.\" Copyright (c) 2001 John H. Baldwin <jhb@FreeBSD.org>
.\" Copyright (c) 2006 Tom Rhodes <trhodes@FreeBSD.org>
.\" Copyright (c) 2021 Mitchell Horne <mhorne@FreeBSD.org>
.\" Copyright (c) 2022 The FreeBSD Foundation
.\"
.\" Portions of this documentation were written by Mitchell Horne
.\" under sponsorship from the FreeBSD Foundation.
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.Dd October 30, 2022
.Dt INTR_EVENT 9
.Os
.Sh NAME
.Nm intr_event_add_handler ,
.Nm intr_event_create ,
.Nm intr_event_destroy ,
.Nm intr_event_handle ,
.Nm intr_event_remove_handler ,
.Nm intr_priority
.Nd "kernel interrupt handler and thread API"
.Sh SYNOPSIS
.In sys/param.h
.In sys/bus.h
.In sys/interrupt.h
.Ft int
.Fo intr_event_add_handler
.Fa "struct intr_event *ie"
.Fa "const char *name"
.Fa "driver_filter_t filter"
.Fa "driver_intr_t handler"
.Fa "void *arg"
.Fa "u_char pri"
.Fa "enum intr_type flags"
.Fa "void **cookiep"
.Fc
.Ft int
.Fo intr_event_create
.Fa "struct intr_event **event"
.Fa "void *source"
.Fa "int flags"
.Fa "int irq"
.Fa "void (*pre_ithread)(void *)"
.Fa "void (*post_ithread)(void *)"
.Fa "void (*post_filter)(void *)"
.Fa "int (*assign_cpu)(void *, int)"
.Fa "const char *fmt"
.Fa "..."
.Fc
.Ft int
.Fn intr_event_destroy "struct intr_event *ie"
.Ft int
.Fn intr_event_handle "struct intr_event *ie" "struct trapframe *frame"
.Ft int
.Fn intr_event_remove_handler "void *cookie"
.Ft u_char
.Fn intr_priority "enum intr_type flags"
.Sh DESCRIPTION
The interrupt event API provides methods to manage the registration and
execution of interrupt handlers and their associated thread contexts.
.Pp
Each interrupt event in the system corresponds to a single hardware or software
interrupt source.
Each interrupt event maintains a list of interrupt handlers, sorted by
priority, which will be invoked when handling the event.
An interrupt event will typically, but not always, have an associated
.Xr kthread 9 ,
known as the interrupt thread.
Finally, each event contains optional callback functions which will be
invoked before and after the handler functions themselves.
.Pp
An interrupt handler contains two distinct handler functions:
the
.Em filter
and the thread
.Em handler .
The
.Em filter
function is run from interrupt context and is intended to perform quick
handling such as acknowledging or masking a hardware interrupt,
and queueing work for the ensuing thread
.Em handler .
Both functions are optional; each interrupt handler may choose to register a
filter, a thread handler, or both.
Each interrupt handler also consists of a name,
a set of flags,
and an opaque argument which will be passed to both the
.Em filter
and
.Em handler
functions.
.Ss Handler Constraints
The
.Em filter
function is executed inside a
.Xr critical 9
section.
Therefore, filters may not yield the CPU for any reason, and may only use spin
locks to access shared data.
Allocating memory within a filter is not permitted.
.Pp
The
.Em handler
function executes from the context of the associated interrupt kernel thread.
Sleeping is not permitted, but the interrupt thread may be preempted by higher
priority threads.
Thus, threaded handler functions may obtain non-sleepable locks, as described
in
.Xr locking 9 .
Any memory or zone allocations in an interrupt thread must specify the
.Dv M_NOWAIT
flag, and any allocation errors must be handled.
.Pp
The exception to these constraints is software interrupt threads, which are
allowed to sleep but should be allocated and scheduled using the
.Xr swi 9
interface.
.Ss Function Descriptions
The
.Fn intr_event_create
function creates a new interrupt event.
The
.Fa event
argument points to a
.Vt struct intr_event
pointer that will reference the newly created event upon success.
The
.Fa source
argument is an opaque pointer which will be passed to the
.Fa pre_ithread ,
.Fa post_ithread ,
and
.Fa post_filter
callbacks.
