- * setup_dma fields rather than determining a dma address themselves. (Note
- * that transfer_buffer and setup_packet must still be set because not all
- * host controllers use DMA, nor do virtual root hubs).
+ * setup_dma fields rather than determining a dma address themselves.
+ *
+ * Note that transfer_buffer must still be set if the controller
+ * does not support DMA (as indicated by bus.uses_dma) and when talking
+ * to root hub. If you have to trasfer between highmem zone and the device
+ * on such controller, create a bounce buffer or bail out with an error.
+ * If transfer_buffer cannot be set (is in highmem) and the controller is DMA
+ * capable, assign NULL to it, so that usbmon knows not to use the value.
+ * The setup_packet must always be set, so it cannot be located in highmem.