usb_callback_flags - USB callback flag definitions


#include <sys/usb/usba.h>


illumos DDI specific (illumos DDI)


If the USB framework detects an error during a request execution, it
calls the client driver's exception callback handler to report what
happened. Callback flags (which are set prior to calling the exception
callback handler) detail errors discovered during the exception recovery
process, and summarize recovery actions taken by the USBA framework.

Information from the callback flags supplements information from the
original transport error. For transfers, the original transport error
status is returned to the callback handler through the original request
(whose completion reason field contains any transport error indication).
For command completion callbacks, the callback's rval argument contains
the transport error status. A completion reason of USB_CR_OK means the
transfer completed with no errors detected.

The usb_cb_flags_t enumerated type contains the following definitions:

No additional errors discovered or recovery
actions taken.

A functional stall occurred during the
transfer. A functional stall is usually
caused by a hardware error, and must be
explicitly cleared. A functional stall is
fatal if it cannot be cleared. The default
control pipe never shows a functional stall.

A functional stall has been cleared by the
USBA framework. This can happen if
USB_ATTRS_AUTOCLEARING is set in the request's
xxxx_attributes field.

A protocol stall has occurred during the
transfer. A protocol stall is caused usually
by an invalid or misunderstood command. It is
cleared automatically when the device is given
its next command. The USBA
framework treats stalls detected on default
pipe transfers as protocol stalls.

A pipe with a stall has been reset
automatically via autoclearing, or via an
explicit call to usb_pipe_reset(9F). Resetting
a pipe consists of stopping all transactions
on a pipe, setting the pipe to the idle state,
and if the pipe is not the default pipe,
flushing all pending requests. The request
which has the error, plus all pending requests
which are flushed, show USB_CB_RESET_PIPE set
in the usb_cb_flags_t when their exception
callback is called.

Resources could not be allocated to process
callbacks asynchronously. Callbacks receiving
this flag must not block, since those
callbacks are executing in a context which
holds resources shared by the rest of the
system. Note that exception callbacks with
USB_CB_ASYNC_REQ_FAILED set may execute out of
order from the requests which preceded them.
Normal callbacks may be already queued when an
exception hits that the USBA is unable to

A queued request was submitted to the host
controller driver and was rejected. The
usb_completion_reason shows why the request
was rejected by the host controller.

Insufficient resources were available for
recovery to proceed.

Callback is executing in interrupt context and
should not block.

The usb_cb_flags_t enumerated type defines a bitmask. Multiple bits can
be set, reporting back multiple statuses to the exception callback


The USBA framework supports callback handling as a way of asynchronous
client driver notification. There are three kinds of callbacks: Normal
completion transfer callback, exception (error) completion transfer
callback, and command completion callback, each described below.

Callback handlers are called whenever they are specified in a request or
command, regardless of whether or not that request or command specifies
the USB_FLAGS_SLEEP flag. (USB_FLAGS_SLEEP tells the request or command
to block until completed.) Callback handlers must be specified whenever
an asynchronous transfer is requested.


Each pipe is associated with a pool of threads that are used to run
callbacks associated with requests on that pipe. All transfer completion
callbacks for a particular pipe are run serially by a single thread.

Pipes taking requests with callbacks which can block must have their pipe
policy properly initialized. If a callback blocks on a condition that is
only met by another thread associated with the same pipe, there must be
sufficient threads available. Otherwise that callback thread will block
forever. Similarly, problems will ensue when callbacks overlap and there
are not enough threads to handle the number of overlapping callbacks.

