1Revised: 2000-Dec-05.
   2Again:   2002-Jul-06
   3Again:   2005-Sep-19
   5    NOTE:
   7    The USB subsystem now has a substantial section in "The Linux Kernel API"
   8    guide (in Documentation/DocBook), generated from the current source
   9    code.  This particular documentation file isn't particularly current or
  10    complete; don't rely on it except for a quick overview.
  131.1. Basic concept or 'What is an URB?'
  15The basic idea of the new driver is message passing, the message itself is 
  16called USB Request Block, or URB for short. 
  18- An URB consists of all relevant information to execute any USB transaction 
  19  and deliver the data and status back. 
  21- Execution of an URB is inherently an asynchronous operation, i.e. the 
  22  usb_submit_urb(urb) call returns immediately after it has successfully
  23  queued the requested action.
  25- Transfers for one URB can be canceled with usb_unlink_urb(urb) at any time. 
  27- Each URB has a completion handler, which is called after the action
  28  has been successfully completed or canceled. The URB also contains a
  29  context-pointer for passing information to the completion handler.
  31- Each endpoint for a device logically supports a queue of requests.
  32  You can fill that queue, so that the USB hardware can still transfer
  33  data to an endpoint while your driver handles completion of another.
  34  This maximizes use of USB bandwidth, and supports seamless streaming
  35  of data to (or from) devices when using periodic transfer modes.
  381.2. The URB structure
  40Some of the fields in an URB are:
  42struct urb
  44// (IN) device and pipe specify the endpoint queue
  45        struct usb_device *dev;         // pointer to associated USB device
  46        unsigned int pipe;              // endpoint information
  48        unsigned int transfer_flags;    // ISO_ASAP, SHORT_NOT_OK, etc.
  50// (IN) all urbs need completion routines
  51        void *context;                  // context for completion routine
  52        void (*complete)(struct urb *); // pointer to completion routine
  54// (OUT) status after each completion
  55        int status;                     // returned status
  57// (IN) buffer used for data transfers
  58        void *transfer_buffer;          // associated data buffer
  59        int transfer_buffer_length;     // data buffer length
  60        int number_of_packets;          // size of iso_frame_desc
  62// (OUT) sometimes only part of CTRL/BULK/INTR transfer_buffer is used
  63        int actual_length;              // actual data buffer length
  65// (IN) setup stage for CTRL (pass a struct usb_ctrlrequest)
  66        unsigned char* setup_packet;    // setup packet (control only)
  68// Only for PERIODIC transfers (ISO, INTERRUPT)
  69    // (IN/OUT) start_frame is set unless ISO_ASAP isn't set
  70        int start_frame;                // start frame
  71        int interval;                   // polling interval
  73    // ISO only: packets are only "best effort"; each can have errors
  74        int error_count;                // number of errors
  75        struct usb_iso_packet_descriptor iso_frame_desc[0];
  78Your driver must create the "pipe" value using values from the appropriate
  79endpoint descriptor in an interface that it's claimed.
  821.3. How to get an URB?
  84URBs are allocated with the following call
  86        struct urb *usb_alloc_urb(int isoframes, int mem_flags)
  88Return value is a pointer to the allocated URB, 0 if allocation failed.
  89The parameter isoframes specifies the number of isochronous transfer frames
  90you want to schedule. For CTRL/BULK/INT, use 0.  The mem_flags parameter
  91holds standard memory allocation flags, letting you control (among other
  92things) whether the underlying code may block or not.
  94To free an URB, use
  96        void usb_free_urb(struct urb *urb)
  98You may free an urb that you've submitted, but which hasn't yet been
  99returned to you in a completion callback.  It will automatically be
 100deallocated when it is no longer in use.
 1031.4. What has to be filled in?
 105Depending on the type of transaction, there are some inline functions 
 106defined in <linux/usb.h> to simplify the initialization, such as
 107fill_control_urb() and fill_bulk_urb().  In general, they need the usb
 108device pointer, the pipe (usual format from usb.h), the transfer buffer,
 109the desired transfer length, the completion  handler, and its context. 
 110Take a look at the some existing drivers to see how they're used.
 113For ISO there are two startup behaviors: Specified start_frame or ASAP.
 114For ASAP set URB_ISO_ASAP in transfer_flags.
 116If short packets should NOT be tolerated, set URB_SHORT_NOT_OK in 
 1201.5. How to submit an URB?
 122Just call
 124        int usb_submit_urb(struct urb *urb, int mem_flags)
 126The mem_flags parameter, such as SLAB_ATOMIC, controls memory allocation,
 127such as whether the lower levels may block when memory is tight.
 129It immediately returns, either with status 0 (request queued) or some
 130error code, usually caused by the following:
 132- Out of memory (-ENOMEM)
 133- Unplugged device (-ENODEV)
 134- Stalled endpoint (-EPIPE)
 135- Too many queued ISO transfers (-EAGAIN)
 136- Too many requested ISO frames (-EFBIG)
 137- Invalid INT interval (-EINVAL)
 138- More than one packet for INT (-EINVAL)
 140After submission, urb->status is -EINPROGRESS; however, you should never
 141look at that value except in your completion callback.
