On Tuesday 27 September 2011, Mark Salter wrote:

+dma_addr_t dma_map_single(struct device *dev, void *ptr, size_t size,
+			  enum dma_data_direction dir)
+{
+	dma_addr_t addr = virt_to_phys(ptr);
+
+	BUG_ON(!valid_dma_direction(dir));
+
+	switch (dir) {
+	case DMA_FROM_DEVICE:
+		L2_cache_block_invalidate(addr, addr + size);
+		break;
+	case DMA_TO_DEVICE:
+		L2_cache_block_writeback(addr, addr + size);
+		break;
+	case DMA_BIDIRECTIONAL:
+		/*
+		 * If area will be used bidirectionally, syncing will
+		 * take place manually in the driver.
+		 */

I think this comment is not strictly correct and you have to do a
writeback+invalidate here. When you map an area, it initially
belongs to the device, until you either do an unmap or sync_*_for_cpu.

+void dma_unmap_single(struct device *dev, dma_addr_t handle,
+		      size_t size, enum dma_data_direction dir)
+{
+	/* nothing to do here */
+	debug_dma_unmap_page(dev, handle, size, dir, true);
+}
+EXPORT_SYMBOL(dma_unmap_single);

+void dma_sync_single_for_cpu(struct device *dev, dma_addr_t handle,
+			     size_t size, enum dma_data_direction dir)
+{
+	unsigned long paddr = handle;
+
+	BUG_ON(!valid_dma_direction(dir));
+
+	switch (dir) {
+	case DMA_FROM_DEVICE:
+		L2_cache_block_invalidate(paddr, paddr + size);
+		break;
+	case DMA_TO_DEVICE:
+		L2_cache_block_writeback(paddr, paddr + size);
+		break;
+	case DMA_BIDIRECTIONAL:
+		L2_cache_block_writeback_invalidate(paddr, paddr + size);
+		break;
+	default:
+		break;
+	}
+
+	debug_dma_sync_single_for_cpu(dev, handle, size, dir);
+}
+EXPORT_SYMBOL(dma_sync_single_for_cpu);

Normally, sync_*_for_cpu is the same as unmap regarding the cache.

+void dma_sync_single_for_device(struct device *dev, dma_addr_t handle,
+				size_t size, enum dma_data_direction dir)
+{
+	unsigned long paddr = handle;
+
+	BUG_ON(!valid_dma_direction(dir));
+
+	/* don't bother invalidating if DMA to device */
+	if (dir != DMA_TO_DEVICE)
+		L2_cache_block_invalidate(paddr, paddr + size);
+
+	debug_dma_sync_single_for_device(dev, handle, size, dir);
+}
+EXPORT_SYMBOL(dma_sync_single_for_device);

And this should do the same as map(). In particular it needs
to writeback the cache when direction is TO_DEVICE or BIDIRECTIONAL.

	Arnd