Hello, On 7/18/2013 9:39 AM, Ricardo Ribalda Delgado wrote:
Hello again: I have started to implemt it, but I think there is more hidden work in this task as it seems. In order to call dma_map_sg and max_dma_segment_size I need acess to the struct device, but (correct me if I am wrong), vb2 is device agnostic. Adding the above functionality will mean not only updating marvell-ccic and solo6x10, but updating all the vb2 buffers.
For getting device pointer, vb2-dma-sg need to be extended with so called 'allocator context'. Please check how it is done in vb2-dma-contig (vb2_dma_contig_init_ctx() function).
Also after some readings, maybe the sg compactation should not be done here, but in dma_map_sg. According to the doc: """ The implementation is free to merge several consecutive sglist entries into one (e.g. if DMA mapping is done with PAGE_SIZE granularity, any consecutive sglist entries can be merged into one provided the first one ends and the second one starts on a page boundary - in fact this is a huge advantage for cards which either cannot do scatter-gather or have very limited number of scatter-gather entries) and returns the actual number of sg entries it mapped them to. On failure 0 is returned. """ So, my proposal would be to alloc with alloc_pages to try to get memory as coherent as possible, then split the page, set the sg in PAGE_SIZE lenghts, and then let the dma_map_sg do its magic. if it doesnt do compactation, fix dma_map_sg, so more driver could take advantage of it.
Right, this approach is probably the best one, but this way you would need to do the compaction in every dma-mapping implementation for every supported architecture. IMHO vb2-dma-sg can help dma-mapping by at least by allocating memory in larger chunks and constructing shorter scatter list. Updating dma-mapping functions across all architectures is a lot of work and testing, so for initial version we should focus on vb2-dma-sg. Memory allocators already do some work to ease mapping a buffer to dma space.
I could also of course fix marvell-ccic and solo6x10 to use sg_table. Does anything of this make sense?
I would also like to help you as much as possible, but for the next 10 days I will be not available for both personal reasons and holidays. If you have any questions, feel free to leave them on my mail, I will reply asap I get back. Best regards -- Marek Szyprowski Samsung R&D Institute Poland -- 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
