urb.txt

《嵌入式系统设计与实例开发实验教材二源码》Linux内核移植与编译实验

Revised: 2000-Dec-05.

1. Specification of the API

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. 

- Ongoing transfers for one URB (e.g. ISO) can simply 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 (INT transfers behave a bit
differently, see below). The URB also contains a context-pointer for free 
usage and information passing to the completion handler.

- URBs can be linked. After completing one URB, the next one can be
automatically submitted. This is especially useful for ISO transfers:
You only have read/write the data from/to the buffers in the completion 
handler, the continuous streaming itself is transparently done by the 
URB-machinery.


1.2. The URB structure

typedef struct urb
{
	spinlock_t lock;		// lock for the URB

// ignore, for host controller/URB machine internal use
	void *hcpriv;                   // private data for host controller
	struct list_head urb_list;      // list pointer to all active urbs 

// This is used for urb linking
	struct urb* next;               // pointer to next URB  
	struct usb_device *dev;         // pointer to associated USB device

// pipe is assembled by the various well-known pipe macros in usb.h
	unsigned int pipe;              // pipe information

// status after each completion
	int status;                     // returned status
	unsigned int transfer_flags;    // ASAP, DISABLE_SPD, etc.

// for data stage (CTRL), BULK, INT and ISO
	void *transfer_buffer;          // associated data buffer

// expected length
	int transfer_buffer_length;     // data buffer length
	int actual_length;              // actual data buffer length    

// setup stage for CTRL (always 8 bytes!)
	unsigned char* setup_packet;    // setup packet (control only)

// with ASAP, start_frame is set to the determined frame
	int start_frame;                // start frame (iso/irq)
	int number_of_packets;          // # of packets (iso/int)
	int interval;                   // polling interval (irq only)
	int error_count;                // number of errors (iso only)
	//
	void *context;                  // context for completion routine
	usb_complete_t complete;        // pointer to completion routine
	//
// specification of the requested data offsets and length for ISO
	iso_packet_descriptor_t iso_frame_desc[0];
} urb_t, *purb_t;


1.3. How to get an URB?

URBs are allocated with the following call

	purb_t usb_alloc_urb(int isoframes)

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.

To free an URB, use

	void usb_free_urb(purb_t purb)

This call also may free internal (host controller specific) memory in the
future.


1.4. What has to be filled in?

Depending on the type of transaction, there are some macros 
(FILL_CONTROL_URB, FILL_CONTROL_URB_TO, FILL_BULK_URB,
FILL_BULK_URB_TO, and FILL_INT_URB, defined in usb.h)
that simplify the URB creation. In general, all macros 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 usb_control_msg function that converts the old API 
into the URB API.

Flags:
For ISO there are two startup behaviors: Specified start_frame or ASAP.
For ASAP set USB_ISO_ASAP in transfer_flags.

If short packets should NOT be tolerated, set USB_DISABLE_SPD in 
transfer_flags.

Usually, to reduce restart time, the completion handler is called
AFTER the URB re-submission.  However, it is called BEFORE URB
re-submission for INT transfers that are being continued.


1.5. How to submit an URB?

Just call

	int usb_submit_urb(purb_t purb)

It immediately returns, either with status 0 (request queued) or some
error code, usually caused by the following:

- Out of memory (-ENOMEM)
- Wrong pipe handle (-ENXIO)
- 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 USB_ST_URB_PENDING (-EINPROGRESS).

For isochronous endpoints, subsequent submitting of URBs to the same endpoint
with the ASAP flag result in a seamless ISO streaming. Exception: The 
execution cannot be scheduled later than 900 frames from the 'now'-time. 
The same applies to INT transfers, but here the seamless continuation is 
independent of the transfer flags (implicitly ASAP).


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(purb_t purb)

It removes the urb from the internal list and frees all allocated
HW descriptors. The status is changed to USB_ST_URB_KILLED. After 
usb_unlink_urb() returns, you can safely free the URB with usb_free_urb(urb)
and all other possibly associated data (urb->context etc.)

There is also an asynchronous unlink mode.  To use this, set the
the USB_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 completion handler is optional, but useful for fast data processing
or wakeup of a sleeping process (as shown in the compatibility wrapper's 
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:  ***** WARNING *****
AVOID using the urb->dev field in your completion handler; it's cleared
as part of URB unlinking.  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 append the iso_packet_descriptor_t structure 
to the URB for each frame you want to schedule. When using usb_alloc_urb(n)
(recommended), the iso_packets parameter can be used to allocate the
structures for iso_packets frames.

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 USB-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).

As can be concluded from above, the UHCI-driver does not care for continuous
data in case of short packet ISO reads! There's no fixup_isoc() like in the 
old driver. There may be a common routine to do this in the future, but this 
has nothing to do with the UHCI-driver!

For scheduling you can choose your own start frame or ASAP. As written above,
queuing more than one ISO frame with ASAP to the same device&endpoint result 
in seamless ISO streaming. For continuous streaming you have to use URB
linking. 


1.9. How to start interrupt (INT) transfers?

INT transfers are currently implemented with different queues for intervals 
for 1, 2, 4,... 128ms. Only one URB is allocated for each interrupt. After
calling the completion handler, that URB is recycled by the host controller
driver (HCD).
With the submission of one URB, the interrupt is scheduled until it is
canceled by usb_unlink_urb.

The usb_submit_urb() call modifies urb->interval to the implemented interval
value that is less than or equal to the requested interval value.