 143For isochronous endpoints, your completion handlers should (re)submit
 144URBs to the same endpoint with the ISO_ASAP flag, using multi-buffering,
 145to get seamless ISO streaming.
 1481.6. How to cancel an already running URB?
 150There are two ways to cancel an URB you've submitted but which hasn't
 151been returned to your driver yet.  For an asynchronous cancel, call
 153        int usb_unlink_urb(struct urb *urb)
 155It removes the urb from the internal list and frees all allocated
 156HW descriptors. The status is changed to reflect unlinking.  Note
 157that the URB will not normally have finished when usb_unlink_urb()
 158returns; you must still wait for the completion handler to be called.
 160To cancel an URB synchronously, call
 162        void usb_kill_urb(struct urb *urb)
 164It does everything usb_unlink_urb does, and in addition it waits
 165until after the URB has been returned and the completion handler
 166has finished.  It also marks the URB as temporarily unusable, so
 167that if the completion handler or anyone else tries to resubmit it
 168they will get a -EPERM error.  Thus you can be sure that when
 169usb_kill_urb() returns, the URB is totally idle.
 171There is a lifetime issue to consider.  An URB may complete at any
 172time, and the completion handler may free the URB.  If this happens
 173while usb_unlink_urb or usb_kill_urb is running, it will cause a
 174memory-access violation.  The driver is responsible for avoiding this,
 175which often means some sort of lock will be needed to prevent the URB
 176from being deallocated while it is still in use.
 178On the other hand, since usb_unlink_urb may end up calling the
 179completion handler, the handler must not take any lock that is held
 180when usb_unlink_urb is invoked.  The general solution to this problem
 181is to increment the URB's reference count while holding the lock, then
 182drop the lock and call usb_unlink_urb or usb_kill_urb, and then
 183decrement the URB's reference count.  You increment the reference
 184count by calling
 186        struct urb *usb_get_urb(struct urb *urb)
 188(ignore the return value; it is the same as the argument) and
 189decrement the reference count by calling usb_free_urb.  Of course,
 190none of this is necessary if there's no danger of the URB being freed
 191by the completion handler.
 1941.7. What about the completion handler?
 196The handler is of the following type:
 198        typedef void (*usb_complete_t)(struct urb *, struct pt_regs *)
 200I.e., it gets the URB that caused the completion call, plus the
 201register values at the time of the corresponding interrupt (if any).
 202In the completion handler, you should have a look at urb->status to
 203detect any USB errors. Since the context parameter is included in the URB,
 204you can pass information to the completion handler. 
 206Note that even when an error (or unlink) is reported, data may have been
 207transferred.  That's because USB transfers are packetized; it might take
 208sixteen packets to transfer your 1KByte buffer, and ten of them might
 209have transferred successfully before the completion was called.
 212NOTE:  ***** WARNING *****
 213NEVER SLEEP IN A COMPLETION HANDLER.  These are normally called
 214during hardware interrupt processing.  If you can, defer substantial
 215work to a tasklet (bottom half) to keep system latencies low.  You'll
 216probably need to use spinlocks to protect data structures you manipulate
 217in completion handlers.
 2201.8. How to do isochronous (ISO) transfers?
 222For ISO transfers you have to fill a usb_iso_packet_descriptor structure,
 223allocated at the end of the URB by usb_alloc_urb(n,mem_flags), for each
 224packet you want to schedule.   You also have to set urb->interval to say
 225how often to make transfers; it's often one per frame (which is once
 226every microframe for highspeed devices).  The actual interval used will
 227be a power of two that's no bigger than what you specify.
 229The usb_submit_urb() call modifies urb->interval to the implemented interval
 230value that is less than or equal to the requested interval value.  If
 231ISO_ASAP scheduling is used, urb->start_frame is also updated.
 233For each entry you have to specify the data offset for this frame (base is
 234transfer_buffer), and the length you want to write/expect to read.
 235After completion, actual_length contains the actual transferred length and 
 236status contains the resulting status for the ISO transfer for this frame.
 237It is allowed to specify a varying length from frame to frame (e.g. for
 238audio synchronisation/adaptive transfer rates). You can also use the length 
 2390 to omit one or more frames (striping).
 241For scheduling you can choose your own start frame or ISO_ASAP. As explained
 242earlier, if you always keep at least one URB queued and your completion
 243keeps (re)submitting a later URB, you'll get smooth ISO streaming (if usb
 244bandwidth utilization allows).
 246If you specify your own start frame, make sure it's several frames in advance
 247of the current frame.  You might want this model if you're synchronizing
 248ISO data with some other event stream.
 2511.9. How to start interrupt (INT) transfers?
 253Interrupt transfers, like isochronous transfers, are periodic, and happen
 254in intervals that are powers of two (1, 2, 4 etc) units.  Units are frames
 255for full and low speed devices, and microframes for high speed ones.
 256The usb_submit_urb() call modifies urb->interval to the implemented interval
 257value that is less than or equal to the requested interval value.
 259In Linux 2.6, unlike earlier versions, interrupt URBs are not automagically
 260restarted when they complete.  They end when the completion handler is
 261called, just like other URBs.  If you want an interrupt URB to be restarted,
 262your completion handler must resubmit it.