Documentation/ioctl/ioctl-decoding.txt

   1 To decode a hex IOCTL code:
   2
   3 Most architectures use this generic format, but check
   4 include/ARCH/ioctl.h for specifics, e.g. powerpc
   5 uses 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
  21 So for example 0x82187201 is a read with arg length of 0x218,
  22 character 'r' function 1. Grepping the source reveals this is:
  23
  24 #define VFAT_IOCTL_READDIR_BOTH         _IOR('r', 1, struct dirent [2])