To decode a hex IOCTL code:

Most architectures use this generic format, but check
include/ARCH/ioctl.h for specifics, e.g. powerpc
uses 3 bits to encode read/write and 13 bits for size.

bits meaning
31-30 00 - no parameters: uses _IO macro
10 - read: _IOR
01 - write: _IOW
11 - read/write: _IOWR

29-16 size of arguments

15-8 ascii character supposedly
unique to each driver

7-0 function #

下面这个解释要看清楚
So for example 0x82187201 is a read with arg length of 0x218,
character ‘r’ function 1. Grepping the source reveals this is: