ALSA project - the C library reference

Use Case Configuration

The ALSA Use Case Configuration. See ALSA Use Case Configuration page for more details.

### ALSA Use Case Configuration

The use case configuration files use Configuration files syntax to define the static configuration tree. This tree is evaluated (modified) at runtime according the conditions and dynamic variables in the configuration tree. The result is parsed and exported to the applications using Use Case Interface API.

### Configuration directory and main filename lookup

The lookup paths are describen in ucm.conf file. The configuration structure looks like:

UseCasePath.path1 {
Directory "conf.virt.d"
File "${OpenName}.conf"
}
UseCasePath.path2 {
Directory "external"
File "${OpenName}.conf"
}

### UCM main configuration file

Each sound card has a master sound card file that lists all the supported use case verbs for that sound card. i.e.:

# Example master file for blah sound card
# By Joe Blogs <joe@bloggs.org>
Syntax 6
# Use Case name for user interface
Comment "Nice Abstracted Soundcard"
# The file is divided into Use case sections. One section per use case verb.
SectionUseCase."Voice Call" {
File "voice_call_blah"
Comment "Make a voice phone call."
}
SectionUseCase."HiFi" {
File "hifi_blah"
Comment "Play and record HiFi quality Music."
}
# Define Value defaults
ValueDefaults {
PlaybackChannels 4
CaptureChannels 4
}
# Define boot / initialization sequence
# This sequence is skipped, when the soundcard was already configured by system
# (alsactl configuration was already created). The purpose is to not alter
# ALSA card controls which may be modified by user after initial settings.
BootSequence [
cset "name='Master Playback Switch',index=2 0,0"
cset "name='Master Playback Volume',index=2 25,25"
msleep 50
cset "name='Master Playback Switch',index=2 1,1"
cset "name='Master Playback Volume',index=2 50,50"
]
# Define fixed boot sequence
# This sequence is always executed on boot (hotplug).
FixedBootSequence [
cset "name='Something to toggle' toggle"
]

### UCM verb configuration file

The verb configuration file defines devices, modifier and initialization sequences. It is something like a sound profile.

# Example Use case verb section for Voice call blah
# By Joe Blogs <joe@blogs.com>
# verb global section
SectionVerb {
# enable and disable sequences are compulsory
EnableSequence [
disdevall "" # run DisableSequence for all devices
]
DisableSequence [
cset "name='Power Save' on"
]
# Optional transition verb
TransitionSequence."ToCaseName" [
disdevall "" # run DisableSequence for all devices
msleep 1
]
# Optional TQ and device values
Value {
TQ HiFi
PlaybackChannels 6
}
}
# Each device is described in new section. N devices are allowed
SectionDevice."Headphones" {
SupportedDevice [
"x"
"y"
]
# or (not both)
ConflictingDevice [
"x"
"y"
]
EnableSequence [
...
]
DisableSequence [
...
]
TransitionSequence."ToDevice" [
...
]
Value {
PlaybackVolume "name='Master Playback Volume',index=2"
PlaybackSwitch "name='Master Playback Switch',index=2"
PlaybackPCM "hw:${CardId},4"
}
}
# Each modifier is described in new section. N modifiers are allowed
SectionModifier."Capture Voice" {
Comment "Record voice call"
SupportedDevice [
"x"
"y"
]
# or (not both)
ConflictingDevice [
"x"
"y"
]
EnableSequence [
...
]
DisableSequence [
...
]
TransitionSequence."ToModifierName" [
...
]
# Optional TQ and ALSA PCMs
Value {
TQ Voice
CapturePCM "hw:${CardId},11"
PlaybackMixerElem "Master"
PlaybackVolume "name='Master Playback Volume',index=2"
PlaybackSwitch "name='Master Playback Switch',index=2"
}
}

### Sequence graphs

### Sequence commands

Command name | Description
--- | ---
enadev2 ARG | execute device enable sequence
disdev2 ARG | execute device disable sequence
disdevall "" | execute device disable sequence for all devices in verb
cdev ARG | ALSA control device name for snd_ctl_open()
cset ARG | ALSA control set - snd_ctl_ascii_elem_id_parse() + snd_ctl_ascii_value_parse()
cset-new ARG | Create new ALSA user control element - snd_ctl_ascii_elem_id_parse() + description
ctl-remove ARG | Remove ALSA user control element - snd_ctl_ascii_elem_id_parse()
sysw ARG | write to sysfs tree
usleep ARG | sleep for specified amount of microseconds
msleep ARG | sleep for specified amount of milliseconds
exec ARG | execute a specific command (without shell - man execv)
shell ARG | execute a specific command (using shell - man system)
cfg-save ARG | save LibraryConfig to a file

# Examples
cset "name='PCM Playback Volue',index=2 99"
cset-new "name='Bool2' type=bool,count=2 1,0"
cset-new "name='Enum' type=enum,labels='L1;L2;L3' 'L2'"
ctl-remove "name='Bool2'"
sysw "-/class/sound/ctl-led/speaker/card${CardNumber}/attach:Speaker Channel Switch"
usleep 10
exec "/bin/echo hello"
shell "set"
cfg-save "/tmp/test.conf:+pcm"

### Naming (devices, verbs)

See the SND_USE_CASE_VERB constains like SND_USE_CASE_VERB_HIFI for the full list of known verbs.