The
.Fa flags
argument is a mask of properties of this thread.
The only valid flag currently for
.Fn intr_event_create
is
.Dv IE_SOFT
to specify that this interrupt thread is a software interrupt.
The
.Fa enable
and
.Fa disable
arguments specify optional functions used to enable and disable this
interrupt thread's interrupt source.
The
.Fa irq
argument is the unique interrupt vector number corresponding to the event.
The
.Fa pre_ithread ,
.Fa post_ithread ,
and
.Fa post_filter
arguments are callback functions that are invoked at different
points while handling an interrupt.
This is described in more detail in the
.Sx Handler Callbacks
section, below.
They may be
.Va NULL
to specify no callback.
The
.Fa assign_cpu
argument points to a callback function that will be invoked when binding
an interrupt to a particular CPU.
It may be
.Va NULL
if binding is unsupported.
The
remaining arguments form a
.Xr printf 9
argument list that is used to build the base name of the new interrupt thread.
The full name of an interrupt thread is formed by concatenating the base
name of the interrupt thread with the names of all of its interrupt handlers.
.Pp
The
.Fn intr_event_destroy
function destroys a previously created interrupt event by releasing its
resources.
.\" The following is not true (yet):
.\"and arranging for the backing kernel thread to terminate.
An interrupt event can only be destroyed if it has no handlers remaining.
.Pp
The
.Fn intr_event_add_handler
function adds a new handler to an existing interrupt event specified by
.Fa ie .
The
.Fa name
argument specifies a name for this handler.
The
.Fa filter
argument provide the filter function to execute.
The
.Fa handler
argument provides the handler function to be executed from the
event's interrupt thread.
The
.Fa arg
argument will be passed to the
.Fa filter
and
.Fa handler
functions when they are invoked.
The
.Fa pri
argument specifies the priority of this handler,
corresponding to the values defined in
.In sys/priority.h .
It determines the order this handler is called relative to the other handlers
for this event, as well as the scheduling priority of the backing kernel
thread.
.Fa flags
argument can be used to specify properties of this handler as defined in
.In sys/bus.h .
If
.Fa cookiep
is not
.Dv NULL ,
then it will be assigned a cookie that can be used later to remove this
handler.
.Pp
The
.Fn intr_event_handle
function is the main entry point into the interrupt handling code.
It must be called from an interrupt context.
The function will execute all filter handlers associated with the interrupt
event
.Fa ie ,
and schedule the associated interrupt thread to run, if applicable.
The
.Fa frame
argument is used to pass a pointer to the
.Vt struct trapframe
containing the machine state at the time of the interrupt.
The main body of this function runs within a
.Xr critical 9
section.
.Pp
The
.Fn intr_event_remove_handler
function removes an interrupt handler from the interrupt event specified by
.Fa ie .
The
.Fa cookie
argument, obtained from
.Fn intr_event_add_handler ,
identifies the handler to remove.
.Pp
The
.Fn intr_priority
function translates the
.Dv INTR_TYPE_*
interrupt flags into interrupt thread scheduling priorities.
.Pp
The interrupt flags not related to the type of a particular interrupt
.Pq Dv INTR_TYPE_*
can be used to specify additional properties of both hardware and software
interrupt handlers.
The
.Dv INTR_EXCL
flag specifies that this handler cannot share an interrupt thread with
another handler.
The
.Dv INTR_MPSAFE
flag specifies that this handler is MP safe in that it does not need the
Giant mutex to be held while it is executed.
The
.Dv INTR_ENTROPY
flag specifies that the interrupt source this handler is tied to is a good
source of entropy, and thus that entropy should be gathered when an interrupt
from the handler's source triggers.
Presently, the
.Dv INTR_ENTROPY
flag is not valid for software interrupt handlers.
.Ss Handler Callbacks
Each
.Vt struct intr_event
is assigned three optional callback functions when it is created:
.Fa pre_ithread ,
.Fa post_ithread ,
and
.Fa post_filter .
These callbacks are intended to be defined by the interrupt controller driver,
to allow for actions such as masking and unmasking hardware interrupt signals.
