man/man4/pcm.4

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.Dd June 1, 2009
.Dt SOUND 4
.Os
.Sh NAME
.Nm sound ,
.Nm pcm ,
.Nm snd
.Nd
.Fx
PCM audio device infrastructure
.Sh SYNOPSIS
To compile this driver into the kernel, place the following line in your
kernel configuration file:
.Bd -ragged -offset indent
.Cd "device sound"
.Ed
.Pp
Non-PnP sound cards require the following lines in
.Xr device.hints 5 :
.Bd -literal -offset indent
hint.pcm.0.at="isa"
hint.pcm.0.irq="5"
hint.pcm.0.drq="1"
hint.pcm.0.flags="0x0"
.Ed
.Sh DESCRIPTION
The
.Nm
driver provides support for
.Tn PCM
audio play and capture.
This driver also supports various
.Tn PCI ,
.Tn ISA ,
.Tn WSS/MSS
compatible
sound cards, AC97 mixer and High Definition Audio.
Once the
.Nm
driver attaches, supported devices provide audio record and
playback channels.
The
.Fx
sound system provides dynamic mixing
.Dq VCHAN
and rate conversion
.Dq soft formats .
True full duplex operation is available on most sound cards.
.Pp
If the sound card is supported by a bridge driver, the
.Nm
driver works in conjunction with the bridge driver.
.Pp
Apart from the usual parameters, the flags field is used to specify
the secondary
.Tn DMA
channel (generally used for capture in full duplex cards).
Flags are set to 0 for cards not using a secondary
.Tn DMA
channel, or to 0x10 + C to specify channel C.
.Pp
The driver does its best to recognize the installed hardware and drive
it correctly so the user is not required to add several lines in
.Pa /boot/device.hints .
For
.Tn PCI
and
.Tn ISA
.Tn PnP
cards this is actually easy
since they identify themselves.
For legacy
.Tn ISA
cards, the driver looks for
.Tn MSS
cards at addresses 0x530 and 0x604 (unless overridden
in
.Pa /boot/device.hints ) .
.Ss Boot Variables
In general, the module
.Pa snd_foo
corresponds to
.Cd "device snd_foo"
and can be
loaded by the boot
.Xr loader 8
via
.Xr loader.conf 5
or from the command line using the
.Xr kldload 8
utility.
Options which can be specified in
.Pa /boot/loader.conf
include:
.Bl -tag -width ".Va snd_emu10k1_load" -offset indent
.It Va snd_driver_load
.Pq Dq Li NO
If set to
.Dq Li YES ,
this option loads all available drivers.
.It Va snd_emu10k1_load
.Pq Dq Li NO
If set to
.Dq Li YES ,
only the SoundBlaster 5.1 driver and dependent modules will be loaded.
.It Va snd_foo_load
.Pq Dq Li NO
If set to
.Dq Li YES ,
load driver for card/chipset foo.
.El
.Pp
To define default values for the different mixer channels,
set the channel to the preferred value using hints, e.g.:
.Va hint.pcm.0.line Ns = Ns Qq Li 0 .
This will mute the input channel per default.
.Ss VCHANs
Each device can optionally support more playback and recording channels
than physical hardware provides by using
.Dq virtual channels
or
.Tn VCHANs .
.Tn VCHAN
options can be configured via the
.Xr sysctl 8
interface but can only be manipulated while the device is inactive.
.Ss Runtime Configuration
There are a number of
.Xr sysctl 8
variables available.
.Va hw.snd.*
tunables are global settings and
.Va dev.pcm.*
are device specific.
.Bl -tag -width ".Va hw.snd.report_soft_formats" -offset indent
.It Va hw.snd.compat_linux_mmap
Enable to allow PROT_EXEC page mappings.
All Linux applications using sound and
.Xr mmap 2
require this.
.It Va hw.snd.default_auto
Enable to automatically assign default sound unit to the most recent
attached device.
.It Va hw.snd.default_unit
Default sound card for systems with multiple sound cards.
