Pull up following revision(s) (requested by nat in ticket #848):

	share/man/man9/Makefile: revision 1.422
	distrib/sets/lists/comp/mi: revision 1.2198
	share/man/man7/Makefile: revision 1.33
	share/man/man9/audio.9: revision 1.47
	share/man/man7/audio.7: file removal
	sys/dev/audio.c: revision 1.456
	sys/dev/audio.c: revision 1.457
	distrib/sets/lists/man/mi: revision 1.1591
	share/man/man9/intro.9: revision 1.20
	share/man/man7/audio.7: revision 1.3
	share/man/man9/audio_system.9: revision 1.1
	share/man/man7/intro.7: revision 1.26
	share/man/man4/audio.4: revision 1.86

Fix a typo with sc_out/inports when setting the hardware recording gain on the
mix ring.  Found by mrg@
XXX pullup-8

Using audioctl without the -p switch defaults to the mix ring.  This allows
setting the hardware gain etc.

Update the audio spec in audio.7 to reflect these changes.
Addresses PR kern/52781.

Move the audio specification to section 9 under audio_system.
Ok wiz@


(martin)

.\"	$NetBSD: audio_system.9,v 1.1.2.2 2018/06/07 18:24:15 martin Exp $
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.Dd May 28, 2018
.Dt AUDIO_SYSTEM 9
.Os
.Sh NAME
.Nm audio_system
.Nd the
.Nx
in-kernel audio mixer specification
.Sh INTRODUCTION
This document aims to describe all aspects of the in-kernel audio mixer
included with
.Nx 8
and onwards, describing its current behavior as of 2018.
.Sh VIRTUAL CHANNEL (VCHAN)
This is the most fundamental element to the mixer.
The vchan has all of the properties of the traditional single open
.Nx
audio channel.
It consists of playback and record rings along with
.Vt audio_info
structures.
.Pp
Upon opening of
.Pa /dev/audio
or
.Pa /dev/sound ,
a new vchan and mixerctl structure is created.
In the case of
.Pa /dev/sound ,
.Vt audio_info
structures are inherited from the last open of
.Pa /dev/audio
or
.Pa /dev/sound .
.Pp
All vchans are up or down sampled into the mix ring (intermediate) format
before being sent to hardware.
.Pp
It is described in the following diagram:
.Bd -literal
	VCHAN1---------\\
			\\   VCHAN0
	VCHAN2-------------MIX RING ---- HARDWARE
		...     /
	VCHANn---------/
.Ed
.Pp
In the case of
.Xr sysctl 8
.Dv usemixer=0
(see below), there is only one vchan whose play
and record rings are the hardware play/record rings.
.Pp
User accessible vchans are numbered starting at one (1).
Vchan 0 is used internally by the mixer for the mix ring and its ring buffers
are not user accessible.
.Pp
The only limit to the number of open vchans is the speed of the computer and the
number of free file descriptors.
.Sh BLOCK - SIZE / LATENCY
A block of audio data is the basic unit for audio data.
Audio applications will not commence playback until three (3) blocks have been
written - this is the source of latency in the mixer along with the size of the
audio data block.
.Pp
For normal uses of audio read/write there will be three blocks of audio data before
playback commences one in the vchan, one in the mix ring and one in the
hardware ring.
.Pp
The size of the audio data block is dependent on the audio format configured
by the application the latency
.Xr sysctl 8
and the underlying audio hardware.
.Pp
Some audio hardware devices only support a static block size, as such the
overall latency of the mixer for these devices cannot be changed.
Other devices such as those supported by
.Xr hdaudio 4
allow the hardware block size
to be changed, allowing the latency of the mixer to change from 4
milliseconds (ms) to 128 ms with the mixer intermediate format being 16 bit,
stereo, 48 kHz.
.Pp
With regard to mmapped audio, blocks are played back immediately so the latency
presented to applications is one third of the latency
.Xr sysctl 8
value.
.Pp
Latency can be calculated by the following formula:
.Bd -literal
	Latency (ms) =	 blocksize(bytes) * num blocks * 1000
			--------------------------------------
			freq(Hz) * bytes per sample * channels
.Ed
.Pp
Latency in the mixer and latency presented to audio applications is consistent,
it will be the same regardless of the audio format requested by the audio
application.
.Pp
The default latency configured at boot time is 150ms and is subject to the above
constraints.
.Sh ADDED IOCTLS
Two new ioctls have been added to accommodate mixing of multiple vchans:
.Bl -tag -width indent
.It Dv AUDIO_SETCHAN :
Allows setting the target vchan to operate on for subsequent
.Xr ioctl 2
calls.
.It Dv AUDIO_GETCHAN :
Returns the current vchan number.
.El
.Pp
These ioctls were necessary as some audio applications like to open an
.Xr audio 4
device and an
.Xr audioctl 4
device so to check on buffer usage and samples played etc.
.Pp
As opening an
.Xr audioctl 4
device would represent vchan 0 (the mix ring), these
ioctls allow setting the target vchan and
.Vt audio_info
structure to that of an existing vchan.
.Sh MIXERCTL INTERFACE / SOFTWARE VOLUME
Mixerctl structures are allocated when a new vchan is created.
The mixer control structure allows for setting the software volume for playback -
.Li vchan.dacN
or recording -
.Li vchan.adcN .
