[alsa-devel] Using UCM with PulseAudio

[Arun Raghavan]

Hello,
I've been working on porting PulseAudio to Android on the OMAP4-based
Galaxy Nexus, and have recently looking at the policy-bits. The basic
porting has been greatly simplified with Wei Feng's work to renew
PulseAudio UCM integration and  Liam's help with fixing up some of the
UCM config for the Galaxy Nexus. I am, however, facing some trouble
mapping how the hardware is used to how UCM presents it (or, perhaps,
the UCM-PA mapping does).

Some simplified background: the devices of interest on the OMAP4 SoC are
the main hifi PCM which can be routed to various outputs, the modem PCM
which is not used for actual output but to enable use of the modem
during calls, and the tones PCM which is intended to be used for playing
ringtones, aiui.

The first problem is mutual exclusivity of verbs. From what I can
understand, verbs are intended to be mutually exclusive -- if you have a
HiFi verb and a VoiceCall verb, only one may be used at a time. We have
mapped verbs to card profiles, which offer the same guarantee. However,
on Android (which is a fair example of the kind of audio policy we might
want), the HiFi verb PCMs maybe used while the VoiceCall PCMs are open.
This is done, for example, to play an end-of-call tone from the CPU
while the modem PCMs are still held open. Is there some way to do this
with UCM?

The second problem is having separate PCMs for modifiers. In the OMAP4
profile, ringtone playback is exposed via a PlayTone modifier which
corresponds to a separate PCM from regular HiFi playback. In the UCM-PA
mapping we decided on, modifiers were implemented as device intended
roles on a sink, so that when a stream with that role came in, we could
enable the modifier, and disable it when such a stream ends. However,
this doesn't account for switching the PCM on which playback is
occurring. Should we be creating a separate sink for such modifiers
(with lower priority, so they're not routed to unless there's a stream
with the required role coming in)? Or should we be reopening the PCM for
this?

Finally, a question also related to modifiers -- is it expected that
there will never be a case where a stream that requires no modifier is
being played while a stream that does require a modifier also exists? If
not, what kind of policy should we have for enabling the modifier or
not?

Cheers,
Arun