When using
.Xr devfs 5 ,
the default device for
.Pa /dev/dsp .
Equivalent to a symlink from
.Pa /dev/dsp
to
.Pa /dev/dsp Ns Va ${hw.snd.default_unit} .
.It Va hw.snd.feeder_rate_max
Maximum allowable sample rate.
.It Va hw.snd.feeder_rate_min
Minimum allowable sample rate.
.It Va hw.snd.feeder_rate_round
Sample rate rounding threshold, to avoid large prime division at the
cost of accuracy.
All requested sample rates will be rounded to the nearest threshold value.
Possible values range between 0 (disabled) and 500.
Default is 25.
.It Va hw.snd.latency
Configure the buffering latency.
Only affects applications that do not explicitly request
blocksize / fragments.
This tunable provides finer granularity than the
.Va hw.snd.latency_profile
tunable.
Possible values range between 0 (lowest latency) and 10 (highest latency).
.It Va hw.snd.latency_profile
Define sets of buffering latency conversion tables for the
.Va hw.snd.latency
tunable.
A value of 0 will use a low and aggressive latency profile which can result
in possible underruns if the application cannot keep up with a rapid irq
rate, especially during high workload.
The default value is 1, which is considered a moderate/safe latency profile.
.It Va hw.snd.maxautovchans
Global
.Tn VCHAN
setting that only affects devices with at least one playback or recording channel available.
The sound system will dynamically create up this many
.Tn VCHANs .
Set to
.Dq 0
if no
.Tn VCHANS
are desired.
Maximum value is 256.
.It Va hw.snd.report_soft_formats
Controls the internal format conversion if it is
available transparently to the application software.
When disabled or not available, the application will
only be able to select formats the device natively supports.
.It Va hw.snd.verbose
Level of verbosity for the
.Pa /dev/sndstat
device.
Higher values include more output and the highest level,
four, should be used when reporting problems.
Other options include:
.Bl -tag -width 2n
.It 0
Installed devices and their allocated bus resources.
.It 1
The number of playback, record, virtual channels, and
flags per device.
.It 2
Channel information per device including the channel's
current format, speed, and pseudo device statistics such as
buffer overruns and buffer underruns.
.It 3
File names and versions of the currently loaded sound modules.
.It 4
Various messages intended for debugging.
.El
.It Va dev.pcm.%d.[play|rec].vchans
The current number of
.Tn VCHANs
allocated per device.
This can be set to preallocate a certain number of
.Tn VCHANs .
Setting this value to
.Dq 0
will disable
.Tn VCHANs
for this device.
.It Va dev.pcm.%d.[play|rec].vchanformat
Format for
.Tn VCHAN
mixing.
All playback paths will be converted to this format before the mixing
process begins.
.It Va dev.pcm.%d.[play|rec].vchanrate
Sample rate speed for
.Tn VCHAN
mixing.
All playback paths will be converted to this sample rate before the mixing
process begins.
.It Va dev.pcm.%d.polling
Experimental polling mode support where the driver operates by querying the
device state on each tick using a
.Xr callout 9
mechanism.
Disabled by default and currently only available for a few device drivers.
.El
.Ss Recording Channels
On devices that have more than one recording source (ie: mic and line),
there is a corresponding
.Pa /dev/dsp%d.r%d
device.
.Ss Statistics
Channel statistics are only kept while the device is open.
So with situations involving overruns and underruns, consider the output
while the errant application is open and running.
.Ss IOCTL Support
The driver supports most of the
.Tn OSS
.Fn ioctl
functions, and most applications work unmodified.
A few differences exist, while memory mapped playback is
supported natively and in
.Tn Linux
emulation, memory mapped recording is
not due to
.Tn VM
system design.
As a consequence, some applications may need to be recompiled
with a slightly modified audio module.
See
.In sys/soundcard.h
for a complete list of the supported
.Fn ioctl
functions.
