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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

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.