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usb_sg_wait(9) [centos man page]

USB_SG_WAIT(9)							   USB Core APIs						    USB_SG_WAIT(9)

usb_sg_wait - synchronously execute scatter/gather request SYNOPSIS
void usb_sg_wait(struct usb_sg_request * io); ARGUMENTS
io request block handle, as initialized with usb_sg_init. some fields become accessible when this call returns. CONTEXT
!in_interrupt () DESCRIPTION
This function blocks until the specified I/O operation completes. It leverages the grouping of the related I/O requests to get good transfer rates, by queueing the requests. At higher speeds, such queuing can significantly improve USB throughput. There are three kinds of completion for this function. (1) success, where io->status is zero. The number of io->bytes transferred is as requested. (2) error, where io->status is a negative errno value. The number of io->bytes transferred before the error is usually less than requested, and can be nonzero. (3) cancellation, a type of error with status -ECONNRESET that is initiated by usb_sg_cancel. When this function returns, all memory allocated through usb_sg_init or this call will have been freed. The request block parameter may still be passed to usb_sg_cancel, or it may be freed. It could also be reinitialized and then reused. DATA TRANSFER RATES
Bulk transfers are valid for full or high speed endpoints. The best full speed data rate is 19 packets of 64 bytes each per frame, or 1216 bytes per millisecond. The best high speed data rate is 13 packets of 512 bytes each per microframe, or 52 KBytes per millisecond. The reason to use interrupt transfers through this API would most likely be to reserve high speed bandwidth, where up to 24 KBytes per millisecond could be transferred. That capability is less useful for low or full speed interrupt endpoints, which allow at most one packet per millisecond, of at most 8 or 64 bytes (respectively). It is not necessary to call this function to reserve bandwidth for devices under an xHCI host controller, as the bandwidth is reserved when the configuration or interface alt setting is selected. COPYRIGHT
Kernel Hackers Manual 3.10 June 2014 USB_SG_WAIT(9)

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USB_SUBMIT_URB(9)						   USB Core APIs						 USB_SUBMIT_URB(9)

