linux-kernel - Re: early_printk accessing __log

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Message-Id: <20070723153437.4fc62253.akpm@linux-foundation.org>
Date:	Mon, 23 Jul 2007 15:34:37 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Robin Getz <rgetz@...ckfin.uclinux.org>
Cc:	"Mike Frysinger" <vapier.adi@...il.com>,
	linux-kernel@...r.kernel.org, "Greg Ungerer" <gerg@...pgear.com>,
	"Russell King" <rmk@....linux.org.uk>,
	"Paul Mundt" <lethal@...ux-sh.org>,
	"Tim Bird" <tim.bird@...sony.com>, bryan.wu@...log.com
Subject: Re: early_printk accessing __log_buf

On Mon, 23 Jul 2007 18:15:37 -0400
Robin Getz <rgetz@...ckfin.uclinux.org> wrote:

> On Mon 23 Jul 2007 16:15, Andrew Morton pondered:
> > This code would be
> > simpler if it did not attempt to read more than one char at a time.  It
> > will be plenty fast enough.
> 
> When systems have NMI kick off due to power failure, and you want to grab the 
> log buffer to write it to flash before power really dies - every cycle 
> counts.
> 
> A single function which does the copy as a loop (existing) is going to be much 
> faster than the overhead of 1024 function calls to copy the last k.

I'd expect the overhead of the (fully-cached) instructions to be
insignificant compared to the time to write to flash?

> > The magical interpretation of len isn't very nice. 
> 
> There are lots of places in the kernel which have magic in them :)

Lots of the kernel is pretty crappy.  One of our main tools for fixing that is to
ensure that new stuff is non-crappy.

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