Re: [Linaro-mm-sig] [RFC 0/2] DMA-mapping & IOMMU - physically contiguous allocations

[Russell King - ARM Linux <linux@arm.linux.org.uk>]

On Tue, Oct 16, 2012 at 12:27:55PM +0200, Hiroshi Doyu wrote:
> Hi Russell,
> 
> Russell King - ARM Linux <linux@arm.linux.org.uk> wrote @ Tue, 16 Oct 2012 10:59:28 +0200:
> 
> > On Tue, Oct 16, 2012 at 09:04:34AM +0300, Hiroshi Doyu wrote:
> > > In addition to those contiguous/discontiguous page allocation, is
> > > there any way to _import_ anonymous pages allocated by a process to be
> > > used in dma-mapping API later?
> > > 
> > > I'm considering the following scenario, an user process allocates a
> > > buffer by malloc() in advance, and then it asks some driver to convert
> > > that buffer into IOMMU'able/DMA'able ones later. In this case, pages
> > > are discouguous and even they may not be yet allocated at
> > > malloc()/mmap().
> > 
> > That situation is covered.  It's the streaming API you're wanting for that.
> > dma_map_sg() - but you may need additional cache handling via
> > flush_dcache_page() to ensure that your code is safe for all CPU cache
> > architectures.
> >
> > Remember that pages allocated into userspace will be cacheable, so a cache
> > flush is required before they can be DMA'd.  Hence the streaming
> > API.
> 
> Is the syscall "cacheflush()" supposed to be the knob for that?
> 
> Or is there any other ones to have more precise control, "clean",
> "invalidate" and "flush", from userland in generic way?

No other syscalls are required - this sequence will do everything you
need to perform DMA on pages mapped into userspace:

	get_user_pages()
	convert array of struct page * to scatterlist
	dma_map_sg()
	perform DMA
	dma_unmap_sg()
	for each page in sg()
		page_cache_release(page);

If you get the list of pages some other way (eg, via shmem_read_mapping_page_gfp)
then additional maintanence may be required (though that may be a bug in
shmem - remember this stuff hasn't been well tested on ARM before.)

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