linux-kernel - Re: [patch v2] Fixes and cleanups for earlyprintk aka boot console.

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Date:	Wed, 11 Apr 2007 20:33:38 +0200
From:	Olaf Dabrunz <od@...e.de>
To:	John Stoffel <john@...ffel.org>
Cc:	Gerd Hoffmann <kraxel@...e.de>, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Paul Mundt <lethal@...ux-sh.org>,
	Ralf Baechle <ralf@...ux-mips.org>, Andi Kleen <ak@...e.de>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Richard Henderson <rth@...ddle.net>,
	Ivan Kokshaysky <ink@...assic.park.msu.ru>,
	Paul Mackerras <paulus@...ba.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Jeremy Fitzhardinge <jeremy@...p.org>
Subject: Re: [patch v2] Fixes and cleanups for earlyprintk aka boot console.

On 03-Apr-07, John Stoffel wrote:
> 
> Sorry, I've been away for the past 10+ days and not reading email at
> all.  

NP. Happens to me too (as you can see).

> Olaf> Try changing the order of the console= parameters:
> 
> Olaf>     kernel ... console=ttyS1,96008N1 console=tty0
> 
> This isn't good for me, since it then drops some output I want to save
> via the serial console to the regular console.  It would be nice if
> all output went to BOTH consoles, though input should only be accepted
> on the first console listed.  

That is actually the underlying problem: the Linux kernel does not
provide the means to duplicate data streams in the kernel to several
devices. This has to be done in user-space. The main reason for this is
that error handling, locking and such things would need to be different
if streams had a fan-in or a fan-out. It tends to make things more
complicated.

But probably it is an interesting project to explore and define the
sematics of devices with fan-in and fan-out.

I can image that user-space programs would be able to add fan-in or
fan-out file-descriptors to an opened file-descriptor by using an
ioctl(), but then they would need to be prepared to deal with different
error handling and behaviour of such a "file".

Some devices (as the console device) could then have a pre-defined
fan-in or fan-out set by default, if opened with a special flag. A
program using this flag would then say: I am prepared to deal with
fan-in / fan-out semantics.

-- 
Olaf Dabrunz (od/odabrunz), SUSE Linux Products GmbH, Nürnberg

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