[patch] Document how to decode an IOCTL number

[Chuck Ebbert <76306.1226@compuserve.com>]

Document how to decode a binary IOCTL number.

Signed-off-by: Chuck Ebbert <76306.1226@compuserve.com>
---
 Documentation/ioctl/ioctl-decoding.txt |   24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

--- /dev/null
+++ 2.6.19.0.5-32smp/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])
-- 
Chuck
"Even supernovas have their duller moments."
-
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