On Thu, Dec 22, 2011 at 5:28 PM, Marek Szyprowski <m.szyprowski@xxxxxxxxxxx> 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@xxxxxxxxxxxxx> > > 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 -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
