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struct_urb(9) - phpMan
STRUCT URB(9) Host-Side Data Types and Macro STRUCT URB(9)
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;
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, the buffer size of each element in the list (except the
last) must be divisible by the endpoint's max packet size if no_sg_constraint isn't
set in 'struct usb_bus'
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.
COPYRIGHT
Kernel Hackers Manual 4.8. January 2017 STRUCT URB(9)
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