Re: [PATCH 8/9] ARM: integrate CMA with DMA-mapping subsystem

Arnd Bergmann
2011-08-12T15:00:47Z

On Friday 12 August 2011, Marek Szyprowski wrote:
> @@ -82,16 +103,16 @@ static struct page *__dma_alloc_buffer(struct device *dev, size_t size, gfp_t gf
> if (mask < 0xffffffffULL)
> gfp |= GFP_DMA;
>
> - page = alloc_pages(gfp, order);
> - if (!page)
> - return NULL;
> -
> /*
> - * Now split the huge page and free the excess pages
> + * Allocate contiguous memory
> */
> - split_page(page, order);
> - for (p = page + (size >> PAGE_SHIFT), e = page + (1 << order); p < e; p++)
> - __free_page(p);
> + if (cma_available())
> + page = dma_alloc_from_contiguous(dev, count, order);
> + else
> + page = __dma_alloc_system_pages(count, gfp, order);
> +
> + if (!page)
> + return NULL;

Why do you need the fallback here? I would assume that CMA now has to be available
on ARMv6 and up to work at all. When you allocate from __dma_alloc_system_pages(),
wouldn't that necessarily fail in the dma_remap_area() stage?

>
> - if (arch_is_coherent() || nommu()) {
> + if (arch_is_coherent() || nommu() ||
> + (cma_available() && !(gfp & GFP_ATOMIC))) {
> + /*
> + * Allocate from system or CMA pages
> + */
> struct page *page = __dma_alloc_buffer(dev, size, gfp);
> if (!page)
> return NULL;
> + dma_remap_area(page, size, area->prot);
> pfn = page_to_pfn(page);
> ret = page_address(page);

Similarly with coherent and nommu. It seems to me that lumping too
many cases together creates extra complexity here.

How about something like

if (arch_is_coherent() || nommu())
ret = alloc_simple_buffer();
else if (arch_is_v4_v5())
ret = alloc_remap();
else if (gfp & GFP_ATOMIC)
ret = alloc_from_pool();
else
ret = alloc_from_contiguous();

This also allows a natural conversion to dma_map_ops when we get there.

> /* reserve any platform specific memblock areas */
> if (mdesc->reserve)
> mdesc->reserve();
>
> + dma_coherent_reserve();
> + dma_contiguous_reserve();
> +
> memblock_analyze();
> memblock_dump_all();
> }

Since we can handle most allocations using CMA on ARMv6+, I would think
that we can have a much smaller reserved area. Have you tried changing
dma_coherent_reserve() to allocate out of the contiguous area instead of
wasting a full 2MB section of memory?

Arnd