Hello, On Friday, December 23, 2011 10:51 AM Ming Lei wrote: > On Fri, Dec 23, 2011 at 5:34 PM, Marek Szyprowski > <m.szyprowski@xxxxxxxxxxx> wrote: > > >> 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. > > > > It can be easily increased for particular boards, there is no problem with this. > > It is not easily to increase it because there is very limited space reserved for > this purpose, see Documentation/arm/memory.txt. Also looks like it is > not configurable. It is really not a big issue to increase it by a few MBytes. > >> > 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] > > > > Your driver doesn't access video data inside kernel space, so this is also not an issue. > > Why not introduce it so that other drivers(include face detection) can > benefit with it? :-) We can get back into this once a driver which really benefits from comes. > >> > > >> > 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 *'. > > > > Then you MUST use cookie for it. vaddr method should return kernel virtual address > > to the buffer video data. Some parts of videobuf2 relies on this - it is used by file > > io emulator (read(), write() calls) and mmap equivalent for non-mmu systems. > > > > Manual casting in the driver is also a bad idea, that's why there are helper functions > > I don't see any casts are needed. The dma address can be got from vaddr with > dma_map_* easily in drivers, see the usage on patch 8/8(media: video: introduce > omap4 face detection module driver). Sorry, but I won't accept such driver/allocator which abuses the defined API. I've already pointed what vaddr method is used for. > > defined for both dma_contig and dma_sg allocators: vb2_dma_contig_plane_dma_addr() and > > vb2_dma_sg_plane_desc(). > > These two helpers are not needed and won't be provided by VIDEOBUF2_PAGE memops. I gave the example. Your allocator should have something similar. Best regards -- Marek Szyprowski Samsung Poland R&D Center -- 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
