Messages in this thread | | | Date | Thu, 18 Jul 2013 15:34:20 +0200 | From | Marek Szyprowski <> | Subject | Re: [PATCH] videobuf2-dma-sg: Minimize the number of dma segments |
| |
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
| |