For the Pinephone, the Quectel firmware supports AT commands for creating a TCP connection through the modem, after which the modem sends wakeups on the bus when it receives data on that socket. So that can be used to implement push notifications without any lag from having to periodically wake up to check.
The problem is that:
a) ... this requires teaching ModemManager about this API since MM is the only thing that talks to the modem.
b) ... this requires teaching clients to talk to MM for a TCP connection, likely using some new dbus-based API to match MM's existing API, instead of just socket() + connect() + read() + write(). So existing TCP-socket using programs will not work without modification. (LD_PRELOAD trickery can work, though ideally only sockets used for push notifications would be routed through MM and the rest use socket(), which can be hard for an LD_PRELOAD'd library to do.)
c) ... one should really be running the OSS firmware instead of Quectel's, and the OSS firmware does not implement those commands.
This was discussed a few months ago in #pinephone. AFAIK no one is working on it.
The problem is that:
a) ... this requires teaching ModemManager about this API since MM is the only thing that talks to the modem.
b) ... this requires teaching clients to talk to MM for a TCP connection, likely using some new dbus-based API to match MM's existing API, instead of just socket() + connect() + read() + write(). So existing TCP-socket using programs will not work without modification. (LD_PRELOAD trickery can work, though ideally only sockets used for push notifications would be routed through MM and the rest use socket(), which can be hard for an LD_PRELOAD'd library to do.)
c) ... one should really be running the OSS firmware instead of Quectel's, and the OSS firmware does not implement those commands.
This was discussed a few months ago in #pinephone. AFAIK no one is working on it.