Re: [PATCH] Sam440ep support

At Tue, 06 May 2008 21:25:53 +1000,
Benjamin Herrenschmidt wrote:

On Tue, 2008-05-06 at 13:14 +0200, Takashi Iwai wrote:

quoted

quoted

sg allocations are usually lists of page, so virt_to_page shouldn't

be a

quoted

problem in the first place, though we still need some way to get the
right prot attributes.

The problem is that ALSA SG buffer is composed from pages allocated
via dma_alloc_coherent(), and the pages are retrieved via virt_to_page
for DMA pages, as found in sound/core/sgbuf.c.

I didn't care about SG-buffer cases in my initial patch, and it's a
bit messy.  The current code requires page lists becasue it calls
vmap() to get a virtually linear buffer.

Hrm... the problem is that you aren't supposed to make up sglists with
the result of dma_alloc_coherent... It might be a limitation of our core
DMA API, but that's we have to deal with today..

If you're going to make up sglists and call vmap, you should allocate
pages with normal GFP. If that is a problem vs. DMA'bility of those
pages, then ... we have a problem :-)

I don't think we can easily update the DMA API at this stage. What we
could do is provide a way to retrieve the struct page array from the
result of dma_alloc_coherent...

In most cases, it can be obtained via pfn_to_page(), I suppose.  But,
it's definitely arch-specific thingy, and a generic solution would be
really appreciated.

Alternatively, we can change the ALSA PCM core code that accesses the
virtual linear buffer and handles SG-buffers as they are.  Maybe it'll
give a bit more useful clean-up in the whole memory-management codes
in ALSA in the end.


Takashi