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-Revised: 2000-Dec-05.
-Again: 2002-Jul-06
-Again: 2005-Sep-19
-
- NOTE:
-
- The USB subsystem now has a substantial section in "The Linux Kernel API"
- guide (in Documentation/DocBook), generated from the current source
- code. This particular documentation file isn't particularly current or
- complete; don't rely on it except for a quick overview.
-
-
-1.1. Basic concept or 'What is an URB?'
-
-The basic idea of the new driver is message passing, the message itself is
-called USB Request Block, or URB for short.
-
-- An URB consists of all relevant information to execute any USB transaction
- 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.
-
-- Transfers for one URB can be canceled with usb_unlink_urb(urb) at any time.
-
-- Each URB has a completion handler, which is called after the action
- has been successfully completed or canceled. The URB also contains a
- context-pointer for passing information to the completion handler.
-
-- Each endpoint for a device logically supports a queue of requests.
- You can fill that queue, so that the USB hardware can still transfer
- data to an endpoint while your driver handles completion of another.
- This maximizes use of USB bandwidth, and supports seamless streaming
- of data to (or from) devices when using periodic transfer modes.
-
-
-1.2. The URB structure
-
-Some of the fields in an URB are:
-
-struct urb
-{
-// (IN) device and pipe specify the endpoint queue
- struct usb_device *dev; // pointer to associated USB device
- unsigned int pipe; // endpoint information
-
- unsigned int transfer_flags; // ISO_ASAP, SHORT_NOT_OK, etc.
-
-// (IN) all urbs need completion routines
- void *context; // context for completion routine
- void (*complete)(struct urb *); // pointer to completion routine
-
-// (OUT) status after each completion
- int status; // returned status
-
-// (IN) buffer used for data transfers
- void *transfer_buffer; // associated data buffer
- int transfer_buffer_length; // data buffer length
- int number_of_packets; // size of iso_frame_desc
-
-// (OUT) sometimes only part of CTRL/BULK/INTR transfer_buffer is used
- int actual_length; // actual data buffer length
-
-// (IN) setup stage for CTRL (pass a struct usb_ctrlrequest)
- unsigned char* setup_packet; // setup packet (control only)
-
-// Only for PERIODIC transfers (ISO, INTERRUPT)
- // (IN/OUT) start_frame is set unless ISO_ASAP isn't set
- int start_frame; // start frame
- int interval; // polling interval
-
- // ISO only: packets are only "best effort"; each can have errors
- int error_count; // number of errors
- struct usb_iso_packet_descriptor iso_frame_desc[0];
-};
-
-Your driver must create the "pipe" value using values from the appropriate
-endpoint descriptor in an interface that it's claimed.
-
-
-1.3. How to get an URB?
-
-URBs are allocated with the following call
-
- struct urb *usb_alloc_urb(int isoframes, int mem_flags)
-
-Return value is a pointer to the allocated URB, 0 if allocation failed.
-The parameter isoframes specifies the number of isochronous transfer frames
-you want to schedule. For CTRL/BULK/INT, use 0. The mem_flags parameter
-holds standard memory allocation flags, letting you control (among other
-things) whether the underlying code may block or not.
-
-To free an URB, use
-
- void usb_free_urb(struct urb *urb)
-
-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?
-
-Depending on the type of transaction, there are some inline functions
-defined in <linux/usb.h> to simplify the initialization, such as
-fill_control_urb() and fill_bulk_urb(). In general, they need the usb
-device pointer, the pipe (usual format from usb.h), the transfer buffer,
-the desired transfer length, the completion handler, and its context.
-Take a look at the some existing drivers to see how they're used.
-
-Flags:
-For ISO there are two startup behaviors: Specified start_frame or ASAP.
-For ASAP set URB_ISO_ASAP in transfer_flags.
-
-If short packets should NOT be tolerated, set URB_SHORT_NOT_OK in
-transfer_flags.
