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+Revised: 2000-Dec-05.
+Again:   2002-Jul-06
+
+    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 not free an urb that you've submitted, but which hasn't yet been
+returned to you in a completion callback.
+
+
+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?
+
+For an URB which you've submitted, but which hasn't been returned to
+your driver by the host controller, 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.
+
+
+1.7. What about the completion handler?
+
+The handler is of the following type:
+
+	typedef void (*usb_complete_t)(struct urb *);
+
+i.e. it gets just 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 succesfully before the completion is 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
+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.
+
+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.