linux-kernel - Re: [RFC][PATCH] add dma_reserve_coherent_memory()/dma_free_reserved

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Date:	Thu, 26 Aug 2010 09:04:14 +0300
From:	Marin Mitov <mitov@...p.bas.bg>
To:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
Cc:	linux-kernel@...r.kernel.org, linux-media@...r.kernel.org,
	akpm@...ux-foundation.org
Subject: Re: [RFC][PATCH] add dma_reserve_coherent_memory()/dma_free_reserved_memory() API

On Thursday, August 26, 2010 08:40:47 am FUJITA Tomonori wrote:
> On Fri, 20 Aug 2010 14:50:12 +0300
> Marin Mitov <mitov@...p.bas.bg> wrote:
> 
> > On Friday, August 20, 2010 11:35:06 am FUJITA Tomonori wrote:
> > > On Fri, 20 Aug 2010 11:13:45 +0300
> > > Marin Mitov <mitov@...p.bas.bg> wrote:
> > > 
> > > > > > This tric is already used in drivers/staging/dt3155v4l.c
> > > > > > dt3155_alloc_coherent()/dt3155_free_coherent()
> > > > > > 
> > > > > > Here proposed for general use by popular demand from video4linux folks.
> > > > > > Helps for videobuf-dma-contig framework.
> > > > > 
> > > > > What you guys exactly want to do? If you just want to pre-allocate
> > > > > coherent memory for latter usage,
> > > > 
> > > > Yes, just to preallocate not coherent, but rather contiguous memory for latter usage.
> > > > We use coherent memory because it turns out to be contiguous.
> > > 
> > > Hmm, you don't care about coherency? You just need contiguous memory?
> > 
> > Yes. We just need contiguous memory. Coherency is important as far as when dma
> > transfer finishes user land is able to see the new data. Could be done by something like
> > dma_{,un}map_single()
> 
> Then, we should avoid using coherent memory as I exaplained before. In
> addition, dma_alloc_coherent can't provide large enough contigous
> memory for some drivers so this patch doesn't help much.

Please, look at drivers/media/video/videobuf-dma-contig.c. Using coherent memory
is inavoidable for now, there is no alternative for it for now. The two new functions,
which I propose are just helpers for those of us who already use coherent memory
(via videobuf-dma-contig API). May be adding these two functions to 
drivers/media/video/videobuf-dma-contig.c will be better solution?

Thanks.

Marin Mitov

> 
> We need the proper API for contiguous memory. Seem that we could have
> something:
> 
> http://lkml.org/lkml/2010/8/20/167
> 
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