue to use the same mechanism: passing major as zero. The mechanism implemented in the patch is not entirely robust, however, and is marked as being temporary.

The third patch just cleans things up a bit, and removes the MAX_CHRDEV macro. For the truly adventurous, there is a fourth patch which actually changes dev_t to 32 bits, using a 16:16 split.

These patches have found their way into the -mm kernel tree, and are now in need of some serious testing. Should things work out, the 32-bit dev_t expansion may finally get crossed off the 2.5 development list.

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