Preemption is an optional policy that you can apply to an audio type. It determines whether the audio stream is suspended when its volume is ducked to zero.
The volume can be ducked to zero either because of the ducking policy of a single audio type or the culmulative effect of the ducking policies of multiple higher-priority audio types, same-priority audio types, or both (see “Understanding the mix policy when using multiple audio types with the same priority”).
You use the audio policy configuration file preemptable key to specify whether an audio stream can be preempted.
For example, to stop playing music when a voice call is received, in the audio policy configuration file:
Preemption puts the audio stream into the SUSPENDED (SND_PCM_STATUS_SUSPENDED) state. In some cases, you can use an API call to move out of the SUSPENDED state. For more information, see “Using APIs to move out of the suspended state”.
When an audio stream is preempted, it moves to the SUSPENDED state (SND_PCM_STATUS_SUSPENDED), which has two modes: soft-suspended and hard-suspended.
The mode determines what happens when you call an API that attempts to transition the stream to the RUNNING mode (for example, snd_pcm_*_resume() or snd_pcm_*_go()).
You can use the ducking_state member of snd_pcm_channel_status_t to determine whether a channel is soft or hard suspended.
When an audio stream is suspended and you call snd_pcm_*_pause, the stream remains in the SUSPENDED state until audio concurrency management clears the SUSPENDED state, and then moves to the PAUSED state.
If an audio stream is in the PREPARED state (call snd_pcm_*_prepare()), you can use the snd_pcm_channel_audio_ducking() function to make audio concurrency management apply ducking polices to an audio stream as if it was RUNNING. This call also enables any preemption policy that applies. (See “Understanding audio ducking” and snd_pcm_channel_audio_ducking().)
Manual ducking remains active until you either use snd_pcm_channel_audio_ducking() to cancel it or reset the parameters using snd_pcm_plugin_params(). However, if the stream transitions out of the PREPARE state (for example, to READY), preemption ends but ducking of same- and lower-priority streams continues.
You can't explicitly move an audio stream to or out of the SUSPENDED (SND_PCM_STATUS_SUSPENDED) state, but you can define the behavior of an audio stream when the suspension ends.
The transient key is set for the interrupting audio stream but configures the behavior of a preempted stream when preemption ends. It makes the interruption appear to be either short-lived (transient) or permanent. When a higher- or same-priority stream with transient enabled begins, the lower priority stream is suspended. When the interrupting stream stops, the lower-priority stream automatically resumes playing, which makes the interruption appear short-lived. An interrupting stream with transient disabled also suspends a lower priority steam from playing, but when the interrupting stream stops, the lower-priority stream moves to the PAUSED state. The interruption is permanent until snd_pcm_*_resume() is called.
This move to the PAUSED state only happens if a single higher-priority stream ducks the stream to 0. If cumulative ducking sets the volume to 0, the behavior is the same as when transient is true (see “Understanding how multiple ducking levels cumulate”).
When preemptable is true:suspend, the audio stream being preempted transitions back its original state when the suspension ends, regardless of the value of transient.
For more information, see “Syntax of the audio policy configuration file.”