291 lines
11 KiB
ReStructuredText
291 lines
11 KiB
ReStructuredText
.. _usb-urb:
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USB Request Block (URB)
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~~~~~~~~~~~~~~~~~~~~~~~
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:Revised: 2000-Dec-05
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:Again: 2002-Jul-06
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:Again: 2005-Sep-19
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:Again: 2017-Mar-29
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.. note::
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The USB subsystem now has a substantial section at :ref:`usb-hostside-api`
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section, generated from the current source code.
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This particular documentation file isn't complete and may not be
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updated to the last version; don't rely on it except for a quick
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overview.
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Basic concept or 'What is an URB?'
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==================================
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The basic idea of the new driver is message passing, the message itself is
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called USB Request Block, or URB for short.
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- An URB consists of all relevant information to execute any USB transaction
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and deliver the data and status back.
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- Execution of an URB is inherently an asynchronous operation, i.e. the
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:c:func:`usb_submit_urb` call returns immediately after it has successfully
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queued the requested action.
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- Transfers for one URB can be canceled with :c:func:`usb_unlink_urb`
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at any time.
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- Each URB has a completion handler, which is called after the action
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has been successfully completed or canceled. The URB also contains a
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context-pointer for passing information to the completion handler.
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- Each endpoint for a device logically supports a queue of requests.
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You can fill that queue, so that the USB hardware can still transfer
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data to an endpoint while your driver handles completion of another.
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This maximizes use of USB bandwidth, and supports seamless streaming
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of data to (or from) devices when using periodic transfer modes.
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The URB structure
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=================
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Some of the fields in struct urb are::
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struct urb
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{
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// (IN) device and pipe specify the endpoint queue
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struct usb_device *dev; // pointer to associated USB device
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unsigned int pipe; // endpoint information
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unsigned int transfer_flags; // URB_ISO_ASAP, URB_SHORT_NOT_OK, etc.
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// (IN) all urbs need completion routines
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void *context; // context for completion routine
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usb_complete_t complete; // pointer to completion routine
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// (OUT) status after each completion
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int status; // returned status
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// (IN) buffer used for data transfers
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void *transfer_buffer; // associated data buffer
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u32 transfer_buffer_length; // data buffer length
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int number_of_packets; // size of iso_frame_desc
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// (OUT) sometimes only part of CTRL/BULK/INTR transfer_buffer is used
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u32 actual_length; // actual data buffer length
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// (IN) setup stage for CTRL (pass a struct usb_ctrlrequest)
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unsigned char *setup_packet; // setup packet (control only)
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// Only for PERIODIC transfers (ISO, INTERRUPT)
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// (IN/OUT) start_frame is set unless URB_ISO_ASAP isn't set
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int start_frame; // start frame
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int interval; // polling interval
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// ISO only: packets are only "best effort"; each can have errors
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int error_count; // number of errors
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struct usb_iso_packet_descriptor iso_frame_desc[0];
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};
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Your driver must create the "pipe" value using values from the appropriate
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endpoint descriptor in an interface that it's claimed.
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How to get an URB?
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==================
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URBs are allocated by calling :c:func:`usb_alloc_urb`::
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struct urb *usb_alloc_urb(int isoframes, int mem_flags)
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Return value is a pointer to the allocated URB, 0 if allocation failed.
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The parameter isoframes specifies the number of isochronous transfer frames
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you want to schedule. For CTRL/BULK/INT, use 0. The mem_flags parameter
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holds standard memory allocation flags, letting you control (among other
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things) whether the underlying code may block or not.
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To free an URB, use :c:func:`usb_free_urb`::
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void usb_free_urb(struct urb *urb)
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You may free an urb that you've submitted, but which hasn't yet been
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returned to you in a completion callback. It will automatically be
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deallocated when it is no longer in use.
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What has to be filled in?
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=========================
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Depending on the type of transaction, there are some inline functions
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defined in ``linux/usb.h`` to simplify the initialization, such as
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:c:func:`usb_fill_control_urb`, :c:func:`usb_fill_bulk_urb` and
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:c:func:`usb_fill_int_urb`. In general, they need the usb device pointer,
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the pipe (usual format from usb.h), the transfer buffer, the desired transfer
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length, the completion handler, and its context. Take a look at the some
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existing drivers to see how they're used.
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Flags:
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- For ISO there are two startup behaviors: Specified start_frame or ASAP.
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- For ASAP set ``URB_ISO_ASAP`` in transfer_flags.
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If short packets should NOT be tolerated, set ``URB_SHORT_NOT_OK`` in
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transfer_flags.
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How to submit an URB?
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=====================
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Just call :c:func:`usb_submit_urb`::
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int usb_submit_urb(struct urb *urb, int mem_flags)
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The ``mem_flags`` parameter, such as ``GFP_ATOMIC``, controls memory
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allocation, such as whether the lower levels may block when memory is tight.
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It immediately returns, either with status 0 (request queued) or some
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error code, usually caused by the following:
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- Out of memory (``-ENOMEM``)
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- Unplugged device (``-ENODEV``)
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- Stalled endpoint (``-EPIPE``)
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- Too many queued ISO transfers (``-EAGAIN``)
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- Too many requested ISO frames (``-EFBIG``)
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- Invalid INT interval (``-EINVAL``)
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- More than one packet for INT (``-EINVAL``)
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After submission, ``urb->status`` is ``-EINPROGRESS``; however, you should
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never look at that value except in your completion callback.
