Binary size#

RMK has included many optimizations by default to reduce binary size. But there are still some tricks to reduce the binary size further. If you get a linker error like:

or some errors occur when writing configs to flash, that means your microcontroller's internal flash is not big enough.

There are several approaches to solve the problem:

Common approaches#

Change DEFMT_LOG level#

Logging is quite useful when debugging the firmware, but it requires a lot of flash. You can change the default logging level to error at .cargo/config.toml, to print only error messages and save flash:

# .cargo/config.toml

[env]
- DEFMT_LOG = "debug"
+ DEFMT_LOG = "error"

Enable unstable feature#

According to embassy's doc, you can set the following in your .cargo/config.toml

[unstable]
build-std = ["core"]
build-std-features = ["panic_immediate_abort"]

And then compile your project with nightly Rust:

For keyboard.toml users#

RMK provides several options that you can use to reduce the binary size:

1. If you don't need storage, you can disable the storage feature to save some flash. To disable storage feature you need to disable default features of rmk crate, and then enable other features you need.

2. You can also fully remove defmt by removing defmt feature from rmk crate and similar feature gates from all other dependencies.

3. If you don't need vial support, you can also disable the vial feature by disabling default features of rmk crate.

# Default features `defmt`, `vial`, and `storage` are disabled
rmk = { version = "...", default-features = false }

If you're using keyboard.toml, you'll also need to disable storage, defmt, and vial in the toml config:

# Disable storage, defmt and vial in keyboard.toml
[storage]
enabled = false

[dependency]
defmt_log = false

[host]
vial_enabled = false

For Rust code users#

Use panic-halt#

By default, RMK uses panic-probe to print error messages if panic occurs. But panic-probe actually takes lots of flash because the panic call can not be optimized. The solution is to use panic-halt instead of panic-probe:

# In your binary's Cargo.toml

- panic-probe = { version = "1.0", features = ["print-defmt"] }
+ panic-halt = "1.0"

Then in main.rs, use panic-halt instead:

// src/main.rs

- use panic_probe as _;
+ use panic_halt as _;

Remove defmt-rtt#

You can also remove the entire defmt-rtt logger to save flash.

# In your binary's Cargo.toml
- defmt-rtt = "1.0"

In this case, you have to implement an empty defmt logger.

# src/main.rs
- use defmt_rtt as _;

+ #[defmt::global_logger]
+ struct Logger;
+
+ unsafe impl defmt::Logger for Logger {
+     fn acquire() {}
+     unsafe fn flush() {}
+     unsafe fn release() {}
+     unsafe fn write(_bytes: &[u8]) {}
+ }

Totally remove storage and vial support#

You can disable storage and vial feature in Cargo.toml:

# Default features `defmt`, `vial`, and `storage` are disabled
rmk = { version = "...", default-features = false }

And then remove anything no longer needed in main.rs.