See the SND_USE_CASE_DEV constants like SND_USE_CASE_DEV_SPEAKER for the full list of known devices. If multiple devices with the same name exists, the number suffixes should be added to these names like HDMI1,HDMI2,HDMI3 etc. No number gaps are allowed. The names with numbers must be continuous. It is allowed to put a whitespace between name and index (like 'Line 1') for the better readability. The device names 'Line 1' and 'Line1' are equal for this purpose.

If EnableSequence/DisableSequence controls independent paths in the hardware it is also recommended to split playback and capture UCM devices and use the number suffixes. Example use case: Use the integrated microphone in the laptop instead the microphone in headphones.

The preference of the devices is determined by the priority value (higher value = higher priority).

See the SND_USE_CASE_MOD constants like SND_USE_CASE_MOD_ECHO_REF for the full list of known modifiers.

### Boot (alsactl)

The FixedBootSequence is executed at each boot. The BootSequence is executed only if the card's configuration is missing. The purpose is to let the users modify the configuration like volumes or switches. The alsactl ensures the persistency (store the state of the controls to the /var tree and loads the previous state in the next boot).

### Device volume

It is expected that the applications handle the volume settings. It is not recommended to set the fixed values for the volume settings in the Enable / Disable sequences for verbs or devices, if the device exports the hardware volume (MixerElem or Volume/Switch values). The default volume settings should be set in the BootSequence. The purpose for this scheme is to allow users to override defaults using the alsactl sound card state management.

Checklist:

1. Set default volume in BootSequence
2. Verb's EnableSequence should ensure that all devices are turned off (mixer paths) to avoid simultaneous device use - the previous state is unknown (see disdevall and disdev2 commands or create a new custom command sequence)

### Dynamic configuration tree

The evaluation order may look a bit different from the user perspective. At first, the standard alsa-lib configuration tree is parsed. All other layers on top are working with this tree. It may shift / move the configuration blocks from the configuration files as they are placed to the tree internally.

Example configuration | Parsed static tree | Identical static tree
----------------------------+-------------------------+-------------------------------
If.1 { | If { | If.1.True.Define.VAR "A"
True.Define.VAR "A" | 1.True.Define.VAR "A" | If.2.True.Define.VAR "C"
} | 2.True.Define.VAR "C" | Define.VAR "B"
Define.VAR "B" | } |
If.2 { | Define.VAR "B" |
True.Define.VAR "C" | |
} | |

Even if one or both conditions are evaluated as true, the variable VAR will be evaluated always as B because the first If block was before the non-nested Define . The second If block was appended to the first If block (before Define in the configuration tree) in the text configuration parser.

### Syntax

Unless described, the syntax version 4 is used.

Syntax 4

### Include

There are two ways to include other configuration files.

#### Static include

The static include is inherited from the standard alsa-lib configuration syntax. It can be placed anywhere in the configuration files. The search path is composed from the root alsa configuration path (usually _/usr/share/alsa_) and ucm2 directory.

<some/path/file.conf> # include file using the search path
</absolute/path/file.conf> # include file using the absolute path

#### Lazy include

The lazy include is evaluated at runtime. The root path is the ucm2 tree. The absolute include appends the ucm2 absolute path to the specified path. The relative include is relative to the file which contains the Include configuration block.

Include.id1.File "/some/path/file.conf" # absolute include (in the ucm2 tree)
Include.id2.File "subdir/file.conf" # relative include to the current configuration directory (UseCasePath)

### Configuration tree evaluation

The evaluation of the static configuration tree is proceed in the specific order (see table bellow). When the dynamic configuration tree changes, the evaluation sequence is restarted to evaluate all possible changes (new Define or Include or If blocks).

Evaluation order | Configuration block | Evaluation restart
--- | --- | ---
1 | Define | No
2 | Include | Yes
3 | If | Yes

### Substitutions

The dynamic tree identifiers and assigned values in the configuration tree are substituted. The substitutes strings are in the table bellow.

