struct urb
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## Name

struct urb — USB Request Block

## Synopsis

    struct urb {
      struct list_head urb_list;
      struct list_head anchor_list;
      struct usb_anchor * anchor;
      struct usb_device * dev;
      struct usb_host_endpoint * ep;
      unsigned int pipe;
      int status;
      unsigned int transfer_flags;
      void * transfer_buffer;
      dma_addr_t transfer_dma;
      int transfer_buffer_length;
      int actual_length;
      unsigned char * setup_packet;
      dma_addr_t setup_dma;
      int start_frame;
      int number_of_packets;
      int interval;
      int error_count;
      void * context;
      usb_complete_t complete;
      struct usb_iso_packet_descriptor iso_frame_desc[0];
    };

## Members

urb_list

  For use by current owner of the URB.

anchor_list

  membership in the list of an anchor

anchor

  to anchor URBs to a common mooring

dev

  Identifies the USB device to perform the request.

ep

  Points to the endpoint's data structure. Will eventually replace `pipe`.

pipe

  Holds endpoint number, direction, type, and more. Create these values with the eight macros available; usb_{snd,rcv}TYPEpipe(dev,endpoint), where the TYPE is “ctrl” (control), “bulk”, “int” (interrupt), or “iso” (isochronous). For example `usb_sndbulkpipe` or `usb_rcvintpipe`. Endpoint numbers range from zero to fifteen. Note that “in” endpoint two is a different endpoint (and pipe) from “out” endpoint two. The current configuration controls the existence, type, and maximum packet size of any given endpoint.

status

  This is read in non-iso completion functions to get the status of the particular request. ISO requests only use it to tell whether the URB was unlinked; detailed status for each frame is in the fields of the iso_frame-desc.

transfer_flags

  A variety of flags may be used to affect how URB submission, unlinking, or operation are handled. Different kinds of URB can use different flags.

transfer_buffer

  This identifies the buffer to (or from) which the I/O request will be performed (unless URB_NO_TRANSFER_DMA_MAP is set). This buffer must be suitable for DMA; allocate it with `kmalloc` or equivalent. For transfers to “in” endpoints, contents of this buffer will be modified. This buffer is used for the data stage of control transfers.

transfer_dma

  When transfer_flags includes URB_NO_TRANSFER_DMA_MAP, the device driver is saying that it provided this DMA address, which the host controller driver should use in preference to the transfer_buffer.

transfer_buffer_length

  How big is transfer_buffer. The transfer may be broken up into chunks according to the current maximum packet size for the endpoint, which is a function of the configuration and is encoded in the pipe. When the length is zero, neither transfer_buffer nor transfer_dma is used.

actual_length

  This is read in non-iso completion functions, and it tells how many bytes (out of transfer_buffer_length) were transferred. It will normally be the same as requested, unless either an error was reported or a short read was performed. The URB_SHORT_NOT_OK transfer flag may be used to make such short reads be reported as errors.

setup_packet

  Only used for control transfers, this points to eight bytes of setup data. Control transfers always start by sending this data to the device. Then transfer_buffer is read or written, if needed.

setup_dma

  For control transfers with URB_NO_SETUP_DMA_MAP set, the device driver has provided this DMA address for the setup packet. The host controller driver should use this in preference to setup_packet.

start_frame

  Returns the initial frame for isochronous transfers.

number_of_packets

  Lists the number of ISO transfer buffers.

interval

  Specifies the polling interval for interrupt or isochronous transfers. The units are frames (milliseconds) for for full and low speed devices, and microframes (1/8 millisecond) for highspeed ones.

error_count

  Returns the number of ISO transfers that reported errors.

context

  For use in completion functions. This normally points to request-specific driver context.

complete

  Completion handler. This URB is passed as the parameter to the completion function. The completion function may then do what it likes with the URB, including resubmitting or freeing it.

iso_frame_desc[0]

  Used to provide arrays of ISO transfer buffers and to collect the transfer status for each buffer.

## Description

This structure identifies USB transfer requests. URBs must be allocated by calling `usb_alloc_urb` and freed with a call to `usb_free_urb`. Initialization may be done using various usb_fill_*`_urb` functions. URBs are submitted using `usb_submit_urb`, and pending requests may be canceled using `usb_unlink_urb` or `usb_kill_urb`.

