[patch 18/18] USB: Update Documentation/usb/URB.txt

Greg KH
2005-09-22T07:50:44Z

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From: Alan Stern <stern@rowland.harvard.edu>

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>

---
Documentation/usb/URB.txt | 76 +++++++++++++++++++---------------------------
1 file changed, 32 insertions(+), 44 deletions(-)

--- scsi-2.6.orig/Documentation/usb/URB.txt 2005-09-20 05:59:35.000000000 -0700
+++ scsi-2.6/Documentation/usb/URB.txt 2005-09-21 17:29:56.000000000 -0700
@@ -1,5 +1,6 @@
Revised: 2000-Dec-05.
Again: 2002-Jul-06
+Again: 2005-Sep-19

NOTE:

@@ -18,8 +19,8 @@
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 @@

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 @@

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().
-
-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.
+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.
+
+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 @@
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 @@
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.

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