linux/Documentation/userspace-api/ioctl/ioctl-decoding.rst
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   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])
  32