linux-kernel - Re: [PATCH 8/8] printk: Add config option for disabling printk offloading

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Message-ID: <20140422112259.5f106a44@alan.etchedpixels.co.uk>
Date:	Tue, 22 Apr 2014 11:22:59 +0100
From:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Jan Kara <jack@...e.cz>, LKML <linux-kernel@...r.kernel.org>,
	pmladek@...e.cz, Frederic Weisbecker <fweisbec@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH 8/8] printk: Add config option for disabling printk
 offloading

On Fri, 18 Apr 2014 11:54:38 -0700
Andrew Morton <akpm@...ux-foundation.org> wrote:

> On Wed, 26 Mar 2014 20:28:15 +0100 Jan Kara <jack@...e.cz> wrote:
> 
> > > All the afflicted consoles are serial, all go via the uart layer as far
> > > as I can see.
> > > 
> > > The uart layer has a queue mechanism that could be used
> >   I'm sorry, I don't follow here - what can the uart queueing be used for?
> 
> Alan, I'm desperate to avoid adding all this complexity to core printk
> code to solve such a rare problem.  It'd be great if you could flesh out
> any alternative ideas please.

It's not worth adding for upstream anyway - not in that form. If it just
used schedule_work it would be way way cleaner anyway.

For the general buffering case we already have tty_write_message(). It's
only really intended for use by the old quota code so it's currently
assuming nul terminated string but thats a trivial detail.

For that matter I don't see why such systems can't implement a queuecon
console which is a queue on the printk side and a fifo on the userspace
side.

The implementation then becomes trivial.

If you want reliable crash logging then we need to be able to set a
printk level mask per console and just set the serial console for
"crit/err" and the queue console for the rest, with a 'cat
>/dev/ttywhatever' running if this feature was in use ?

Alan
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