Re: DMA API issues
From: Takashi Iwai (tiwai_at_suse.de)
Date: 06/21/04
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Date: Mon, 21 Jun 2004 15:35:42 +0200 To: Russell King <rmk+lkml@arm.linux.org.uk>
At Fri, 18 Jun 2004 20:43:22 +0100,
Russell King wrote:
>
> On Fri, Jun 18, 2004 at 12:21:12PM -0700, Matt Porter wrote:
> > > page_to_dma so that device specific dma addresses can be constructed.
> >
> > A struct device argument to page_to_dma seems like a no brainer to be
> > included.
>
> Tony Lindgren recently submitted a patch for this:
>
> http://www.arm.linux.org.uk/developer/patches/viewpatch.php?id=1931/1
>
> which now pending for Linus. ARM platforms now have three macros to
> define if they want to override the default struct page to DMA address
> translation.
Wouldn't it be nicer to define a more generic style like
struct page *dma_to_page(struct device *, void *, dma_addr_t)
??
>
> > I see that's somewhat like what David Brownell suggested before...a single
> > pointer to a set of dma ops from struct device. hppa_dma_ops translated
> > into a generic dma_ops entity with fields corresponding to existing
> > DMA API calls would be a good starting point. We can get rid of some
> > address translation hacks in a lot of custom embedded PPC drivers
> > with something like this.
>
> I really don't think we need to go this far.
>
> As I understand it, the issue seems to surround DMA coherent memory
> for USB descriptors, and what happens when we call the streaming DMA
> API.
>
> We have the latter solved with Deepak's DMA bounce code (already merged)
> provided it's given the right information to determine when bounce
> buffers are needed.
>
> The bounce code only needs a way to get at the "safe" DMA memory, and
> it uses DMA pools for that. DMA pools in turn take their memory from
> dma_alloc_coherent.
>
> So, we just need a way to make dma_alloc_coherent do the right thing.
> One suggestion from James yesterday was to have a way to register
> device-private "coherent DMA" backing memory with dma_alloc_coherent,
> which it would use in preference to calling alloc_pages(). However,
> this memory would have no struct page pointer associated with it, and
> our dma_alloc_coherent implementation currently relies completely on
> that condition existing - so it would mean a complete rewrite of that.
>
> Note also that some drivers (notably ALSA) assume that memory returned
> from dma_alloc_coherent() does have struct page pointers, so this would
> also break ALSA (which in turn provides much more of a justification
> for the DMA MMAP API I was trying (and failed) to propose a few months
> back.)
Yes, the struct page pointer is needed for vma_ops.nopage in mmap on
ALSA. So far, this is broken on some architectures like ARM. We need
a proper conversion from virtual/bus pointer to a page struct.
Of course, mmap is not mandatory, and we have a fallback via
ioctl/read/write. But it's important to _know_ whether remapping is
available...
-- Takashi Iwai <tiwai@suse.de> ALSA Developer - www.alsa-project.org - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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