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Many multi-mode Qualcomm devices report all available modes in their AT+GCAP response (for example, CDMA/EVDO and GSM/UMTS) but they cannot actually function in all these modes at the same time. The modem's actual current capabilities are expressed by the QCDM NV ModePref item, which is not reflected in the AT+GCAP response. Reading the current capabilities from the NV ModePref item ensures that ModemManager does not create interfaces for the modem which the modem cannot actually implement. Because the generic modem plugin does not implement the Modem Capabilities hook (because there is no standard way to determine what access technologies a modem supports), the Current Capabilities are copied to the Modem Capabilities. For devices that support QCDM this means that Modem Capabilies which used to be created from the GCAP response and thus would contain all available capabilities now contain only current capabilities. This isn't a problem though since there was no way to switch the devices to use any of their other capabilities, becuase there aren't any standard commands for it. Plugins that know how to switch their modem's capabilities should (and they already do) override load_current_capabilities and load_modem_capabilities to get the correct information.
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 udev 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 port 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!