Re: DMA API issues

[Takashi Iwai]

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@xxxxxxx>		ALSA Developer - www.alsa-project.org
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