STRUCT URB(9) Host-Side Data Types and Macro STRUCT URB(9)NAMEstruct_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;
unsigned int stream_id;
int status;
unsigned int transfer_flags;
void * transfer_buffer;
dma_addr_t transfer_dma;
struct scatterlist * sg;
int num_mapped_sgs;
int num_sgs;
u32 transfer_buffer_length;
u32 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.
stream_id
the endpoint's stream ID for bulk streams
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 (however, do not
leave garbage in transfer_buffer even then). 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.
sg
scatter gather buffer list
num_mapped_sgs
(internal) number of mapped sg entries
num_sgs
number of entries in the sg list
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
DMA pointer for the setup packet. The caller must not use this
field; setup_packet must point to a valid buffer.
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 full and low
speed devices, and microframes (1/8 millisecond) for highspeed and
SuperSpeed devices.
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_TRANSFER_DMA_MAP transfer
flag, which tells the host controller driver that no such mapping is
needed for the transfer_buffer since the device driver is DMA-aware.
For example, a device driver might allocate a DMA buffer with
usb_alloc_coherent or call usb_buffer_map. When this transfer flag is
provided, host controller drivers will attempt to use the dma address
found in the transfer_dma field 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.
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 valid pointer in the setup_packet field.
Unlike the transfer_buffer, the setup_packet may not be mapped for DMA
beforehand.
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.)
If an isochronous endpoint queue isn't already running, the host
controller will schedule a new URB to start as soon as bandwidth
utilization allows. If the queue is running then a new URB will be
scheduled to start in the first transfer slot following the end of the
preceding URB, if that slot has not already expired. If the slot has
expired (which can happen when IRQ delivery is delayed for a long
time), the scheduling behavior depends on the URB_ISO_ASAP flag. If the
flag is clear then the URB will be scheduled to start in the expired
slot, implying that some of its packets will not be transferred; if the
flag is set then the URB will be scheduled in the first unexpired slot,
breaking the queue's synchronization. Upon URB completion, the
start_frame field will be set to the (micro)frame number in which the
transfer was scheduled. Ranges for frame counter values are HC-specific
and can go from as low as 256 to as high as 65536 frames.
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.
COPYRIGHTKernel Hackers Manual 3.11 November 2013 STRUCT URB(9)
