[RFC][PATCH] add dma_reserve_coherent_memory()/dma_free_reserved_memory() API

[Guennadi Liakhovetski]

On Thu, 26 Aug 2010, FUJITA Tomonori wrote:

> On Thu, 26 Aug 2010 09:04:14 +0300
> Marin Mitov <mitov at issp.bas.bg> wrote:
> 
> > On Thursday, August 26, 2010 08:40:47 am FUJITA Tomonori wrote:
> > > On Fri, 20 Aug 2010 14:50:12 +0300
> > > Marin Mitov <mitov at issp.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 at issp.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?
> 
> If you add something to the videobuf-dma-contig API, that's fine by me
> because drivers/media/video/videobuf-dma-contig.c uses the own
> structure and plays with dma_alloc_coherent. As long as a driver
> doesn't touch device->dma_mem directly, it's fine, I think (that is,
> dt3155v4l driver is broken). There are already some workarounds for
> contigous memory in several drivers anyway.

No, this will not work - this API has to be used from board code and 
videobuf can be built modular.

> We will have the proper API for contiguous memory. I don't think that
> adding such workaround to the DMA API is a good idea.

We have currently a number of boards broken in the mainline. They must be 
fixed for 2.6.36. I don't think the mentioned API will do this for us. So, 
as I suggested earlier, we need either this or my patch series

http://thread.gmane.org/gmane.linux.ports.sh.devel/8595

for 2.6.36.

Thanks
Guennadi
---
Guennadi Liakhovetski, Ph.D.
Freelance Open-Source Software Developer
http://www.open-technology.de/