Tribblix: manual page: usb_callback

USB_CALLBACK_FLAGS(9S) Data Structures for Drivers USB_CALLBACK_FLAGS(9S)

NAME

usb_callback_flags - USB callback flag definitions

SYNOPSIS

#include <sys/usb/usba.h>

INTERFACE LEVEL

illumos DDI specific (illumos DDI)

DESCRIPTION

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:

USB_CB_NO_INFO
No additional errors discovered or
recovery actions taken.

USB_CB_FUNCTIONAL_STALL
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.

USB_CB_STALL_CLEARED
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.

USB_CB_PROTOCOL_STALL
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.

USB_CB_RESET_PIPE
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.

USB_CB_ASYNC_REQ_FAILED
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 queue.

USB_CB_SUBMIT_FAILED
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.

USB_CB_NO_RESOURCES
Insufficient resources were available for
recovery to proceed.

USB_CB_INTR_CONTEXT
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 handler.

CALLBACK HANDLER

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.

PIPE POLICY

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 usb_pipe_open(9F).

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 context.

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 thread.)

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 completion.

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
passed.

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.

COMMAND COMPLETION CALLBACKS

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
usb_clr_feature(9F).

The signature of a command completion callback is as follows:

command_cb(
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.

ATTRIBUTES