[Replicant] Replicant on Nexus One

[Denis 'GNUtoo' Carikli]

On Tue, 23 Jan 2018 23:26:54 +1100
Simon Shields <simon at lineageos.org> wrote:

> It indicates that the "2MIC" (which is an
> Audience EarSmart ES305B[0])
> has both I2C, reset and PCM lines wired directly to the modem.
This most probably means that the modem control this "Audience EarSmart
ES305B", which according to the link, is a chip that is used to improve
the sound quality of the voice:
> * Wideband and narrowband non-stationary noise suppression, both
>   transmit and receive
> * Handheld speakerphone (HHS) non-stationary noise suppression,
>   transmit and receive
> * Acoustic echo cancellation (AEC)
> * Automatic receive voice equalization (VEQ) to boost voice above the
>   ambient acoustic environment
[...]

So if this chip doesn't have a microphone it should be ok.

It typically works like that:
1. The modem sees an incoming call
2. If the processor running Android sleeps, the modem send it a
   notification so it would wake up, and then the modem will notify
   this processor that it has a call. On Replicant libsamsung-ipc and
   samsung-ril probably receives a message indicating that there is a
   call.
3. The android libraries (run by Replicant) talks to the Linux
   sound driver to route the audio from the current microphone[1] to
   the modem (hence the I2S link to the modem).
4. When the call terminates the libraries and sound driver change the
   route again. Since it's also a good idea to power off unused
   components (to save some more battery), the microphone is probably
   powered off when it's not used.

There is some documentation about all that in Documentation/sound/soc
in the Linux kernel source code. That documentation can be found
online, in the git of the Linux kernel here for instance:
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git/tree/Documentation/sound/soc?h=v4.14.14

References:
-----------
[1]If there there is a connected headset with a mic, it routes it to
   the modem, If the Bluetooth chip is connected to the CODEC and that
   the person uses a Bluetooth headset, it routes that, else it routes
   the internal microphone (the one connected to the CODEC) to the
   modem. Depending on the hardware it can route digital or analog
   sound. Some Bluetooth chip can have analog/digital input/output, but
   the sound can also probably be sent/retrieved by software
   communicating with the chip, for instance with Linux drivers and
   Bluez or equivalent.

Denis.
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