1==============================
 2Decoding an IOCTL Magic Number
 3==============================
 4
 5To decode a hex IOCTL code:
 6
 7Most architectures use this generic format, but check
 8include/ARCH/ioctl.h for specifics, e.g. powerpc
 9uses 3 bits to encode read/write and 13 bits for size.
10
11 ====== ==================================
12 bits   meaning
13 ====== ==================================
14 31-30	00 - no parameters: uses _IO macro
15	10 - read: _IOR
16	01 - write: _IOW
17	11 - read/write: _IOWR
18
19 29-16	size of arguments
20
21 15-8	ascii character supposedly
22	unique to each driver
23
24 7-0	function #
25 ====== ==================================
26
27
28So for example 0x82187201 is a read with arg length of 0x218,
29character 'r' function 1. Grepping the source reveals this is::
30
31	#define VFAT_IOCTL_READDIR_BOTH         _IOR('r', 1, struct dirent [2])