On Thursday, August 19, 2010 08:09:27 pm Janusz Krzysztofik wrote:
> Thursday 19 August 2010 14:16:21 Marin Mitov napisał(a):
> > On Thursday, August 19, 2010 02:39:47 pm Guennadi Liakhovetski wrote:
> > >
> > > No, I don't think you should go to the next power of 2 - that's too
> > > crude. Try rounding your buffer size to the page size, that should
> > > suffice.
>
> Guennadi,
> If you have a look at how a device reserved memory is next allocated to a
> driver with drivers/base/dma-coherent.c::dma_alloc_from_coherent(), then than
> you may find my conclusion on a power of 2 as true:
>
> int dma_alloc_from_coherent(struct device *dev, ssize_t size,
> dma_addr_t *dma_handle, void **ret)
> {
> ...
> int order = get_order(size);
> ...
> pageno = bitmap_find_free_region(mem->bitmap, mem->size, order);
> ...
> }
>
>
> > Allocated coherent memory is always a power of 2.
>
> Marin,
> For ARM, this seems true as long as allocated with the above from a device
> assigned pool, but not true for a (pre)allocation from a generic system RAM.
> See arch/arm/mm/dma-mapping.c::__dma_alloc_buffer(), where it looks like
> extra
> pages are freed:
>
> static struct page *__dma_alloc_buffer(struct device *dev, size_t size, gfp_t
> gfp)
> {
> unsigned long order = get_order(size);
> ...
> page = alloc_pages(gfp, order);
> ...
> split_page(page, order);
> for (p = page + (size >> PAGE_SHIFT), e = page + (1 << order); p < e;
> p++)
> __free_page(p);
> ...
> }
Thanks for the clarification.
Marin Mitov
>
>
> Thanks,
> Janusz
>
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