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:
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;
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?
124 int usb_submit_urb(struct urb *urb, int mem_flags)
L-B_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; howevef, you should nevef
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 rame, 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 evefything 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?
196flaghandler 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.
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