## Data Transfer Buffers

Normally drivers provide I/O buffers allocated with `kmalloc` or otherwise taken from the general page pool. That is provided by transfer_buffer (control requests also use setup_packet), and host controller drivers perform a dma mapping (and unmapping) for each buffer transferred. Those mapping operations can be expensive on some platforms (perhaps using a dma bounce buffer or talking to an IOMMU), although they're cheap on commodity x86 and ppc hardware.

Alternatively, drivers may pass the URB_NO_xxx_DMA_MAP transfer flags, which tell the host controller driver that no such mapping is needed since the device driver is DMA-aware. For example, a device driver might allocate a DMA buffer with `usb_buffer_alloc` or call `usb_buffer_map`. When these transfer flags are provided, host controller drivers will attempt to use the dma addresses found in the transfer_dma and/or 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).

## Initialization

All URBs submitted must initialize the dev, pipe, transfer_flags (may be zero), and complete fields. All URBs must also initialize transfer_buffer and transfer_buffer_length. They may provide the URB_SHORT_NOT_OK transfer flag, indicating that short reads are to be treated as errors; that flag is invalid for write requests.

Bulk URBs may use the URB_ZERO_PACKET transfer flag, indicating that bulk OUT transfers should always terminate with a short packet, even if it means adding an extra zero length packet.

Control URBs must provide a setup_packet. The setup_packet and transfer_buffer may each be mapped for DMA or not, independently of the other. The transfer_flags bits URB_NO_TRANSFER_DMA_MAP and URB_NO_SETUP_DMA_MAP indicate which buffers have already been mapped. URB_NO_SETUP_DMA_MAP is ignored for non-control URBs.

Interrupt URBs must provide an interval, saying how often (in milliseconds or, for highspeed devices, 125 microsecond units) to poll for transfers. After the URB has been submitted, the interval field reflects how the transfer was actually scheduled. The polling interval may be more frequent than requested. For example, some controllers have a maximum interval of 32 milliseconds, while others support intervals of up to 1024 milliseconds. Isochronous URBs also have transfer intervals. (Note that for isochronous endpoints, as well as high speed interrupt endpoints, the encoding of the transfer interval in the endpoint descriptor is logarithmic. Device drivers must convert that value to linear units themselves.)

Isochronous URBs normally use the URB_ISO_ASAP transfer flag, telling the host controller to schedule the transfer as soon as bandwidth utilization allows, and then set start_frame to reflect the actual frame selected during submission. Otherwise drivers must specify the start_frame and handle the case where the transfer can't begin then. However, drivers won't know how bandwidth is currently allocated, and while they can find the current frame using usb_get_current_frame_number () they can't know the range for that frame number. (Ranges for frame counter values are HC-specific, and can go from 256 to 65536 frames from “now”.)

Isochronous URBs have a different data transfer model, in part because the quality of service is only “best effort”. Callers provide specially allocated URBs, with number_of_packets worth of iso_frame_desc structures at the end. Each such packet is an individual ISO transfer. Isochronous URBs are normally queued, submitted by drivers to arrange that transfers are at least double buffered, and then explicitly resubmitted in completion handlers, so that data (such as audio or video) streams at as constant a rate as the host controller scheduler can support.

## Completion Callbacks

The completion callback is made `in_interrupt`, and one of the first things that a completion handler should do is check the status field. The status field is provided for all URBs. It is used to report unlinked URBs, and status for all non-ISO transfers. It should not be examined before the URB is returned to the completion handler.

The context field is normally used to link URBs back to the relevant driver or request state.

When the completion callback is invoked for non-isochronous URBs, the actual_length field tells how many bytes were transferred. This field is updated even when the URB terminated with an error or was unlinked.

ISO transfer status is reported in the status and actual_length fields of the iso_frame_desc array, and the number of errors is reported in error_count. Completion callbacks for ISO transfers will normally (re)submit URBs to ensure a constant transfer rate.

Note that even fields marked “public” should not be touched by the driver when the urb is owned by the hcd, that is, since the call to `usb_submit_urb` till the entry into the completion routine.

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