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If a port returns EAGAIN on write attempts, previously the code would spin and attempt to resend the failed byte after send_delay microseconds. This resulted in up to 3 second hard blocks in the serial code when sending to ports that don't respond. While in this blocking loop no other events or dbus commands could be processed. Instead, send each byte and reschedule sending the next byte in send_delay microseconds, so that we can process other events in between attempts to write to stupid ports. This doesn't hugely decrease the amount of time that probing requires, since we still need to probe all ports of the device before exporting the modem to D-Bus, but it does let MM find responsive ports much more quickly, and ensures that MM doesn't block any D-Bus requests.
ModemManager. The problem ModemManager tries to solve is to provide a unified high level API for communicating with (mobile broadband) modems. While the basic commands are standardized, the more advanced operations (like signal quality monitoring while connected) varies a lot. Using. ModemManager is a system daemon and is not meant to be used directly from the command line. However, since it provides DBus API, it is possible to use 'dbus-send' command to control it from the terminal. There's an example program (tests/mm-test.py) that demonstrates the basic API usage. Implementation. ModemManager is a DBus system bus activated service (meaning it's started automatically when a request arrives). It is written in C. The devices are queried from HAL and automatically updated based on hardware events. There's a GInterface (MMModem) that defines the modem interface and any device specific implementation must implement it. There are two generic MMModem implementations to support the basic operations (one for GSM, one for CDMA,) which are common for all cards. Plugins. Plugins are loaded on startup, and must implement the MMPlugin interface. It consists of a couple of methods which tell the daemon whether the plugin supports a HAL UDI and to create custom MMModem implementations. It most likely makes sense to derive custom modem implementations from one of the generic classes and just add (or override) operations which are not standard. There's a fully working plugin in the plugins/ directory for Huawei cards that can be used as an example for writing new plugins. Writing new plugins is highly encouraged! API. The API is open for changes, so if you're writing a plugin and need to add or change some public method, feel free to suggest it!