From cf1b939e41c28a31f2052e5c0a6619b5711ad7d0 Mon Sep 17 00:00:00 2001 From: Chuck Ebbert <76306.1226@compuserve.com> Date: Sun, 10 Dec 2006 02:18:57 -0800 Subject: [PATCH] [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 Signed-off-by: Linus Torvalds --- Documentation/ioctl/ioctl-decoding.txt | 24 ++++++++++++++++++++++++ 1 file changed, 24 insertions(+) create mode 100644 Documentation/ioctl/ioctl-decoding.txt diff --git a/Documentation/ioctl/ioctl-decoding.txt b/Documentation/ioctl/ioctl-decoding.txt new file mode 100644 index 000000000000..bfdf7f3ee4f0 --- /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]) -- 2.20.1