-
-
-1.5. How to submit an URB?
-
-Just call
-
- int usb_submit_urb(struct urb *urb, int mem_flags)
-
-The mem_flags parameter, such as SLAB_ATOMIC, controls memory allocation,
-such as whether the lower levels may block when memory is tight.
-
-It immediately returns, either with status 0 (request queued) or some
-error code, usually caused by the following:
-
-- Out of memory (-ENOMEM)
-- Unplugged device (-ENODEV)
-- Stalled endpoint (-EPIPE)
-- Too many queued ISO transfers (-EAGAIN)
-- Too many requested ISO frames (-EFBIG)
-- Invalid INT interval (-EINVAL)
-- More than one packet for INT (-EINVAL)
-
-After submission, urb->status is -EINPROGRESS; however, you should never
-look at that value except in your completion callback.
-
-For isochronous endpoints, your completion handlers should (re)submit
-URBs to the same endpoint with the ISO_ASAP flag, using multi-buffering,
-to get seamless ISO streaming.
-
-
-1.6. How to cancel an already running URB?
-
-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. 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.
-
-There is a lifetime issue to consider. An URB may complete at any
-time, and the completion handler may free the URB. If this happens
-while usb_unlink_urb or usb_kill_urb is running, it will cause a
-memory-access violation. The driver is responsible for avoiding this,
-which often means some sort of lock will be needed to prevent the URB
-from being deallocated while it is still in use.
-
-On the other hand, since usb_unlink_urb may end up calling the
-completion handler, the handler must not take any lock that is held
-when usb_unlink_urb is invoked. The general solution to this problem
-is to increment the URB's reference count while holding the lock, then
-drop the lock and call usb_unlink_urb or usb_kill_urb, and then
-decrement the URB's reference count. You increment the reference
-count by calling
-
- struct urb *usb_get_urb(struct urb *urb)
-
-(ignore the return value; it is the same as the argument) and
-decrement the reference count by calling usb_free_urb. Of course,
-none of this is necessary if there's no danger of the URB being freed
-by the completion handler.
-
-
-1.7. What about the completion handler?
-
-The handler is of the following type:
-
- typedef void (*usb_complete_t)(struct urb *, struct pt_regs *)
-
-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.
-
-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 successfully before the completion was called.
-
-
-NOTE: ***** WARNING *****
-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
-in completion handlers.
-
-
-1.8. How to do isochronous (ISO) transfers?
-
-For ISO transfers you have to fill a usb_iso_packet_descriptor structure,
-allocated at the end of the URB by usb_alloc_urb(n,mem_flags), for each
-packet you want to schedule. You also have to set urb->interval to say
-how often to make transfers; it's often one per frame (which is once
-every microframe for highspeed devices). The actual interval used will
-be a power of two that's no bigger than what you specify.
-
-The usb_submit_urb() call modifies urb->interval to the implemented interval
-value that is less than or equal to the requested interval value. If
-ISO_ASAP scheduling is used, urb->start_frame is also updated.
-
-For each entry you have to specify the data offset for this frame (base is
-transfer_buffer), and the length you want to write/expect to read.
-After completion, actual_length contains the actual transferred length and
-status contains the resulting status for the ISO transfer for this frame.
-It is allowed to specify a varying length from frame to frame (e.g. for
-audio synchronisation/adaptive transfer rates). You can also use the length
-0 to omit one or more frames (striping).
-
-For scheduling you can choose your own start frame or ISO_ASAP. As explained
-earlier, if you always keep at least one URB queued and your completion
-keeps (re)submitting a later URB, you'll get smooth ISO streaming (if usb
-bandwidth utilization allows).
-
-If you specify your own start frame, make sure it's several frames in advance
-of the current frame. You might want this model if you're synchronizing
-ISO data with some other event stream.
-
-
-1.9. How to start interrupt (INT) transfers?
-
-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.
-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.