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For isochronous endpoints, your completion handlers should (re)submit
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URBs to the same endpoint with the ``URB_ISO_ASAP`` flag, using
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multi-buffering, to get seamless ISO streaming.
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How to cancel an already running URB?
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=====================================
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There are two ways to cancel an URB you've submitted but which hasn't
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been returned to your driver yet. For an asynchronous cancel, call
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:c:func:`usb_unlink_urb`::
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int usb_unlink_urb(struct urb *urb)
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It removes the urb from the internal list and frees all allocated
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HW descriptors. The status is changed to reflect unlinking. Note
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that the URB will not normally have finished when :c:func:`usb_unlink_urb`
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returns; you must still wait for the completion handler to be called.
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To cancel an URB synchronously, call :c:func:`usb_kill_urb`::
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void usb_kill_urb(struct urb *urb)
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It does everything :c:func:`usb_unlink_urb` does, and in addition it waits
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until after the URB has been returned and the completion handler
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has finished. It also marks the URB as temporarily unusable, so
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that if the completion handler or anyone else tries to resubmit it
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they will get a ``-EPERM`` error. Thus you can be sure that when
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:c:func:`usb_kill_urb` returns, the URB is totally idle.
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There is a lifetime issue to consider. An URB may complete at any
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time, and the completion handler may free the URB. If this happens
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while :c:func:`usb_unlink_urb` or :c:func:`usb_kill_urb` is running, it will
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cause a memory-access violation. The driver is responsible for avoiding this,
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which often means some sort of lock will be needed to prevent the URB
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from being deallocated while it is still in use.
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On the other hand, since usb_unlink_urb may end up calling the
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completion handler, the handler must not take any lock that is held
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when usb_unlink_urb is invoked. The general solution to this problem
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is to increment the URB's reference count while holding the lock, then
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drop the lock and call usb_unlink_urb or usb_kill_urb, and then
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decrement the URB's reference count. You increment the reference
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count by calling :c:func`usb_get_urb`::
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struct urb *usb_get_urb(struct urb *urb)
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(ignore the return value; it is the same as the argument) and
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decrement the reference count by calling :c:func:`usb_free_urb`. Of course,
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none of this is necessary if there's no danger of the URB being freed
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by the completion handler.
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What about the completion handler?
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==================================
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The handler is of the following type::
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typedef void (*usb_complete_t)(struct urb *)
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I.e., it gets the URB that caused the completion call. In the completion
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handler, you should have a look at ``urb->status`` to detect any USB errors.
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Since the context parameter is included in the URB, you can pass
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information to the completion handler.
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Note that even when an error (or unlink) is reported, data may have been
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transferred. That's because USB transfers are packetized; it might take
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sixteen packets to transfer your 1KByte buffer, and ten of them might
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have transferred successfully before the completion was called.
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.. warning::
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NEVER SLEEP IN A COMPLETION HANDLER.
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These are often called in atomic context.
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In the current kernel, completion handlers run with local interrupts
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disabled, but in the future this will be changed, so don't assume that
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local IRQs are always disabled inside completion handlers.
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How to do isochronous (ISO) transfers?
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======================================
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Besides the fields present on a bulk transfer, for ISO, you also
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have to set ``urb->interval`` to say how often to make transfers; it's
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often one per frame (which is once every microframe for highspeed devices).
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The actual interval used will be a power of two that's no bigger than what
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you specify. You can use the :c:func:`usb_fill_int_urb` macro to fill
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most ISO transfer fields.
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For ISO transfers you also have to fill a :c:type:`usb_iso_packet_descriptor`
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structure, allocated at the end of the URB by :c:func:`usb_alloc_urb`, for
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each packet you want to schedule.
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The :c:func:`usb_submit_urb` call modifies ``urb->interval`` to the implemented
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interval value that is less than or equal to the requested interval value. If
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``URB_ISO_ASAP`` scheduling is used, ``urb->start_frame`` is also updated.
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For each entry you have to specify the data offset for this frame (base is
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transfer_buffer), and the length you want to write/expect to read.
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After completion, actual_length contains the actual transferred length and
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status contains the resulting status for the ISO transfer for this frame.
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It is allowed to specify a varying length from frame to frame (e.g. for
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audio synchronisation/adaptive transfer rates). You can also use the length
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0 to omit one or more frames (striping).
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For scheduling you can choose your own start frame or ``URB_ISO_ASAP``. As
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explained earlier, if you always keep at least one URB queued and your
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completion keeps (re)submitting a later URB, you'll get smooth ISO streaming
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(if usb bandwidth utilization allows).
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If you specify your own start frame, make sure it's several frames in advance
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of the current frame. You might want this model if you're synchronizing
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ISO data with some other event stream.
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How to start interrupt (INT) transfers?
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=======================================
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Interrupt transfers, like isochronous transfers, are periodic, and happen
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in intervals that are powers of two (1, 2, 4 etc) units. Units are frames
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for full and low speed devices, and microframes for high speed ones.
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You can use the :c:func:`usb_fill_int_urb` macro to fill INT transfer fields.
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The :c:func:`usb_submit_urb` call modifies ``urb->interval`` to the implemented
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interval value that is less than or equal to the requested interval value.
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In Linux 2.6, unlike earlier versions, interrupt URBs are not automagically
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restarted when they complete. They end when the completion handler is
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called, just like other URBs. If you want an interrupt URB to be restarted,
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your completion handler must resubmit it.
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s
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