Messages in this thread | | | Date | Fri, 23 Dec 2011 17:22:23 +0800 | Subject | Re: [RFC PATCH v2 4/8] media: videobuf2: introduce VIDEOBUF2_PAGE memops | From | Ming Lei <> |
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On Thu, Dec 22, 2011 at 5:28 PM, Marek Szyprowski <m.szyprowski@samsung.com> wrote: > Hello, > > On Wednesday, December 14, 2011 3:00 PM Ming Lei wrote: > >> DMA contig memory resource is very limited and precious, also >> accessing to it from CPU is very slow on some platform. >> >> For some cases(such as the comming face detection driver), DMA Streaming >> buffer is enough, so introduce VIDEOBUF2_PAGE to allocate continuous >> physical memory but letting video device driver to handle DMA buffer mapping >> and unmapping things. >> >> Signed-off-by: Ming Lei <ming.lei@canonical.com> > > Could you elaborate a bit why do you think that DMA contig memory resource > is so limited? If dma_alloc_coherent fails because of the memory fragmentation, > the alloc_pages() call with order > 0 will also fail.
For example, on ARM, there is very limited kernel virtual address space reserved for DMA coherent buffer mapping, the default size is about 2M if I don't remember mistakenly.
> > I understand that there might be some speed issues with coherent (uncached) > userspace mappings, but I would solve it in completely different way. The interface
Also there is poor performance inside kernel space, see [1]
> for both coherent/uncached and non-coherent/cached contig allocator should be the > same, so exchanging them is easy and will not require changes in the driver. > I'm planning to introduce some design changes in memory allocator api and introduce > prepare and finish callbacks in allocator ops. I hope to post the rfc after > Christmas. For your face detection driver using standard dma-contig allocator > shouldn't be a big issue. > > Your current implementation also abuses the design and api of videobuf2 memory > allocators. If the allocator needs to return a custom structure to the driver
I think returning vaddr is enough.
> you should use cookie method. vaddr is intended to provide only a pointer to > kernel virtual mapping, but you pass a struct page * there.
No, __get_free_pages returns virtual address instead of 'struct page *'.
thanks, -- Ming Lei
[1], http://marc.info/?t=131198148500001&r=1&w=2
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