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