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| -rw-r--r-- | docs/4a-framework/README.md | 48 |
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diff --git a/docs/4a-framework/README.md b/docs/4a-framework/README.md index c9a7d8f..77eccde 100644 --- a/docs/4a-framework/README.md +++ b/docs/4a-framework/README.md @@ -44,3 +44,51 @@ The high level API uses the hal-manager to list enabled HALs then it uses directly those HALs. HALs rely directly on drivers and/or softmixer. + +### Softmixer + +The 4a softmixer is a sound server. It is a more secured, more CPU-friendly +server than PulseAudio. + +It is mandatory to have a sound server to share a sound card between +several applications, that may write to the sound card at different +sound rates and formats. + +It is the responsibility of the 4a-softmixer to instantiate the dmix +alsa plugin, based on the 4A config files. + +#### snd-aloop + +snd-aloop, which has recently been replaced by a more flexible implementation +(called avirt), proxifies the access from playback application to sound cards. + +The 4a-softmixer reads from snd-aloop (or avirt) captures, and will perform +the rate conversion, soft volume control for hardware that does not implement +it, routing to configured audio channels, and mixing at the end. + +A security label (SMACK) can be put on the snd-aloop PCM playback devices, +and since each these devices matches an audio role, that applications have to +claim to the 4A high level API, this makes a gatekeeper for audio apps. + +snd-aloop introduces one more buffer copy; but the measured performances and +CPU load on the tested boards do not have revealed any unacceptable levels, up +to now. + +The CPU load (which could be even better with some unimplemented optimizations +today) is typically less than 5% on the Renesas RCAR board, and similar +numbers have been seen on Intel Atom boards. + +#### avirt + +Introduced recently on the master branch of AGL, it provides to 4A high level +a common interface for both softmixing, and DSP-accelerated mixing. I also +sets user-readable names to the dynamically-configured playback devices, that +match the 4A audio roles (aka streams). + +The avirt-loop backend is a loop implementation that provides the same +functionality as snd-aloop, with enhanced security (each playback has a +dedicated PCM device, on which a SMACK label can be put), and with the +flexibility offered by the top avirt layer. + +It is not limited to 8 channels like snd-aloop. + |