Substituted string | Value
--- | ---
${OpenName} | Original UCM card name (passed to snd_use_case_mgr_open())
${ConfLibDir} | Library top-level configuration directory (e.g. /usr/share/alsa)
${ConfTopDir} | Top-level UCM configuration directory (e.g. /usr/share/alsa/ucm2)
${ConfDir} | Card's UCM configuration directory (e.g. /usr/share/alsa/ucm2/conf.d/USB-Audio)
${ConfName} | Configuration name (e.g. USB-Audio.conf)
${CardNumber} | Real ALSA card number (or empty string for the virtual UCM card)
${CardId} | ALSA card identifier (see snd_ctl_card_info_get_id())
${CardDriver} | ALSA card driver (see snd_ctl_card_info_get_driver())
${CardName} | ALSA card name (see snd_ctl_card_info_get_name())
${CardLongName} | ALSA card long name (see snd_ctl_card_info_get_longname())
${CardComponents} | ALSA card components (see snd_ctl_card_info_get_components())
${env:<str>} | Environment variable <str>
${sys:<str>} | Contents of sysfs file <str>
${var:<str>} | UCM parser variable (set using a Define block)
${eval:<str>} | Evaluate expression like *($var+2)/3* [Syntax 5]
${find-card:<str>} | Find a card - see Find card substitution section
${find-device:<str>} | Find a device - see Find device substitution section

#### Special whole string substitution

Substituted string | Value
--- | ---
${evali:<str>} | Evaluate expression like *($var+2)/3* [Syntax 6]; target node will be integer; substituted only in the LibraryConfig subtree

#### Find card substitution

This substitutions finds the ALSA card and returns the appropriate identifier or the card number (see return argument).

Usage example:

${find-card:field=name,regex='^acp$',return=number}

Arguments:

Argument | Description
--- | ---
return | return value type (id, number), id is the default
field | field for the lookup (id, driver, name, longname, mixername, components)
regex | regex string for the field match

#### Find device substitution

Usage example:

${find-device:type=pcm,field=name,regex='DMIC'}

Arguments:

Argument | Description
--- | ---
type | device type (pcm)
stream | stream type (playback, capture), playback is default
field | field for the lookup (id, name, subname)
regex | regex string for the field match

### Variable defines

The variables can be defined and altered with the Define or DefineRegex blocks. The Define block looks like:

Define {
variable1 "a"
variable2 "b"
}

The DefineRegex allows substring extraction like:

DefineRegex.rval {
Regex "(hello)|(regex)"
String "hello, it's my regex"
}

The result will be stored to variables rval1 as hello and rval2 as regex (every matched substrings are stored to a separate variable with the sequence number postfix.

Variables can be substituted using the `${var:rval1}` reference for example.

### Macros

Macros were added for Syntax version 6. The DefineMacro defines new macro like:

DefineMacro.macro1 {
Define.a "${var:__arg1}"
Define.b "${var:__other}"
# Device or any other block may be defined here...
}

The arguments in the macro are refered as the variables with the double underscore name prefix (like *__variable*). The configuration block in the DefineMacro subtree is always evaluated (including arguments and variables) at the time of the instantiation.

The macros can be instantiated (expanded) using:

# short version
Macro.id1.macro1 "arg1='something 1',other='other x'"
# long version
Macro.id1.macro1 {
arg1 'something 1'
other 'other x'
}

The second identifier (in example as id1) must be unique, but the contents is ignored. It just differentiate the items in the subtree (allowing multiple instances for one macro).

### Conditions

The configuration tree evaluation supports the conditions - If blocks. Each If blocks must define a Condition block and True or False blocks or both. The True or False blocks will be merged to the parent tree (where the If block is defined) when the Condition is evaluated.

Example:

If.uniqueid {
Condition {
Type String
Haystack "abcd"
Needle "a"
}
True {
Define.a a
define.b b
}
}

#### True (Type AlwaysTrue)

Execute only True block. It may be used to change the evaluation order as explained in the Configuration Tree paragraph.

#### String is empty (Type String)

Field | Description
--- | ---
Empty | string

#### Strings are equal (Type String)

Field | Description
--- | ---
String1 | string
String2 | substring in string

#### Substring is present (Type String)

Field | Description
--- | ---
Haystack | string
Needle | substring in string

#### Regex match (Type RegexMatch)

Field | Description
--- | ---
String | string
Regex | regex expression (extended posix, ignore case)

#### ALSA control element exists (Type ControlExists)

Field | Description
--- | ---
Device | ALSA control device (see snd_ctl_open())
Control | control in ASCII (parsed using snd_ctl_ascii_elem_id_parse())
ControlEnum | value for the enum control (optional)

Example:

If.fmic {
Condition {
Type ControlExists
Control "name='Front Mic Playback Switch'"
}
True {
...
}
}

### Variants

To avoid duplication of the many configuration files for the cases with minimal configuration changes, there is the variant extension. Variants were added for Syntax version 6.

The bellow example will create two verbs - "HiFi" and "HiFi 7.1" with the different playback channels (2 and 8) for the "Speaker" device.

Example (main configuration file):

SectionUseCase."HiFi" {
File "HiFi.conf"
Variant."HiFi" {
Comment "HiFi"
}
Variant."HiFi 7+1" {
Comment "HiFi 7.1"
}
}

Example (verb configuration file - HiFi.conf):

SectionDevice."Speaker" {
Value {
PlaybackChannels 2
}
Variant."HiFi 7+1".Value {
PlaybackChannels 8
}
}