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struct_usb_request
Langue: en
Version: October 2010 (fedora - 01/12/10)
Section: 9 (Appels noyau Linux)
NAME
struct_usb_request - describes one i/o requestSYNOPSIS
struct usb_request { void * buf; unsigned length; dma_addr_t dma; unsigned no_interrupt:1; unsigned zero:1; unsigned short_not_ok:1; void (* complete) (struct usb_ep *ep,struct usb_request *req); void * context; struct list_head list; int status; unsigned actual; };
MEMBERS
buf
- Buffer used for data. Always provide this; some controllers only use PIO, or don't use DMA for some endpoints.
length
- Length of that data
dma
- DMA address corresponding to 'buf'. If you don't set this field, and the usb controller needs one, it is responsible for mapping and unmapping the buffer.
no_interrupt
- If true, hints that no completion irq is needed. Helpful sometimes with deep request queues that are handled directly by DMA controllers.
zero
- If true, when writing data, makes the last packet be lqshortrq by adding a zero length packet as needed;
short_not_ok
- When reading data, makes short packets be treated as errors (queue stops advancing till cleanup).
complete
- Function called when request completes, so this request and its buffer may be re-used. The function will always be called with interrupts disabled, and it must not sleep. Reads terminate with a short packet, or when the buffer fills, whichever comes first. When writes terminate, some data bytes will usually still be in flight (often in a hardware fifo). Errors (for reads or writes) stop the queue from advancing until the completion function returns, so that any transfers invalidated by the error may first be dequeued.
context
- For use by the completion callback
list
- For use by the gadget driver.
status
- Reports completion code, zero or a negative errno. Normally, faults block the transfer queue from advancing until the completion callback returns. Code lq-ESHUTDOWNrq indicates completion caused by device disconnect, or when the driver disabled the endpoint.
actual
- Reports bytes transferred to/from the buffer. For reads (OUT transfers) this may be less than the requested length. If the short_not_ok flag is set, short reads are treated as errors even when status otherwise indicates successful completion. Note that for writes (IN transfers) some data bytes may still reside in a device-side FIFO when the request is reported as complete.
DESCRIPTION
These are allocated/freed through the endpoint they're used with. The hardware's driver can add extra per-request data to the memory it returns, which often avoids separate memory allocations (potential failures), later when the request is queued.
Request flags affect request handling, such as whether a zero length packet is written (the lqzerorq flag), whether a short read should be treated as an error (blocking request queue advance, the lqshort_not_okrq flag), or hinting that an interrupt is not required (the lqno_interruptrq flag, for use with deep request queues).
Bulk endpoints can use any size buffers, and can also be used for interrupt transfers. interrupt-only endpoints can be much less functional.
NOTE
this is analagous to 'struct urb' on the host side, except that it's thinner and promotes more pre-allocation.
AUTHOR
David Brownell <dbrownell@users.sourceforge.net>
- Author.
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