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Deploying to mFi Devices

The mFi persistent flash storage is only 64 KB — far too small for the binaries themselves. The recommended approach is to store binaries in /tmp (RAM disk) and use a small boot script in persistent storage to download them on startup.

Binary Size Optimization

Release builds (mips-release) use -Os, LTO, -fno-rtti, -fno-unwind-tables, -ffunction-sections, -fdata-sections, --gc-sections, -fmerge-all-constants, -fvisibility=hidden, and -Wl,--exclude-libs,ALL to minimize binary size. Typical stripped sizes for MIPS static builds:

Binary On disk In memory (text+data+bss)
mfi-mqtt-client 1.14 MB 1.16 MB
mfi-rest-server 741 KB 768 KB
mfi-cli 692 KB 718 KB

For further on-disk savings, enable UPX compression at build time (off by default):

cmake --preset mips-release -DMFI_UPX=ON
cmake --build --preset mips-release

This automatically runs upx --best on each executable after linking. UPX-compressed binaries are self-extracting — they decompress into memory at startup with no additional tooling needed on the device. This reduces download time over the network and storage on the HTTP server hosting them. In-memory size is unchanged.

Deployment via Boot Script

  1. Build with mips-release preset.
  2. Host the binaries on a local HTTP server or NAS accessible from the device.
  3. Create /var/etc/persistent/rc.poststart to download and run on boot:
#!/bin/sh

BINARY_URL="http://your-server/mfi"
INSTALL_DIR="/tmp/mfi-bin"

mkdir -p "$INSTALL_DIR"

# Download binaries
wget -q -O "$INSTALL_DIR/mfi-mqtt-client" "$BINARY_URL/mfi-mqtt-client"
chmod +x "$INSTALL_DIR/mfi-mqtt-client"

# Start the MQTT client
"$INSTALL_DIR/mfi-mqtt-client" --config /var/etc/persistent/mqtt.conf &
  1. Place your configuration file at /var/etc/persistent/mqtt.conf (this is small enough for persistent storage).
  2. Run save on the device to persist the script and config across reboots.

Direct SCP Deployment (Non-Persistent)

For testing, you can SCP binaries directly to /tmp:

scp build/mips-release/mfi-mqtt-client/mfi-mqtt-client admin@mfi-device:/tmp/
ssh admin@mfi-device '/tmp/mfi-mqtt-client --config /var/etc/persistent/mqtt.conf &'

Note: /tmp is cleared on reboot, so this approach requires re-copying after each restart.