linux/Documentation/ioctl/ioctl-decoding.rst
Mauro Carvalho Chehab 5c04dceaa1 docs: ioctl: convert to ReST
Rename the iio documentation files to ReST, add an
index for them and adjust in order to produce a nice html
output via the Sphinx build system.

The cdrom.txt and hdio.txt have their own particular syntax.
In order to speedup the conversion, I used a small ancillary
perl script:

	my $d;
	$d .= $_ while(<>);
	$d =~ s/(\nCDROM\S+)\s+(\w[^\n]*)/$1\n\t$2\n/g;
	$d =~ s/(\nHDIO\S+)\s+(\w[^\n]*)/$1\n\t$2\n/g;
	$d =~ s/(\n\s*usage:)[\s\n]*(\w[^\n]*)/$1:\n\n\t  $2\n/g;
	$d =~ s/(\n\s*)(E\w+[\s\n]*\w[^\n]*)/$1- $2/g;
	$d =~ s/(\n\s*)(inputs|outputs|notes):\s*(\w[^\n]*)/$1$2:\n\t\t$3\n/g;
	print $d;

It basically add blank lines on a few interesting places. The
script is not perfect: still several things require manual work,
but it saved quite some time doing some obvious stuff.

At its new index.rst, let's add a :orphan: while this is not linked to
the main index.rst file, in order to avoid build warnings.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
2019-07-15 09:20:26 -03:00

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ReStructuredText

==============================
Decoding an IOCTL Magic Number
==============================
To decode a hex IOCTL code:
Most architectures 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])