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,
Janusz
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