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User Configuration
Various parameters are configurable by the user. These settings can be stored in the NVM on the device itself, or come from an SD card. In general the device does not write to the SD card (exception: see Export section below) in order to avoid filesystem corruption. This means that only settings read from or stored in NVM are user changeable via the head unit. Settings loaded from the SD card are effectively read-only during operation.
In the case of no SD card, all settings are configured on the device itself, using the screen and buttons. When saved, these settings are saved to the NVM on the device and reloaded the next time it starts.
With an SD card inserted, some (or all) settings can be loaded from a configuration file on the card. These settings are contained with a file called config.txt and written in plain ASCII using an INI file syntax. Each key/value pair is predefined and corresponds to a setting on the device.
If a particular setting has no entry in config.txt, then it behaves as if there was no SD card (for that particular setting). In other words, the value can be viewed and changed using the screen and buttons and the value is saved to the device's NVM. Upon a reboot, if this setting is still not contained on the SD card, then the value from NVM will be loaded and used.
When a setting is found in the config.txt file, and it is valid, then that setting value will be loaded and used by the device, ignoring any value stored in NVM for that particular setting. The screen will show the value loaded from the SD card, but it will be read-only and not changeable. Some indication should be used to show the user that the setting is read-only and/or from the SD card. To change such a setting, the SD card would have to be removed and edited on a computer.
The one case where the device can write to the SD card is during a full setting export. When activated, the user will be warned to not power off the device while it is writing to the SD card. A config.txt file will then be written to the card, containing all user configurable settings. This allows cloning a particular setup onto the card for further editing and/or copying to other devices.
A confirmation screen is probably wise to confirm overwriting an existing config.txt file. Perhaps also rename the existing file to something like config-old.txt.