.Sh FILES
The
.Nm
drivers may create the following
device nodes:
.Pp
.Bl -tag -width ".Pa /dev/audio%d.%d" -compact
.It Pa /dev/audio%d.%d
Sparc-compatible audio device.
.It Pa /dev/dsp%d.%d
Digitized voice device.
.It Pa /dev/dspW%d.%d
Like
.Pa /dev/dsp ,
but 16 bits per sample.
.It Pa /dev/dsp%d.p%d
Playback channel.
.It Pa /dev/dsp%d.r%d
Record channel.
.It Pa /dev/dsp%d.vp%d
Virtual playback channel.
.It Pa /dev/dsp%d.vr%d
Virtual recording channel.
.It Pa /dev/sndstat
Current
.Nm
status, including all channels and drivers.
.El
.Pp
The first number in the device node
represents the unit number of the
.Nm
device.
All
.Nm
devices are listed
in
.Pa /dev/sndstat .
Additional messages are sometimes recorded when the
device is probed and attached, these messages can be viewed with the
.Xr dmesg 8
utility.
.Pp
The above device nodes are only created on demand through the dynamic
.Xr devfs 5
clone handler.
Users are strongly discouraged to access them directly.
For specific sound card access, please instead use
.Pa /dev/dsp
or
.Pa /dev/dsp%d .
.Sh DIAGNOSTICS
.Bl -diag
.It pcm%d:play:%d:dsp%d.p%d: play interrupt timeout, channel dead
The hardware does not generate interrupts to serve incoming (play)
or outgoing (record) data.
.It unsupported subdevice XX
A device node is not created properly.
.El
.Sh SEE ALSO
.Xr snd_ad1816 4 ,
.Xr snd_als4000 4 ,
.Xr snd_atiixp 4 ,
.Xr snd_audiocs 4 ,
.Xr snd_cmi 4 ,
.Xr snd_cs4281 4 ,
.Xr snd_csa 4 ,
.Xr snd_ds1 4 ,
.Xr snd_emu10k1 4 ,
.Xr snd_emu10kx 4 ,
.Xr snd_envy24 4 ,
.Xr snd_envy24ht 4 ,
.Xr snd_es137x 4 ,
.Xr snd_ess 4 ,
.Xr snd_fm801 4 ,
.Xr snd_gusc 4 ,
.Xr snd_hda 4 ,
.Xr snd_ich 4 ,
.Xr snd_maestro 4 ,
.Xr snd_maestro3 4 ,
.Xr snd_mss 4 ,
.Xr snd_neomagic 4 ,
.Xr snd_sbc 4 ,
.Xr snd_solo 4 ,
.Xr snd_spicds 4 ,
.Xr snd_t4dwave 4 ,
.Xr snd_uaudio 4 ,
.Xr snd_via8233 4 ,
.Xr snd_via82c686 4 ,
.Xr snd_vibes 4 ,
.Xr devfs 5 ,
.Xr device.hints 5 ,
.Xr loader.conf 5 ,
.Xr dmesg 8 ,
.Xr kldload 8 ,
.Xr sysctl 8
.Rs
.%T "The OSS API"
.%O "http://www.opensound.com/pguide/oss.pdf"
.Re
.Sh HISTORY
The
.Nm
device driver first appeared in
.Fx 2.2.6
as
.Nm pcm ,
written by
.An Luigi Rizzo .
It was later
rewritten in
.Fx 4.0
by
.An Cameron Grant .
The API evolved from the VOXWARE
standard which later became OSS standard.
.Sh AUTHORS
.An -nosplit
.An Luigi Rizzo Aq luigi@iet.unipi.it
initially wrote the
.Nm pcm
device driver and this manual page.
.An Cameron Grant Aq gandalf@vilnya.demon.co.uk
later revised the device driver for
.Fx 4.0 .
.An Seigo Tanimura Aq tanimura@r.dl.itc.u-tokyo.ac.jp
revised this manual page.
It was then rewritten for
.Fx 5.2 .
.Sh BUGS
Some features of your sound card (e.g., global volume control) might not
be supported on all devices.