一致性DMA --- linux kernel 实现赏析

void *dma_alloc_attrs(struct device *dev, size_t size, dma_addr_t *dma_handle,
		gfp_t flag, unsigned long attrs)
{
	const struct dma_map_ops *ops = get_dma_ops(dev);
	void *cpu_addr;

	WARN_ON_ONCE(!dev->coherent_dma_mask);

	if (dma_alloc_from_dev_coherent(dev, size, dma_handle, &cpu_addr))
		return cpu_addr;

	/* let the implementation decide on the zone to allocate from: */
	flag &= ~(__GFP_DMA | __GFP_DMA32 | __GFP_HIGHMEM);

	if (dma_alloc_direct(dev, ops))
		cpu_addr = dma_direct_alloc(dev, size, dma_handle, flag, attrs);
	else if (ops->alloc)
		cpu_addr = ops->alloc(dev, size, dma_handle, flag, attrs);
	else
		return NULL;

	debug_dma_alloc_coherent(dev, size, *dma_handle, cpu_addr);
	return cpu_addr;
}