.Pp
When an interrupt is triggered, all filters are run to determine if any
threaded interrupt handlers should be scheduled for execution by the associated
interrupt thread. If no threaded handlers are scheduled, the
.Fa post_filter
callback is invoked which should acknowledge the interrupt and permit it to
trigger in the future.
If any threaded handlers are scheduled, the
.Fa pre_ithread
callback is invoked instead.
This handler should acknowledge the interrupt, but it should also ensure that
the interrupt will not fire continuously until after the threaded handlers have
executed.
Typically this callback masks level-triggered interrupts in an interrupt
controller while leaving edge-triggered interrupts alone.
Once all threaded handlers have executed,
the
.Fa post_ithread
callback is invoked from the interrupt thread to enable future interrupts.
Typically this callback unmasks level-triggered interrupts in an interrupt
controller.
.Sh RETURN VALUES
The
.Fn intr_event_add_handler ,
.Fn intr_event_create ,
.Fn intr_event_destroy ,
.Fn intr_event_handle ,
and
.Fn intr_event_remove_handler
functions return zero on success and non-zero on failure.
The
.Fn intr_priority
function returns a process priority corresponding to the passed in interrupt
flags.
.Sh EXAMPLES
The
.Xr swi_add 9
function demonstrates the use of
.Fn intr_event_create
and
.Fn intr_event_add_handler .
.Bd -literal -offset indent
int
swi_add(struct intr_event **eventp, const char *name, driver_intr_t handler,
    void *arg, int pri, enum intr_type flags, void **cookiep)
{
	struct intr_event *ie;
	int error = 0;

	if (flags & INTR_ENTROPY)
		return (EINVAL);

	ie = (eventp != NULL) ? *eventp : NULL;

	if (ie != NULL) {
		if (!(ie->ie_flags & IE_SOFT))
			return (EINVAL);
	} else {
		error = intr_event_create(&ie, NULL, IE_SOFT, 0,
		    NULL, NULL, NULL, swi_assign_cpu, "swi%d:", pri);
		if (error)
			return (error);
		if (eventp != NULL)
			*eventp = ie;
	}
	if (handler != NULL) {
		error = intr_event_add_handler(ie, name, NULL, handler, arg,
		    PI_SWI(pri), flags, cookiep);
	}
	return (error);
}
.Ed
.Sh ERRORS
The
.Fn intr_event_add_handler
function will fail if:
.Bl -tag -width Er
.It Bq Er EINVAL
The
.Fa ie
or
.Fa name
arguments are
.Dv NULL .
.It Bq Er EINVAL
The
.Fa handler
and
.Fa filter
arguments are both
.Dv NULL .
.It Bq Er EINVAL
The
.Dv IH_EXCLUSIVE
flag is specified and the interrupt thread
.Fa ie
already has at least one handler, or the interrupt thread
.Fa ie
already has an exclusive handler.
.El
.Pp
The
.Fn intr_event_create
function will fail if:
.Bl -tag -width Er
.It Bq Er EINVAL
A flag other than
.Dv IE_SOFT
was specified in the
.Fa flags
parameter.
.El
.Pp
The
.Fn intr_event_destroy
function will fail if:
.Bl -tag -width Er
.It Bq Er EINVAL
The
.Fa ie
argument is
.Dv NULL .
.It Bq Er EBUSY
The interrupt event pointed to by
.Fa ie
has at least one handler which has not been removed with
.Fn intr_event_remove_handler .
.El
.Pp
The
.Fn intr_event_handle
function will fail if:
.Bl -tag -width Er
.It Bq Er EINVAL
The
.Fa ie
argument is
.Dv NULL .
.It Bq Er EINVAL
There are no interrupt handlers assigned to
.Fa ie .
.It Bq Er EINVAL
The interrupt was not acknowledged by any filter and has no associated thread
handler.
.El
.Pp
The
.Fn intr_event_remove_handler
function will fail if:
.Bl -tag -width Er
.It Bq Er EINVAL
The
.Fa cookie
argument is
.Dv NULL .
.El
.Sh SEE ALSO
.Xr critical 9 ,
.Xr kthread 9 ,
.Xr locking 9 ,
.Xr malloc 9 ,
.Xr swi 9 ,
.Xr uma 9
.Sh HISTORY
Interrupt threads and their corresponding API first appeared in
.Fx 5.0 .