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[PATCH] USB: Update Documentation/usb/URB.txt

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This patch (as564) updates Documentation/usb/URB.txt, bringing it roughly
up to the current level.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
This commit is contained in:
Alan Stern 2005-09-22 00:49:51 -07:00 committed by Linus Torvalds
parent c6c88834b2
commit 0fc084eaff
1 changed files with 31 additions and 43 deletions
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74
Documentation/usb/URB.txt
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@ -1,5 +1,6 @@
Revised: 2000-Dec-05.
Again: 2002-Jul-06
Again: 2005-Sep-19
NOTE:
@ -18,8 +19,8 @@ called USB Request Block, or URB for short.
and deliver the data and status back.
- Execution of an URB is inherently an asynchronous operation, i.e. the
usb_submit_urb(urb) call returns immediately after it has successfully queued
the requested action.
usb_submit_urb(urb) call returns immediately after it has successfully
queued the requested action.
- Transfers for one URB can be canceled with usb_unlink_urb(urb) at any time.
@ -94,8 +95,9 @@ To free an URB, use
void usb_free_urb(struct urb *urb)
You may not free an urb that you've submitted, but which hasn't yet been
returned to you in a completion callback.
You may free an urb that you've submitted, but which hasn't yet been
returned to you in a completion callback. It will automatically be
deallocated when it is no longer in use.
1.4. What has to be filled in?
@ -145,30 +147,36 @@ to get seamless ISO streaming.
1.6. How to cancel an already running URB?
For an URB which you've submitted, but which hasn't been returned to
your driver by the host controller, call
There are two ways to cancel an URB you've submitted but which hasn't
been returned to your driver yet. For an asynchronous cancel, call
int usb_unlink_urb(struct urb *urb)
It removes the urb from the internal list and frees all allocated
HW descriptors. The status is changed to reflect unlinking. After
usb_unlink_urb() returns with that status code, you can free the URB
with usb_free_urb().
HW descriptors. The status is changed to reflect unlinking. Note
that the URB will not normally have finished when usb_unlink_urb()
returns; you must still wait for the completion handler to be called.
There is also an asynchronous unlink mode. To use this, set the
the URB_ASYNC_UNLINK flag in urb->transfer flags before calling
usb_unlink_urb(). When using async unlinking, the URB will not
normally be unlinked when usb_unlink_urb() returns. Instead, wait
for the completion handler to be called.
To cancel an URB synchronously, call
void usb_kill_urb(struct urb *urb)
It does everything usb_unlink_urb does, and in addition it waits
until after the URB has been returned and the completion handler
has finished. It also marks the URB as temporarily unusable, so
that if the completion handler or anyone else tries to resubmit it
they will get a -EPERM error. Thus you can be sure that when
usb_kill_urb() returns, the URB is totally idle.
1.7. What about the completion handler?
The handler is of the following type:
typedef void (*usb_complete_t)(struct urb *);
typedef void (*usb_complete_t)(struct urb *, struct pt_regs *)
i.e. it gets just the URB that caused the completion call.
I.e., it gets the URB that caused the completion call, plus the
register values at the time of the corresponding interrupt (if any).
In the completion handler, you should have a look at urb->status to
detect any USB errors. Since the context parameter is included in the URB,
you can pass information to the completion handler.
@ -176,17 +184,11 @@ you can pass information to the completion handler.
Note that even when an error (or unlink) is reported, data may have been
transferred. That's because USB transfers are packetized; it might take
sixteen packets to transfer your 1KByte buffer, and ten of them might
have transferred succesfully before the completion is called.
have transferred succesfully before the completion was called.
NOTE: ***** WARNING *****
Don't use urb->dev field in your completion handler; it's cleared
as part of giving urbs back to drivers. (Addressing an issue with
ownership of periodic URBs, which was otherwise ambiguous.) Instead,
use urb->context to hold all the data your driver needs.
NOTE: ***** WARNING *****
Also, NEVER SLEEP IN A COMPLETION HANDLER. These are normally called
NEVER SLEEP IN A COMPLETION HANDLER. These are normally called
during hardware interrupt processing. If you can, defer substantial
work to a tasklet (bottom half) to keep system latencies low. You'll
probably need to use spinlocks to protect data structures you manipulate
@ -229,24 +231,10 @@ ISO data with some other event stream.
Interrupt transfers, like isochronous transfers, are periodic, and happen
in intervals that are powers of two (1, 2, 4 etc) units. Units are frames
for full and low speed devices, and microframes for high speed ones.
Currently, after you submit one interrupt URB, that urb is owned by the
host controller driver until you cancel it with usb_unlink_urb(). You
may unlink interrupt urbs in their completion handlers, if you need to.
After a transfer completion is called, the URB is automagically resubmitted.
THIS BEHAVIOR IS EXPECTED TO BE REMOVED!!
Interrupt transfers may only send (or receive) the "maxpacket" value for
the given interrupt endpoint; if you need more data, you will need to
copy that data out of (or into) another buffer. Similarly, you can't
queue interrupt transfers.
THESE RESTRICTIONS ARE EXPECTED TO BE REMOVED!!
Note that this automagic resubmission model does make it awkward to use
interrupt OUT transfers. The portable solution involves unlinking those
OUT urbs after the data is transferred, and perhaps submitting a final
URB for a short packet.
The usb_submit_urb() call modifies urb->interval to the implemented interval
value that is less than or equal to the requested interval value.
In Linux 2.6, unlike earlier versions, interrupt URBs are not automagically
restarted when they complete. They end when the completion handler is
called, just like other URBs. If you want an interrupt URB to be restarted,
your completion handler must resubmit it.