linux-kernel - Re: [patch] Document how to decode an IOCTL number

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Date:	Thu, 7 Dec 2006 20:42:58 -0800
From:	Randy Dunlap <randy.dunlap@...cle.com>
To:	Chuck Ebbert <76306.1226@...puserve.com>
Cc:	linux-kernel <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...l.org>
Subject: Re: [patch] Document how to decode an IOCTL number

On Thu, 7 Dec 2006 23:02:04 -0500 Chuck Ebbert wrote:

> Document how to decode a binary IOCTL number.
> 
> Signed-off-by: Chuck Ebbert <76306.1226@...puserve.com>
> ---
>  Documentation/ioctl/ioctl-decoding.txt |   24 ++++++++++++++++++++++++
>  1 file changed, 24 insertions(+)

Thanks...  could this be added to Documentation/ioctl-number.txt
instead?  and maybe move that file into Documentation/ioctl/ ?


> --- /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])
> -- 

---
~Randy
-
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