Scsi - Index of

716 * Bulk URBs may
717 * use the URB_ZERO_PACKET transfer flag, indicating that bulk OUT transfers
718 * should always terminate with a short packet, even if it means adding an
719 * extra zero length packet.
720 *
721 * Control URBs must provide a setup_packet. The setup_packet and
722 * transfer_buffer may each be mapped for DMA or not, independently of
723 * the other. The transfer_flags bits URB_NO_TRANSFER_DMA_MAP and
724 * URB_NO_SETUP_DMA_MAP indicate which buffers have already been mapped.
725 * URB_NO_SETUP_DMA_MAP is ignored for non-control URBs.
726 *
727 * Interrupt URBs must provide an interval, saying how often (in milliseconds
728 * or, for highspeed devices, 125 microsecond units)
729 * to poll for transfers. After the URB has been submitted, the interval
730 * field reflects how the transfer was actually scheduled.
731 * The polling interval may be more frequent than requested.
732 * For example, some controllers have a maximum interval of 32 microseconds,
733 * while others support intervals of up to 1024 microseconds.
734 * Isochronous URBs also have transfer intervals. (Note that for isochronous
735 * endpoints, as well as high speed interrupt endpoints, the encoding of
736 * the transfer interval in the endpoint descriptor is logarithmic.
737 * Device drivers must convert that value to linear units themselves.)
738 *
739 * Isochronous URBs normally use the URB_ISO_ASAP transfer flag, telling
740 * the host controller to schedule the transfer as soon as bandwidth
741 * utilization allows, and then set start_frame to reflect the actual frame
742 * selected during submission. Otherwise drivers must specify the start_frame
743 * and handle the case where the transfer can't begin then. However, drivers
744 * won't know how bandwidth is currently allocated, and while they can
745 * find the current frame using usb_get_current_frame_number () they can't
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746 * know the range for that frame number. (Ranges for frame counter values
747 * are HC-specific, and can go from 256 to 65536 frames from "now".)
748 *
749 * Isochronous URBs have a different data transfer model, in part because
750 * the quality of service is only "best effort". Callers provide specially
751 * allocated URBs, with number_of_packets worth of iso_frame_desc structures
752 * at the end. Each such packet is an individual ISO transfer. Isochronous
753 * URBs are normally queued, submitted by drivers to arrange that
754 * transfers are at least double buffered, and then explicitly resubmitted
755 * in completion handlers, so
756 * that data (such as audio or video) streams at as constant a rate as the
757 * host controller scheduler can support.
758 *