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

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

Hello,

On 7/17/2013 11:43 AM, Ricardo Ribalda Delgado wrote:
> Hi Marek
>
>   alloc_pages_exact returns pages of order 0, every single page is
> filled into buf->pages, that then is used by vb2_dma_sg_mmap(), that
> also expects order 0 pages (its loops increments in PAGE_SIZE). The
> code has been tested on real HW.
>
> Your concern is that that alloc_pages_exact splits the higher order pages?
>
> Do you want that videobuf2-dma-sg to have support for higher order pages?

Ah... My fault. I didn't notice that you recalculate req_pages at the
begginning of each loop iteration, so the code is correct, buf->pages is
filled correctly with order 0 pages.

So now the only issue I see is the oversized sglist allocation (the size
of sg list is computed for worst case, 0 order pages) and lack of the max
segment size support. Sadly there are devices which can handle single sg
chunk up to some predefined size (see dma_get_max_seg_size() function).

For some reference code, please check __iommu_map_sg() and maybe
__iommu_alloc_buffer() functions in arch/arm/mm/dma-mapping.c

Best regards
-- 
Marek Szyprowski
Samsung R&D Institute Poland