Re: [PATCH] Sam440ep support

["Gerhard Pircher" <gerhard_pircher@gmx.net>]

-------- Original-Nachricht --------
> Datum: Tue, 06 May 2008 09:44:18 +1000
> Von: Benjamin Herrenschmidt <benh@kernel.crashing.org>
> An: Gerhard Pircher <gerhard_pircher@gmx.net>
> CC: Kumar Gala <galak@kernel.crashing.org>, cjg@cruxppc.org, linuxppc-dev@ozlabs.org
> Betreff: Re: [PATCH] Sam440ep support

> 
> On Mon, 2008-05-05 at 21:50 +0200, Gerhard Pircher wrote:
> > This is a (bad) hack that I also use on the AmigaOne to get the ALSA
> > sound drivers working with DMA, because ALSA doesn't work with
> > dma-noncoherent.c. The problem is the "nopage" mechanism, which fails
> > with non coherent DMA allocations due to their own virtual address
> > space (correct me, if I'm wrong).
> > 
> > See this thread for more info:
> > http://readlist.com/lists/vger.kernel.org/linux-kernel/45/226541.html
> > 
> > This is a general problem that affects all powerpc boards that use
> > dma-noncoherent.c with ALSA PCI drivers.
> 
> The link above doesn't provide any useful information on the problem and
> it contains itself a non working link...
Sorry, I didn't check the embedded link. This one should work:
http://www.uwsg.iu.edu/hypermail/linux/kernel/0406.2/0801.html

IIRC the problem is the mmaping of non coherent DMA allocations, as you
already know. The link above points to a very old (from 2004) and quite
long thread where the correct DMA API for mmaping DMA allocations was
discussed.

> Can somebody explains exactly what's going on ? That shouldn't be hard
> to fix. I can't believe the problem has been around for 2 years and
> nobody actually bothered fixing it properly.
Takashi Iwai posted a preliminary patch a long time ago. I tested it on my
machine and it failed with non coherent scatter-gather DMA allocations
(I guess almost all ALSA PCI drivers use SG DMA?).

Thanks!

Gerhard

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