diff options
Diffstat (limited to '')
| -rw-r--r-- | Documentation/driver-api/usb/URB.rst | 290 |
1 files changed, 290 insertions, 0 deletions
diff --git a/Documentation/driver-api/usb/URB.rst b/Documentation/driver-api/usb/URB.rst new file mode 100644 index 000000000..61a54da9f --- /dev/null +++ b/Documentation/driver-api/usb/URB.rst @@ -0,0 +1,290 @@ +.. _usb-urb: + +USB Request Block (URB) +~~~~~~~~~~~~~~~~~~~~~~~ + +:Revised: 2000-Dec-05 +:Again: 2002-Jul-06 +:Again: 2005-Sep-19 +:Again: 2017-Mar-29 + + +.. note:: + + The USB subsystem now has a substantial section at :ref:`usb-hostside-api` + section, generated from the current source code. + This particular documentation file isn't complete and may not be + updated to the last version; don't rely on it except for a quick + overview. + +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 + :c:func:`usb_submit_urb` call returns immediately after it has successfully + queued the requested action. + +- Transfers for one URB can be canceled with :c:func:`usb_unlink_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. + + +The URB structure +================= + +Some of the fields in struct :c:type:`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; // URB_ISO_ASAP, URB_SHORT_NOT_OK, etc. + + // (IN) all urbs need completion routines + void *context; // context for completion routine + usb_complete_t complete; // 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 + u32 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 + u32 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 URB_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. + + +How to get an URB? +================== + +URBs are allocated by calling :c:func:`usb_alloc_urb`:: + + 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 :c:func:`usb_free_urb`:: + + 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. + + +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 +:c:func:`usb_fill_control_urb`, :c:func:`usb_fill_bulk_urb` and +:c:func:`usb_fill_int_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. + + +How to submit an URB? +===================== + +Just call :c:func:`usb_submit_urb`:: + + int usb_submit_urb(struct urb *urb, int mem_flags) + +The ``mem_flags`` parameter, such as ``GFP_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 ``URB_ISO_ASAP`` flag, using +multi-buffering, to get seamless ISO streaming. + + +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 +:c:func:`usb_unlink_urb`:: + + 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 :c:func:`usb_unlink_urb` +returns; you must still wait for the completion handler to be called. + +To cancel an URB synchronously, call :c:func:`usb_kill_urb`:: + + void usb_kill_urb(struct urb *urb) + +It does everything :c:func:`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 +:c:func:`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 :c:func:`usb_unlink_urb` or :c:func:`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 :c:func`usb_get_urb`:: + + 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 :c:func:`usb_free_urb`. Of course, +none of this is necessary if there's no danger of the URB being freed +by the completion handler. + + +What about the completion handler? +================================== + +The handler is of the following type:: + + typedef void (*usb_complete_t)(struct urb *) + +I.e., it gets the URB that caused the completion call. 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. + + +.. warning:: + + NEVER SLEEP IN A COMPLETION HANDLER. + + These are often called in atomic context. + +In the current kernel, completion handlers run with local interrupts +disabled, but in the future this will be changed, so don't assume that +local IRQs are always disabled inside completion handlers. + +How to do isochronous (ISO) transfers? +====================================== + +Besides the fields present on a bulk transfer, for ISO, you also +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. You can use the :c:func:`usb_fill_int_urb` macro to fill +most ISO transfer fields. + +For ISO transfers you also have to fill a :c:type:`usb_iso_packet_descriptor` +structure, allocated at the end of the URB by :c:func:`usb_alloc_urb`, for +each packet you want to schedule. + +The :c:func:`usb_submit_urb` call modifies ``urb->interval`` to the implemented +interval value that is less than or equal to the requested interval value. If +``URB_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 ``URB_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. + + +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. +You can use the :c:func:`usb_fill_int_urb` macro to fill INT transfer fields. + +The :c:func:`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. +s |