usb/URB.txt: convert to ReST and update it

The URB doc describes the Kernel mechanism that do USB transfers.
While the functions are already described at urb.h, there are a
number of concepts and theory that are important for USB driver
developers.

Convert it to ReST and use C ref links to point to the places
at usb.h where each function and struct is located.

A few of those descriptions were incomplete. While here, update
to reflect the current API status.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/driver-api/usb/URB.rst

@@ -0,0 +1,288 @@
+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

Documentation/driver-api/usb/index.rst

@@ -10,6 +10,7 @@ Linux USB API
    bulk-streams
    callbacks
    dma
+   URB
    power-management
    hotplug
    persist

Documentation/driver-api/usb/usb.rst

@@ -1,3 +1,5 @@
+.. _usb-hostside-api:
+
 ===========================
 The Linux-USB Host Side API
 ===========================