linux/Documentation/ioctl/ioctl-decoding.txt
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   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])
  25
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