Embedded Rust Basics.

defmt is an efficient logging framework. Just like log, the defmt crate enables defmt::info, defmt::warn and other levels. It also has a defmt::println! macro.

The name defmt comes from deferred formatting:

(…) instead of formatting 255u8 into "255" and sending the string, the single-byte binary data is sent to a second machine, the host, and the formatting happens there.

So the formatting is deferred to the host. The other bit improving efficiency is compression:

defmt’s string compression consists of building a table of string literals, like "Hello, world" or "The answer is {:?}", at compile time. At runtime the logging machine sends indices instead of complete strings.

Source: defmt docs.

defmt Ecosystem

esp-println, esp-backtrace and espflash provide mechanisms to use defmt:

• espflash has support for different logging formats, one of them being defmt.

• esp-println needs defmt-espflash feature. As their docs state:

  Using the defmt-espflash feature, esp-println will install a defmt global logger. Updated the Cargo.toml’s features.

• esp-backtrace needs a defmt feature that uses defmt logging to print panic and exception handler messages.

Exercise

Go to exercises/defmt directory.

1. Update the runner’s logger in .cargo/config.toml:

   [target.riscv32imac-unknown-none-elf]
   - runner = "espflash flash --monitor"
   + runner = "espflash flash --monitor -L defmt"
   

   so that espflash monitor can decode the format of the defmt messages received.

2. Update Cargo.toml to include the needed features:

   esp-println = { version = "0.16.0", features = [
       "esp32c6",
   -     "log-04",
   +     "defmt-espflash",
   ] }
   # (...)
   esp-backtrace = { version = "0.18.0", features = [
       "esp32c6",
       "panic-handler",
   +   "defmt",
   ]}
   

3. Due to the linking process we need to add defmt linker script to cargo/config.toml:

   rustflags = [
     # ....
   +  "-C", "link-arg=-Tdefmt.x",
   ]
   

4. Add defmt to the dependencies.

5. Logging level: Use the defmt::println! and some defmt macros to print a few messages.

   • When building the app, set DEFMT_LOG level as done for ESP_LOG earlier.

   • An alternative to changing .cargo/config.toml is using DEFMT_LOG=<value> cargo run --release; the same is valid for ESP_LOG.

6. Add a panic! macro to trigger a panic with a defmt message.

exercises/defmt/examples/defmt.rs contains a solution. You can run it with the following command: cargo run --example defmt --release. You will need to have the settings above done correctly though!

Suggested reading

Short articles that give more context:

• defmt linking process for setting the compilation-time linker up
• defmt DEFMT_LOG environment variable
• esp-println logging formats