ALSA vs. non coherent DMA
benh at kernel.crashing.org
Tue May 6 10:08:28 EST 2008
Hi Takashi !
I'm bringing up an old thread as I'm just discovering that the problem
still hasn't been fixed.
There seem to be a few issues with ALSA current usage of mmap vs. non
cache coherent architecture, such as embedded PowerPC's.
I can see at least two with a quick look to pcm-native.c, one I don't
understand and one I think I do:
- The control/status mapping. Can you elaborate a bit on what this is
actually doing and why it shouldn't be done on "non coherent"
architectures ? Currently this -is- done on all powerpc's, whether they
are coherent or not and I want to understand what the underlying issue
- The mmap of DMA pages. Here, the problem appears two fold:
* Use of virt_to_page() on virtual addresses returned by
* No using the appropriate page protection for a DMA coherent mapping
It seems like you have solved that in part with implementing a generic
dma_mmap_coherent() in the past that for some reason you never merged
upstream (I can track that to about 2 years ago). Is there a reason ?
I think we need to at least apply a band-aid today as it's becoming a
nasty issue for several non-coherent powerpc platforms. It could be in
the form of implementing dma_mmap_coherent() and changing Alsa to use it
with the appropriate ifdef, or just adding an ifdef CONFIG_PPC with the
right code in there for now until a better solution is found.
It should be trivial though. Getting the PFN from the DMA address is
easy if we have the dma handle and the virtual address, though that -is-
definitely platform specific. I can implement a function for that if you
need. As for the pgprot, we can come up with something like
pgprot_mmap_dma(). Either that or I can fold it all in a powerpc wide
implementation of a dma_mmap_coherent() like we envisioned initially.
Let me know what approach is preferred here and I'll come up with
patches ASAP. As far as I'm concerned, this is a bug and thus must be
fixed now for .26 and possibly backported to stable even if we can come
up with a non invasive solution). I'm annoyed because it represents a
trivial amount of code, this problem should have been fixed a long time
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