NAME¶
struct_usb_request - describes one i/o request
SYNOPSIS¶
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
“short” 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 “-ESHUTDOWN” 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 “zero” flag), whether a short
read should be treated as an error (blocking request queue advance, the
“short_not_ok” flag), or hinting that an interrupt is not
required (the “no_interrupt” 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.