[PATCH] Document how to decode an IOCTL number

Document how to decode a binary IOCTL number.

Signed-off-by: Chuck Ebbert <76306.1226@compuserve.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

diff --git a/Documentation/ioctl/ioctl-decoding.txt b/Documentation/ioctl/ioctl-decoding.txt
new file mode 100644 (file)
index 0000000..bfdf7f3
--- /dev/null
+++ b/Documentation/ioctl/ioctl-decoding.txt
@@ -0,0 +1,24 @@
+To decode a hex IOCTL code:
+
+Most architecures 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 #
+
+
+ So for example 0x82187201 is a read with arg length of 0x218,
+character 'r' function 1. Grepping the source reveals this is:
+
+#define VFAT_IOCTL_READDIR_BOTH         _IOR('r', 1, struct dirent [2])