linux-kernel - Re: [PATCH v3 2/4] videobuf2-dma-streaming: new videobuf2 memory allocator

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Message-ID: <20130109074838.GH13335@pengutronix.de>
Date:	Wed, 9 Jan 2013 08:48:38 +0100
From:	Michael Olbrich <m.olbrich@...gutronix.de>
To:	Jonathan Corbet <corbet@....net>
Cc:	Marek Szyprowski <m.szyprowski@...sung.com>,
	Mauro Carvalho Chehab <mchehab@...hat.com>,
	Alessandro Rubini <rubini@...dd.com>, federico.vaga@...il.com,
	mchehab@...radead.org, pawel@...iak.com, hans.verkuil@...co.com,
	giancarlo.asnaghi@...com, linux-media@...r.kernel.org,
	linux-kernel@...r.kernel.org, s.nawrocki@...sung.com
Subject: Re: [PATCH v3 2/4] videobuf2-dma-streaming: new videobuf2 memory
 allocator

On Tue, Jan 08, 2013 at 07:31:30AM -0700, Jonathan Corbet wrote:
> On Tue, 08 Jan 2013 07:50:41 +0100
> Marek Szyprowski <m.szyprowski@...sung.com> wrote:
> 
> > > Couldn't this performance difference be due to the usage of GFP_DMA inside
> > > the VB2 code, like Federico's new patch series is proposing?
> > >
> > > If not, why are there a so large performance penalty?  
> > 
> > Nope, this was caused rather by a very poor CPU access to non-cached (aka
> > 'coherent') memory and the way the video data has been accessed/read 
> > with CPU.
> 
> Exactly.  Uncached memory *hurts*, especially if you're having to touch it
> all with the CPU.

Even worse, on ARMv7 (at least) the cache implements or is necessary for
(I'm not an expert here) unaligned access. I've seen applications crash
on non-cached memory with a bus error because gcc assumes unaligned access
works. And there isn't even a exception handler in the kernel, probably for
the same reason.

Michael

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