These are 8 bit values and the this value is applied during mixing into the mix
ring.
.Pp
The software volume is applied to all channels (1, 2, 4 etc.) in the vchan and at
present (2018-05-04) there are no balance controls for user accessible vchans.
.Pp
The first vchan corresponds to the
.Li vchan.dac1/adc1
mixer controls.
.Pp
All vchan mixer controls only have effect upon its own volume and writing to
.Li outputs.master
(or equivalent) control is required to change the volume of the hardware.
.Pp
Mixer controls are only present whilst the chan is in use and numbering starts
at one (1).
Mixer control numbers i.e.
.Li dac/adc1
correspond to their vchan number.
.Sh AUDIOCTL / AUDIO_INFO INTERFACE
Audioctl allows access to the
.Vt audio_info
structure of a given device.
Due to the audio mixer a
.Fl p
flag was added to allow access to a given vchan's
.Vt audio_info
structure.
The values for
.Fl p
are numbered starting at zero (0).
.Pp
Not specifying
.Fl p
is the same as specifying
.Fl p Ar 0
and will result in working with vchan 0 (the mix ring).
This will display the audio parameters of the mix ring and allow
setting the hardware gain and balance.
.Pp
This is for compatibility with existing applications and shell scripts
that are unaware of the
.Fl p
switch.
.Pp
The parameters for playback and recording only effect the particular vchan
being operated on (gain, sample rate, channels, encoding etc), except
.Fl p Ar 0
(the mix ring).
.Sh ADDED SYSCTLS
With the introduction of the audio mixer the following
.Xr sysctl 7 Ns s
have been added:
.Bl -tag -width indent
.It Li hw.driverN.frequency :
.It Li hw.driverN.precision :
.It Li hw.driverN.channels :
Intermediate mixing format.
(see below)
.It Li hw.driverN.latency :
Expressed in milliseconds.
(see above)
.It Li hw.driverN.multiuser :
Off/On (0/1), defaults to off.
This
.Xr sysctl 7
determines if multiple users are allowed to access the sound hardware.
The root user is always allowed access (i.e., for wsbell).
The first user to open the audio device has full control of the audio device
if this sysctl is set to off.
There currently is an outstanding PR about affecting a privileged process -
PR/52627.
.Pp
Ideally if root intervenes with the audio device, it should do so unaffected.
.Pp
If this control is set to on, then all users' audio data are mixed and all users
have access to the audio hardware.
.It Li hw.driverN.usemixer :
Off/On (0/1), defaults to on.
This
.Xr sysctl 7
enables or disables the audio mixer.
When set to off, the audio device can support only one vchan.
This vchan's play and record ring buffers are the hardware ring buffers.
.Pp
This option was added to aid older/slower systems where the extra overhead of
the audio mixer might pose a problem.
.El
.Sh INTERMEDIATE / MIXING FORMAT
The initial concept was to handle incoming audio data similarly to that of a
superheterodyne radio receiver:
.Pp
.Dl		RF -> IF -> AF
.Pp
So the corresponding mixing concept is:
.Pp
.Dl		vchan -> mixing format -> hardware
.Pp
The
.Xr sysctl 7 Ns s
described above determine the format for mixing.
All vchans are up or down sampled to this format before mixing takes place.
.Pp
On most systems this defaults to 16 bit stereo 48kHz.
The
.Xr sysctl 7 Ns s
governing the mixing format may only be changed when there are no
vchans in use.
.Pp
On faster systems the precision (8, 16, 32 bits) may be changed along with the
sample rate and number of channels (mono, stereo, 4 etc.).
.Pp
On older/slower systems utilizing audio mixing, it may be required to lower the
quality of this format to ease the amount of data processing whilst mixing.
.Pp
All possible audio formats (mulaw, alaw, slinear, ulinear, 8, 16, and 32 bit
precision) are converted for use by the audio mixer.
.Sh MEMORY MAPPED PLAYBACK
It is possible to use mmap for audio playback, achieving reduced latency.
However the audio applications selected format must match the
mixing/intermediate format (see above).
.Pp
It is possible to obtain the
.Vt audio_info
for vchan0 which contains the
intermediate/mixing format to ease applications configuring for mmapped audio.
.Pp
At present most applications don't use the mix ring's
.Vt audio_info
structure to obtain the requiredplay back parameters and some user
intervention is required to set the audio format for the application.
.Sh HARDWARE DRIVER REQUIREMENTS
Audio mixing requires signed linear support in the host's endianness.
Driver authors should support slinear_le and slinear_be formats.
.Pp
If the audio hardware is intended to be used with the mixer disabled, mulaw 1ch
8000 hz needs to be supported also.
.Pp
This is easily achievable with the auconv framework/filters.
All new drivers should consider the use of auconv where possible.
.Sh SEE ALSO
.Xr audioctl 1 ,
.Xr mixerctl 1 ,
.Xr audio 4 ,
.Xr audio 9
.Sh AUTHORS
.An Nathanial Sloss
.Sh SPECIAL THANKS
Great appreciation goes to Onno van der Linden, isaki@, maya@, jmcneill@,
pgoyette@, mrg@, riastradh@ and christos@ \(em without their input, this code would
not be what it is currently.