Re: Re: RFC: dma_mmap_coherent() for powerpc/ppc architecture and ALSA?

-------- Original-Nachricht --------
Datum: Fri, 09 Jun 2006 20:46:32 -0400
Von: Lee Revell [off-list ref]
An: Benjamin Herrenschmidt [off-list ref]
Betreff: Re: RFC: dma_mmap_coherent() for powerpc/ppc architecture and 
ALSA?

On Sat, 2006-06-10 at 10:34 +1000, Benjamin Herrenschmidt wrote:

quoted

quoted

This leads me to the question, if there are any plans to include the 
dma_mmap_coherent() function (for powerpc/ppc and/or any other
platform) in one of the next kernel versions and if an adapation of
the ALSA drivers is planned. Or is there a simple way (hack) to fix
this problem?

You are welcome to do a patch implementing this :)

Please cc: alsa-devel when you do so.

:)

Well, implementing the dma_mmap_coherent() function isn't the problem, because it is already implemented for the ARM architecture. But as far as I understand this would require a rewrite of all the ALSA drivers (or at least a rewrite of the ALSA's DMA helper functions).

Original mail included for alsa-devel mailing list:

I'm trying to adapt Linux for the AmigaOne, which is a G3/G4 PPC desktop 
system with a non cache coherent northbridge (MAI ArticiaS), a VIA82C686B 
southbridge and the U-boot firmware. Due to the cache coherency problem I 
compiled in the CONFIG_NOT_COHERENT_CACHE option
(arch/ppc/kernel/dma-mapping.c) in the AmigaOne Linux kernel.

While that fixes the DMA data corruption problem, it causes a kernel oops 
or a complete system lookup after starting sound playback. With kernel
versions =<2.6.14 the oops messages refered to a BUG() entry in
mm/rmap.c. Therefore I tried out a newer kernel (2.6.16.15), where the
oops refers to the ALSA function snd_pcm_mmap_data_nopage() implemented
in pcm_native.c.

Well, after searching a while in some old linux kernel threads, I found
this thread here:
http://www.thisishull.net/showthread.php?t=22080&page=3&pp=10

Based on the information in this thread, I came to the conclusion that
ALSA simply won't work on non cache coherent architectures (except ARM),
because the generic DMA API was never expanded to support the
functionality required by ALSA (namely mapping dma pages into user space > with dma_mmap_coherent()).

This leads me to the question, if there are any plans to include the
dma_mmap_coherent() function (for powerpc/ppc and/or any other platform)
in one of the next kernel versions and if an adapation of the ALSA
drivers is planned. Or is there a simple way (hack) to fix this problem?

Gerhard

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