[E3-hacking] [RFC] [PATCH 1/6] SoC Camera: add driver for OMAP1 camera interface

[Janusz Krzysztofik]

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