dma_alloc_coherent

[rohan puri]

On Wed, Oct 12, 2011 at 11:47 PM, bob jonny <ilikepie420 at live.com> wrote:

>  In dma_alloc_coherent(), where is it allocating memory from, and how does
> it know that that memory is cache coherent? Does every device have it's
> cache coherent memory? I got lost at function pointer struct dma_map_ops
> ops, is there an easy way to figure out what ops->alloc_coherent() points
> to?
>
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> Hi Bob,

dma_alloc_coherent() invokes the alloc_coherent method for the particular
device which is the first parameter.

So to see from where the memory is getting allocated, you need to see that
device's dma_map_op's alloc_coherent method.

I went through some of them, they tend to alloc memory by calling either
__get_free_pages() or alloc_pages() functions.

To maintain cache coherency, the pages which are allocated in these
functions have to be uncached and if they where cached earlier then they
have to be flushed.

Following function explains all these : -

void *dma_generic_alloc_coherent(struct device *dev, size_t size,
                                 dma_addr_t *dma_handle, gfp_t gfp)
{
        void *ret, *ret_nocache;
        int order = get_order(size);

        gfp |= __GFP_ZERO;

        ret = (void *)__get_free_pages(gfp, order);
        if (!ret)
                return NULL;

        /*
         * Pages from the page allocator may have data present in
         * cache. So flush the cache before using uncached memory.
         */
        dma_cache_sync(dev, ret, size, DMA_BIDIRECTIONAL);

        ret_nocache = (void __force *)ioremap_nocache(virt_to_phys(ret),
size);
        if (!ret_nocache) {
                free_pages((unsigned long)ret, order);
                return NULL;
        }

        split_page(pfn_to_page(virt_to_phys(ret) >> PAGE_SHIFT), order);

        *dma_handle = virt_to_phys(ret);

        return ret_nocache;
}

Regards,
Rohan Puri
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