Re: [PATCH] videobuf2-dma-sg: Minimize the number of dma segments

[Marek Szyprowski <m.szyprowski@samsung.com>]

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