Re: [RFC] lazy allocation of struct block_device

[Jamie Lokier <lk@tantalophile.demon.co.uk>]

Andries.Brouwer@cwi.nl wrote:
>     From viro@math.psu.edu Sat Sep  1 18:26:53 2001
>     > A kdev_t is a pointer to a struct that has the info now found in
>     > the arrays (and major, minor fields, and a name function..).
>     > This struct is allocated by the driver.
> 
>     Umm... Apply the arguments from the char_device thread - pointers to
>     unions are rather bad idea.  IOW, kdev_t must die - kernel always
>     knows which kind we are dealing with.
>[...]
> However, a union is not so bad. It seems a pity to avoid unions
> and waste 4 bytes for every inode with separate i_bdev and i_cdev
> instead of a single i_bcdev.

Please, a union of different pointer types is much nicer.  You can have
i_bdev and i_cdev without wasting any bytes.

This form works with GCC 2.96:

		union {
			struct char_device * i_cdev;
			struct block_device * i_bdev;
		};

If you're using a really old compiler that doesn't support anonymous unions,
(GCC 2.95 might be in this category, I'm not sure), then you'll need this:

	#define i_bdev __i_bcdev_union.i_bdev
	#define i_cdev __i_bcdev_union.i_cdev
		union {
			struct char_device * i_cdev;
			struct block_device * i_bdev;
		} __i_bcdev_union;

Either way, you avoid pointers to unions and you also avoid having a
named union type which contains pointers.

-- Jamie