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Subject: Re: [Patch] fix MTD CFI/LPDDR flash driver huge latency bug
From: Stefani Seibold <stefani@seibold.net>
To: Jamie Lokier <jamie@shareable.org>
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Date: Mon, 15 Mar 2010 07:15:54 +0100
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Cc: linux-mtd@lists.infradead.org, Andrew Morton <akpm@linux-foundation.org>,
	David Woodhouse <dwmw2@infradead.org>, "Kreuzer,
	Michael \(NSN - DE/Ulm\)" <michael.kreuzer@nsn.com>,
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Am Montag, den 15.03.2010, 03:03 +0000 schrieb Jamie Lokier:
> Stefani Seibold wrote:
> > Am Freitag, den 12.03.2010, 23:38 +0000 schrieb Jamie Lokier:
> > > Andrew Morton wrote:
> > > > On Sat, 06 Mar 2010 17:48:57 +0100
> > > > Stefani Seibold <stefani@seibold.net> wrote:
> > > > 
> > 
> > 
> > > > > The patch change all the use of spin_lock operations for xxxx->mutex
> > > > > into mutex operations, which is exact what the name says and means.
> > > 
> > > It would be even better if it also split the critical sections into
> > > smaller ones with cond_resched() between, so that non-preemptible
> > > kernels benefit too.
> > 
> > The problem is the memcpy operation which is very slow. A cond_resched
> > wouldn't help, since the cpu bus is blocked during the transfer of the
> > word.
> 
> I mean split the memcpy into multiple smaller memcpys, so that the
> total time in each memcpy is limited to something reasonable.
> 
> The check in cond_resched() is fast, especially once cached.  memcpy
> speed depends a lot on the attached flash and how everything's
> configured, varying from 2.5MB/s up to hundreds of MB/s.  So how about
> doing cond_resched() every 256 bytes?
> 
> -- Jamie

I thoght about this aporoach and i don't like this idea. Why not using a
preemptible kernel?

Stefani