+/*
+ * Some usb host controllers can only perform dma using a small SRAM area.
+ * The usb core itself is however optimized for host controllers that can dma
+ * using regular system memory - like pci devices doing bus mastering.
+ *
+ * To support host controllers with limited dma capabilites we provide dma
+ * bounce buffers. This feature can be enabled using the HCD_LOCAL_MEM flag.
+ * For this to work properly the host controller code must first use the
+ * function dma_declare_coherent_memory() to point out which memory area
+ * that should be used for dma allocations.
+ *
+ * The HCD_LOCAL_MEM flag then tells the usb code to allocate all data for
+ * dma using dma_alloc_coherent() which in turn allocates from the memory
+ * area pointed out with dma_declare_coherent_memory().
+ *
+ * So, to summarize...
+ *
+ * - We need "local" memory, canonical example being
+ * a small SRAM on a discrete controller being the
+ * only memory that the controller can read ...
+ * (a) "normal" kernel memory is no good, and
+ * (b) there's not enough to share
+ *
+ * - The only *portable* hook for such stuff in the
+ * DMA framework is dma_declare_coherent_memory()
+ *
+ * - So we use that, even though the primary requirement
+ * is that the memory be "local" (hence addressible
+ * by that device), not "coherent".
+ *
+ */
+
+static int hcd_alloc_coherent(struct usb_bus *bus,
+ gfp_t mem_flags, dma_addr_t *dma_handle,
+ void **vaddr_handle, size_t size,
+ enum dma_data_direction dir)