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

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

Hello,

On Friday, August 12, 2011 5:01 PM Arnd Bergmann wrote:

> 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?

It is not a fallback - I've just merged 2 cases together (CMA case and
coheren/nommu
arch). I agree that such mixed code might be confusing.

> >
> > -	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.

Ok. Is it ok to enable CMA permanently for ARMv6+? If CMA is left conditional
the dma pool code will be much more complicated, because it will need to support
both CMA and non-CMA cases.

> >  	/* 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?

I will move the reserved pool directly into CMA area, so it can be shrunk below
2MiB.

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


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