Re: [PATCH] Sam440ep support

-------- Original-Nachricht --------

Datum: Tue, 06 May 2008 09:44:18 +1000
Von: Benjamin Herrenschmidt [off-list ref]
An: Gerhard Pircher [off-list ref]
CC: Kumar Gala <redacted>, <redacted>, <redacted>
Betreff: Re: [PATCH] Sam440ep support

On Mon, 2008-05-05 at 21:50 +0200, Gerhard Pircher wrote:

quoted

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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