dma-mapping: Allow mixing bypass and mapped DMA operation

At the moment we allow bypassing DMA ops only when we can do this for
the entire RAM. However there are configs with mixed type memory
where we could still allow bypassing IOMMU in most cases;
POWERPC with persistent memory is one example.

This adds an arch hook to determine where bypass can still work and
we invoke direct DMA API. The following patch checks the bus limit
on POWERPC to allow or disallow direct mapping.

This adds a ARCH_HAS_DMA_MAP_DIRECT config option to make the arch_xxxx
hooks no-op by default.

Signed-off-by: Alexey Kardashevskiy <[e-mail]>
Signed-off-by: Christoph Hellwig <[e-mail]>

include/linux/dma-map-ops.h

···
 void *arch_dma_set_uncached(void *addr, size_t size);
 void arch_dma_clear_uncached(void *addr, size_t size);
 
+#ifdef CONFIG_ARCH_HAS_DMA_MAP_DIRECT
+bool arch_dma_map_page_direct(struct device *dev, phys_addr_t addr);
+bool arch_dma_unmap_page_direct(struct device *dev, dma_addr_t dma_handle);
+bool arch_dma_map_sg_direct(struct device *dev, struct scatterlist *sg,
+		int nents);
+bool arch_dma_unmap_sg_direct(struct device *dev, struct scatterlist *sg,
+		int nents);
+#else
+#define arch_dma_map_page_direct(d, a)		(false)
+#define arch_dma_unmap_page_direct(d, a)	(false)
+#define arch_dma_map_sg_direct(d, s, n)		(false)
+#define arch_dma_unmap_sg_direct(d, s, n)	(false)
+#endif
+
 #ifdef CONFIG_ARCH_HAS_SETUP_DMA_OPS
 void arch_setup_dma_ops(struct device *dev, u64 dma_base, u64 size,
 		const struct iommu_ops *iommu, bool coherent);

kernel/dma/Kconfig

···
 config DMA_OPS_BYPASS
 	bool
 
+# Lets platform IOMMU driver choose between bypass and IOMMU
+config ARCH_HAS_DMA_MAP_DIRECT
+	bool
+
 config NEED_SG_DMA_LENGTH
 	bool
 

kernel/dma/mapping.c

···
 	if (WARN_ON_ONCE(!dev->dma_mask))
 		return DMA_MAPPING_ERROR;
 
-	if (dma_map_direct(dev, ops))
+	if (dma_map_direct(dev, ops) ||
+	    arch_dma_map_page_direct(dev, page_to_phys(page) + offset + size))
 		addr = dma_direct_map_page(dev, page, offset, size, dir, attrs);
 	else
 		addr = ops->map_page(dev, page, offset, size, dir, attrs);
···
 	const struct dma_map_ops *ops = get_dma_ops(dev);
 
 	BUG_ON(!valid_dma_direction(dir));
-	if (dma_map_direct(dev, ops))
+	if (dma_map_direct(dev, ops) ||
+	    arch_dma_unmap_page_direct(dev, addr + size))
 		dma_direct_unmap_page(dev, addr, size, dir, attrs);
 	else if (ops->unmap_page)
 		ops->unmap_page(dev, addr, size, dir, attrs);
···
 	if (WARN_ON_ONCE(!dev->dma_mask))
 		return 0;
 
-	if (dma_map_direct(dev, ops))
+	if (dma_map_direct(dev, ops) ||
+	    arch_dma_map_sg_direct(dev, sg, nents))
 		ents = dma_direct_map_sg(dev, sg, nents, dir, attrs);
 	else
 		ents = ops->map_sg(dev, sg, nents, dir, attrs);
···
 
 	BUG_ON(!valid_dma_direction(dir));
 	debug_dma_unmap_sg(dev, sg, nents, dir);
-	if (dma_map_direct(dev, ops))
+	if (dma_map_direct(dev, ops) ||
+	    arch_dma_unmap_sg_direct(dev, sg, nents))
 		dma_direct_unmap_sg(dev, sg, nents, dir, attrs);
 	else if (ops->unmap_sg)
 		ops->unmap_sg(dev, sg, nents, dir, attrs);