usb_submit_urb - issue an asynchronous transfer request for an endpoint SYNOPSIS
int usb_submit_urb(struct urb * urb, gfp_t mem_flags); ARGUMENTS
urb pointer to the urb describing the request mem_flags the type of memory to allocate, see kmalloc for a list of valid options for this. DESCRIPTION
This submits a transfer request, and transfers control of the URB describing that request to the USB subsystem. Request completion will be indicated later, asynchronously, by calling the completion handler. The three types of completion are success, error, and unlink (a software-induced fault, also called "request cancellation"). URBs may be submitted in interrupt context. The caller must have correctly initialized the URB before submitting it. Functions such as usb_fill_bulk_urb and usb_fill_control_urb are available to ensure that most fields are correctly initialized, for the particular kind of transfer, although they will not initialize any transfer flags. If the submission is successful, the complete callback from the URB will be called exactly once, when the USB core and Host Controller Driver (HCD) are finished with the URB. When the completion function is called, control of the URB is returned to the device driver which issued the request. The completion handler may then immediately free or reuse that URB. With few exceptions, USB device drivers should never access URB fields provided by usbcore or the HCD until its complete is called. The exceptions relate to periodic transfer scheduling. For both interrupt and isochronous urbs, as part of successful URB submission urb->interval is modified to reflect the actual transfer period used (normally some power of two units). And for isochronous urbs, urb->start_frame is modified to reflect when the URB's transfers were scheduled to start. Not all isochronous transfer scheduling policies will work, but most host controller drivers should easily handle ISO queues going from now until 10-200 msec into the future. Drivers should try to keep at least one or two msec of data in the queue; many controllers require that new transfers start at least 1 msec in the future when they are added. If the driver is unable to keep up and the queue empties out, the behavior for new submissions is governed by the URB_ISO_ASAP flag. If the flag is set, or if the queue is idle, then the URB is always assigned to the first available (and not yet expired) slot in the endpoint's schedule. If the flag is not set and the queue is active then the URB is always assigned to the next slot in the schedule following the end of the endpoint's previous URB, even if that slot is in the past. When a packet is assigned in this way to a slot that has already expired, the packet is not transmitted and the corresponding usb_iso_packet_descriptor's status field will return -EXDEV. If this would happen to all the packets in the URB, submission fails with a -EXDEV error code. For control endpoints, the synchronous usb_control_msg call is often used (in non-interrupt context) instead of this call. That is often used through convenience wrappers, for the requests that are standardized in the USB 2.0 specification. For bulk endpoints, a synchronous usb_bulk_msg call is available. RETURN
0 on successful submissions. A negative error number otherwise. REQUEST QUEUING
URBs may be submitted to endpoints before previous ones complete, to minimize the impact of interrupt latencies and system overhead on data throughput. With that queuing policy, an endpoint's queue would never be empty. This is required for continuous isochronous data streams, and may also be required for some kinds of interrupt transfers. Such queuing also maximizes bandwidth utilization by letting USB controllers start work on later requests before driver software has finished the completion processing for earlier (successful) requests. As of Linux 2.6, all USB endpoint transfer queues support depths greater than one. This was previously a HCD-specific behavior, except for ISO transfers. Non-isochronous endpoint queues are inactive during cleanup after faults (transfer errors or cancellation). RESERVED BANDWIDTH TRANSFERS
Periodic transfers (interrupt or isochronous) are performed repeatedly, using the interval specified in the urb. Submitting the first urb to the endpoint reserves the bandwidth necessary to make those transfers. If the USB subsystem can't allocate sufficient bandwidth to perform the periodic request, submitting such a periodic request should fail. For devices under xHCI, the bandwidth is reserved at configuration time, or when the alt setting is selected. If there is not enough bus bandwidth, the configuration/alt setting request will fail. Therefore, submissions to periodic endpoints on devices under xHCI should never fail due to bandwidth constraints. Device drivers must explicitly request that repetition, by ensuring that some URB is always on the endpoint's queue (except possibly for short periods during completion callacks). When there is no longer an urb queued, the endpoint's bandwidth reservation is canceled. This means drivers can use their completion handlers to ensure they keep bandwidth they need, by reinitializing and resubmitting the just-completed urb until the driver longer needs that periodic bandwidth. MEMORY FLAGS
The general rules for how to decide which mem_flags to use are the same as for kmalloc. There are four different possible values; GFP_KERNEL, GFP_NOFS, GFP_NOIO and GFP_ATOMIC. GFP_NOFS is not ever used, as it has not been implemented yet. GFP_ATOMIC is used when (a) you are inside a completion handler, an interrupt, bottom half, tasklet or timer, or (b) you are holding a spinlock or rwlock (does not apply to semaphores), or (c) current->state != TASK_RUNNING, this is the case only after you've changed it. GFP_NOIO is used in the block io path and error handling of storage devices. All other situations use GFP_KERNEL. Some more specific rules for mem_flags can be inferred, such as (1) start_xmit, timeout, and receive methods of network drivers must use GFP_ATOMIC (they are called with a spinlock held); (2) queuecommand methods of scsi drivers must use GFP_ATOMIC (also called with a spinlock held); (3) If you use a kernel thread with a network driver you must use GFP_NOIO, unless (b) or (c) apply; (4) after you have done a down you can use GFP_KERNEL, unless (b) or (c) apply or your are in a storage driver's block io path; (5) USB probe and disconnect can use GFP_KERNEL unless (b) or (c) apply; and (6) changing firmware on a running storage or net device uses GFP_NOIO, unless b) or c) apply COPYRIGHT
Kernel Hackers Manual 3.10 June 2014 USB_SUBMIT_URB(9)
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