x86: Introduce CONFIG_X86_DMA_REMAP

The default functions phys_to_dma, dma_to_phys implement identity
mapping as fast inline functions.  Some systems, however, may need a
custom function to implement its own mapping between CPU addresses and
device addresses. This new configuration option allows the functions
to be external when needed (such as for the ConneXt device)

Signed-off-by: Alessandro Rubini <rubini@gnudd.com>
Link: http://lkml.kernel.org/r/6e4329b772df675f1c442f68e59e844e4dd8c965.1333560789.git.rubini@gnudd.com
Acked-by: Giancarlo Asnaghi <giancarlo.asnaghi@st.com>
Cc: Alan Cox <alan@linux.intel.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>

arch/x86/Kconfig

@@ -2220,6 +2220,9 @@ config X86_DEV_DMA_OPS
 	bool
 	depends on X86_64
 
+config X86_DMA_REMAP
+	bool
+
 source "net/Kconfig"
 
 source "drivers/Kconfig"

arch/x86/include/asm/dma-mapping.h

@@ -62,6 +62,12 @@ extern void *dma_generic_alloc_coherent(struct device *dev, size_t size,
 					dma_addr_t *dma_addr, gfp_t flag,
 					struct dma_attrs *attrs);
 
+#ifdef CONFIG_X86_DMA_REMAP /* Platform code defines bridge-specific code */
+extern bool dma_capable(struct device *dev, dma_addr_t addr, size_t size);
+extern dma_addr_t phys_to_dma(struct device *dev, phys_addr_t paddr);
+extern phys_addr_t dma_to_phys(struct device *dev, dma_addr_t daddr);
+#else
+
 static inline bool dma_capable(struct device *dev, dma_addr_t addr, size_t size)
 {
 	if (!dev->dma_mask)
@@ -79,6 +85,7 @@ static inline phys_addr_t dma_to_phys(struct device *dev, dma_addr_t daddr)
 {
 	return daddr;
 }
+#endif /* CONFIG_X86_DMA_REMAP */
 
 static inline void
 dma_cache_sync(struct device *dev, void *vaddr, size_t size,