diff options
Diffstat (limited to 'Documentation')
| -rw-r--r-- | Documentation/usb/URB.txt | 74 |
1 files changed, 31 insertions, 43 deletions
diff --git a/Documentation/usb/URB.txt b/Documentation/usb/URB.txt index d59b95cc6f1..a49e5f2c2b4 100644 --- a/Documentation/usb/URB.txt +++ b/Documentation/usb/URB.txt @@ -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. |