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Add verbage on some of the rules for a 'filter' vs an 'ithread'

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interrupt handler.
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imp 2007-12-19 01:26:34 +00:00
parent d198b4b8bb
commit 5dbc9c7397
1 changed files with 81 additions and 2 deletions
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83
share/man/man9/BUS_SETUP_INTR.9
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@ -24,7 +24,7 @@
.\"
.\" $FreeBSD$
.\"
.Dd March 1, 2007
.Dd December 18, 2007
.Dt BUS_SETUP_INTR 9
.Os
.Sh NAME
@ -90,6 +90,9 @@ To define a time-critical handler (previously known as
that will not execute any potentially blocking operation, use the
.Fa filter
argument.
See the
.Sx "Filter Routine"
section below for information on writing a filter.
Otherwise, use the
.Fa ithread
argument.
@ -97,6 +100,9 @@ The defined handler
will be called with the value
.Fa arg
as its only argument.
See the
.Sx "ithread Routine"
section below for more information on writing an interrupt handler.
.Pp
The
.Fa cookiep
@ -121,13 +127,86 @@ returns, it is guaranteed that the interrupt function is not active and
will no longer be called.
.Pp
Mutexes are not allowed to be held across calls to these functions.
.Ss "Filter Routines"
A filter runs in a context very similar to what was known as an
.Dv INTR_FAST
routine in previous versions of
.Fx .
In this context, normal mutexes cannot be used.
Only the spin lock version of these can be used (specified by passing
.Dv MTX_SPIN
to
.Fn mtx_init
when initializing the mutex).
.Xr wakeup 9
and similar routines can be called.
Atomic operations from
.Pa machine/atomic
may be used.
Reads and writes to hardware through
.Xr bus_space 9
may be used.
PCI configuration registers may be read and written.
All other kernel interfaces cannot be used.
.Pp
In this restricted environment, care must be taken to account for all
races.
A careful analysis of races should be done as well.
It is generally cheaper to take an extra interrupt, for example, than
to protect variables with spinlocks.
Read, modify, write cycles of hardware registers need to be carefully
analyzed if other threads are accessing the same registers.
.Pp
Generally, a filter routine will use one of two strategies.
The first strategy is to simply mask the interrupt in hardware and
allow the
.Dv ithread
routine to read the state from the hardware and then reenable
interrupts.
The
.Dv ithread
also acknowledges the interrupt before re-enabling the interrupt
source in hardware.
Most PCI hardware can mask its interrupt source.
.Pp
The second common approach is to use a filter with multiple
.Xr taskqueue 9
tasks.
In this case, the filter acknowledges the interrupts and queues the
work to the appropriate taskqueue.
Where one has to multiplex different kinds of interrupt sources, like
a network card's transmit and receive paths, this can reduce lock
contention and increase performance.
.Pp
You should not
.Xr malloc 9
from inside a filter.
You may not call anything that uses a normal mutex.
Witness may complain about these.
.Ss "ithread Routines"
You can do whatever you want in an ithread routine, except sleep.
Care must be taken not to sleep in an ithread.
In addition, one should minimize lock contention in an ithread routine
because contested locks ripple over to all other ithread routines on
that interrupt.
.Ss "Sleeping"
Sleeping is voluntarily giving up control of your thread.
All the sleep routine found in
.Xr msleep 9
sleep.
Waiting for a condition variable described in
.Xr condvar 9
is sleeping.
Calling any function that does any of these things is sleeping.
.Sh RETURN VALUES
Zero is returned on success,
otherwise an appropriate error is returned.
.Sh SEE ALSO
.Xr random 4 ,
.Xr device 9 ,
.Xr driver 9
.Xr driver 9 ,
.Xr mtx_init 9 ,
.Xr wakeup
.Sh AUTHORS
.An -nosplit
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