linux-kernel - Re: [Linaro-mm-sig] [RFC 0/2] DMA-mapping & IOMMU

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Message-ID: <20121016103702.GB21164@n2100.arm.linux.org.uk>
Date:	Tue, 16 Oct 2012 11:37:02 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Hiroshi Doyu <hdoyu@...dia.com>
Cc:	"m.szyprowski@...sung.com" <m.szyprowski@...sung.com>,
	"inki.dae@...sung.com" <inki.dae@...sung.com>,
	"arnd@...db.de" <arnd@...db.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linaro-mm-sig@...ts.linaro.org" <linaro-mm-sig@...ts.linaro.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"kyungmin.park@...sung.com" <kyungmin.park@...sung.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>
Subject: Re: [Linaro-mm-sig] [RFC 0/2] DMA-mapping & IOMMU - physically
	contiguous allocations

On Tue, Oct 16, 2012 at 12:27:55PM +0200, Hiroshi Doyu wrote:
> Hi Russell,
> 
> Russell King - ARM Linux <linux@....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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