The pp_max_async_reqs field of the pipe_policy provides a hint of how
many threads to allocate for asynchronous processing of request callbacks
on a pipe. Set this value high enough per pipe to accommodate all of the
pipe's possible asynchronous conditions. The pipe_policy is passed to

Transfer completion callbacks (normal completion and exception):

Most transfer completion callbacks are allowed to block, but only under
certain conditions:

1. No callback is allowed to block if the callback flags show
USB_CB_INTR_CONTEXT set, since that flag indicates that the
callback is running in interrupt context instead of kernel
context. Isochronous normal completion callbacks, plus those
with USB_CB_ASYNC_REQ_FAILED set, execute in interrupt

2. Any callback except for isochronous normal completion can
block for resources (for example to allocate memory).

3. No callback can block for synchronous completion of a command
(for example, a call to usb_pipe_close(9F) with the
USB_FLAGS_SLEEP flag passed) done on the same pipe. The
command could wait for all callbacks to complete, including
the callback which issued that command, causing all operations
on the pipe to deadlock. Note that asynchronous commands can
start from a callback, providing that the pipe's policy
pp_max_async_reqs field is initialized to accommodate them.

4. Avoid callbacks that block for synchronous completion of
commands done on other pipes. Such conditions can cause
complex dependencies and unpredictable results.

5. No callback can block waiting for a synchronous transfer
request to complete. (Note that making an asynchronous request
to start a new transfer or start polling does not block, and
is OK.)

6. No callback can block waiting for another callback to
complete. (This is because all callbacks are done by a single

7. Note that if a callback blocks, other callbacks awaiting
processing can backup behind it, impacting system resources.

A transfer request can specify a non-null normal-completion callback.
Such requests conclude by calling the normal-completion callback when the
transfer completes normally. Similarly, a transfer request can specify a
non-null exception callback. Such requests conclude by calling the
exception callback when the transfer completes abnormally. Note that the
same callback can be used for both normal completion and exception
callback handling. A completion reason of USB_CR_OK defines normal

All request-callbacks take as arguments a usb_pipe_handle_t and a pointer
to the request:

xxxx_cb(usb_pipe_handle_t ph, struct usb_ctrl_req *req);

Such callbacks can retrieve saved state or other information from the
private area of the pipe handle. (See usb_pipe_set_private(9F).)
Handlers also have access to the completion reason (usb_cr_t) and
callback flags (usb_cb_flags_t) through the request argument they are

Request information follows. In the data below, xxxx below represents the
type of request (ctrl, intr, isoc or bulk.)

Request structure name is usb_xxxx_req_t.

Normal completion callback handler field is xxxx_cb.

Exception callback handler field is xxxx_exc_cb.

Completion reason field is xxxx_completion_reason.

Callback flags field is xxxx_cb_flags.


Calls to some non-transfer functions can be set up for callback
notification. These include usb_pipe_close(9F), usb_pipe_reset(9F),
usb_pipe_drain_reqs(9F), usb_set_cfg(9F), usb_set_alt_if(9F) and

The signature of a command completion callback is as follows:

usb_pipe_handle_t cb_pipe_handle,
usb_opaque_t arg,
int rval,
usb_cb_flags_t flags);

As with transfer completion callbacks, command completion callbacks take
a usb_pipe_handle_t to retrieve saved state or other information from the
pipe's private area. Also, command completion callbacks are provided with
an additional user-definable argument (usb_opaque_t arg), the return
status of the executed command (int rval), and the callback flags
(usb_cb_flags_t flags).

The rval argument is roughly equivalent to the completion reason of a
transfer callback, indicating the overall status. See the return values
of the relevant function for possible rval values which can be passed to
the callback.

The callback flags can be checked when rval indicates failure status.
Just as for transfer completion callbacks, callback flags return
additional information on execution events.


See attributes(7) for descriptions of the following attributes:

|Architecture | PCI-based systems |
|Interface stability | Committed |


usb_alloc_request(9F), usb_pipe_bulk_xfer(9F), usb_pipe_ctrl_xfer(9F),
usb_pipe_intr_xfer(9F), usb_pipe_isoc_xfer(9F), usb_bulk_request(9S),
usb_ctrl_request(9S), usb_intr_request(9S), usb_isoc_request(9S)

illumos January 5, 2004 USB_CALLBACK_FLAGS(9S)