linux-kernel - Re: [PATCH 1/1] Implement /dev/byte (a generic byte source similiar to /dev/zero)

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Date:	Wed, 20 Apr 2011 20:07:58 +0200
From:	Alexander Holler <holler@...oftware.de>
To:	Pavel Machek <pavel@....cz>
CC:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] Implement /dev/byte (a generic byte source similiar
 to /dev/zero)

Am 20.04.2011 12:57, schrieb Pavel Machek:
> On Mon 2011-04-18 13:37:56, Alexander Holler wrote:
>> This device outputs by default 0xff instead 0 which makes more sense
>> than 0 to clear e.g. FLASH based devices.
>
> Well, now you should provide example where you mmap /dev/byte, then
> write() the flash directly from the mapping.
>
> ... hmm, that brings good question: what happens on existing mappings
> when the byte is changed?

I never used mmap (never had a need to use it) explicit and barely know 
what it does. And I don't see a reason to use mmap on /dev/byte. 
Otherwise I would have known before the reason why /dev/zero exists. ;)

So I can't answer what happens when someone uses mmap on /dev/byte and 
changes the value while the map exists (without testing it by myself).

>> To make the device more general usable, the value it outputs is changeable
>> on a per file descriptor basis through simple writes to it.
>> Values can be decimal (0 - 255), octal (00 - 0377) or hex (0x0 - 0xff).
>> For other values (or strings) written to it, the write operation returns an
>> error and the subsequent output is undefined.
> ...
>>   # Create a file of size 10GB and filled with 0xaa.
>> exec 5<>/dev/byte # Open /dev/byte and assign fd 5 to it
>> echo 0xaa>&5     # Instruct the device to output 0xaa
>
> That's seriously strange. /dev/byte should be changeable... by writing
> bytes.

As I've written before, that was the only solution I could come up with 
which makes it possible to change the output of /dev/byte on the fly 
without introducing race conditions (except ioctl). I know, it's ugly, 
but works, at least for common (read) file operations.

So it seems the only proper solution would be to use minors which would 
make such a device static.
So I better stick to something in userland, even when I would like to 
have at least /dev/byteFF. Chances seem to be minimal to get such 
included in the kernel if no one else sees a usage pattern for such.

Regards,

Alexander
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