embassy/docs/modules/ROOT/pages/faq.adoc

= Frequently Asked Questions

These are a list of unsorted, commonly asked questions and answers.

Please feel free to add items to link:https://github.com/embassy-rs/embassy/edit/main/docs/modules/ROOT/pages/faq.adoc[this page], especially if someone in the chat answered a question for you!

== How to deploy to RP2040 without a debugging probe.

Install link:https://github.com/JoNil/elf2uf2-rs[elf2uf2-rs] for converting the generated elf binary into a uf2 file.

Configure the runner to use this tool, add this to `.cargo/config.toml`:
[source,toml]
----
[target.'cfg(all(target_arch = "arm", target_os = "none"))']
runner = "elf2uf2-rs --deploy --serial --verbose"
----

The command-line parameters `--deploy` will detect your device and upload the binary, `--serial` starts a serial connection. See the documentation for more info.

== Missing main macro

If you see an error like this:

[source,rust]
----
#[embassy_executor::main]
|                   ^^^^ could not find `main` in `embassy_executor`
----

You are likely missing some features of the `embassy-executor` crate.

For Cortex-M targets, consider making sure that ALL of the following features are active in your `Cargo.toml` for the `embassy-executor` crate:

* `arch-cortex-m`
* `executor-thread`
* `nightly`

For Xtensa ESP32, consider using the executors and `#[main]` macro provided by your appropriate link:https://crates.io/crates/esp-hal-common[HAL crate].

== Why is my binary so big?

The first step to managing your binary size is to set up your link:https://doc.rust-lang.org/cargo/reference/profiles.html[profiles].

[source,toml]
----
[profile.release]
debug = false
lto = true
opt-level = "s"
incremental = true
----

All of these flags are elaborated on in the Rust Book page linked above.

=== My binary is still big... filled with `std::fmt` stuff!

This means your code is sufficiently complex that `panic!` invocation's formatting requirements could not be optimized out, despite your usage of `panic-halt` or `panic-reset`.

You can remedy this by adding the following to your `.cargo/config.toml`:

[source,toml]
----
[unstable]
build-std = ["core"]
build-std-features = ["panic_immediate_abort"]
----

This replaces all panics with a `UDF` (undefined) instruction.

Depending on your chipset, this will exhibit different behavior.

Refer to the spec for your chipset, but for `thumbv6m`, it results in a hardfault. Which can be configured like so:

[source,rust]
----
#[exception]
unsafe fn HardFault(_frame: &ExceptionFrame) -> ! {
    SCB::sys_reset() // <- you could do something other than reset
}
----

Refer to cortex-m's link:https://docs.rs/cortex-m-rt/latest/cortex_m_rt/attr.exception.html[exception handling] for more info.