fix #628

firmware/defmt-rtt/Cargo.toml

@@ -12,4 +12,5 @@ version = "0.3.0"
 
 [dependencies]
 cortex-m = "0.7"
+konst = "0.2"
 defmt = { version = "0.3", path = "../../defmt" }

firmware/defmt-rtt/README.md

@@ -12,6 +12,10 @@ The fastest way to get started with `defmt` is to use our [app-template] to set
 
 For more details about the framework check the book at https://defmt.ferrous-systems.com
 
+## Customization
+
+The RTT buffer size (default: 1024) can be configured with the `DEFMT_RTT_BUFFER_SIZE` environment variable in a tight memory situation. Use a power of 2 for best performance.
+
 ## Support
 
 `defmt-rtt` is part of the [Knurling] project, [Ferrous Systems]' effort at

firmware/defmt-rtt/build.rs

@@ -0,0 +1,23 @@
+fn main() {
+ println!("cargo:rerun-if-env-changed=DEFMT_RTT_BUFFER_SIZE");
+
+ let size = option_env!("DEFMT_RTT_BUFFER_SIZE")
+ .map(|s| s.parse().ok())
+ .flatten()
+ .unwrap_or(1024_usize);
+
+ let non_power_of_two = (size & (size - 1)) != 0;
+
+ if non_power_of_two {
+ println!("cargo:warning=RTT buffer size of {} is not a power of two, performance will be degraded.", size)
+ }
+
+ let out_dir_path = std::path::PathBuf::from(std::env::var_os("OUT_DIR").unwrap());
+ let out_file_path = out_dir_path.join("consts.rs");
+
+ std::fs::write(
+ out_file_path,
+ format!("pub(crate) const BUF_SIZE: usize = {};", size),
+ )
+ .unwrap();
+}

firmware/defmt-rtt/src/channel.rs

@@ -3,7 +3,7 @@ use core::{
  sync::atomic::{AtomicUsize, Ordering},
 };
 
-use crate::{MODE_BLOCK_IF_FULL, MODE_MASK, SIZE};
+use crate::{consts::BUF_SIZE, MODE_BLOCK_IF_FULL, MODE_MASK};
 
 #[repr(C)]
 pub(crate) struct Channel {
@@ -45,9 +45,9 @@ impl Channel {
  let available = if read > write {
  read - write - 1
  } else if read == 0 {
- SIZE - write - 1
+ BUF_SIZE - write - 1
  } else {
- SIZE - write
+ BUF_SIZE - write
  };
 
  if available == 0 {
@@ -58,9 +58,9 @@ impl Channel {
  let len = bytes.len().min(available);
 
  unsafe {
- if cursor + len > SIZE {
+ if cursor + len > BUF_SIZE {
  // split memcpy
- let pivot = SIZE - cursor;
+ let pivot = BUF_SIZE - cursor;
  ptr::copy_nonoverlapping(bytes.as_ptr(), self.buffer.add(cursor), pivot);
  ptr::copy_nonoverlapping(bytes.as_ptr().add(pivot), self.buffer, len - pivot);
  } else {
@@ -69,21 +69,21 @@ impl Channel {
  }
  }
  self.write
- .store(write.wrapping_add(len) % SIZE, Ordering::Release);
+ .store(write.wrapping_add(len) % BUF_SIZE, Ordering::Release);
 
  len
  }
 
  fn nonblocking_write(&self, bytes: &[u8]) -> usize {
  let write = self.write.load(Ordering::Acquire);
  let cursor = write;
- // NOTE truncate at SIZE to avoid more than one "wrap-around" in a single `write` call
- let len = bytes.len().min(SIZE);
+ // NOTE truncate atBUF_SIZE to avoid more than one "wrap-around" in a single `write` call
+ let len = bytes.len().min(BUF_SIZE);
 
  unsafe {
- if cursor + len > SIZE {
+ if cursor + len > BUF_SIZE {
  // split memcpy
- let pivot = SIZE - cursor;
+ let pivot = BUF_SIZE - cursor;
  ptr::copy_nonoverlapping(bytes.as_ptr(), self.buffer.add(cursor), pivot);
  ptr::copy_nonoverlapping(bytes.as_ptr().add(pivot), self.buffer, len - pivot);
  } else {
@@ -92,7 +92,7 @@ impl Channel {
  }
  }
  self.write
- .store(write.wrapping_add(len) % SIZE, Ordering::Release);
+ .store(write.wrapping_add(len) % BUF_SIZE, Ordering::Release);
 
  len
  }

firmware/defmt-rtt/src/consts.rs

@@ -0,0 +1,2 @@
+// see `build.rs` for contents
+include!(concat!(env!("OUT_DIR"), "/consts.rs"));

firmware/defmt-rtt/src/lib.rs

@@ -28,6 +28,12 @@ use cortex_m::{interrupt, register};
 
 use crate::channel::Channel;
 
+mod consts;
+
+/// RTT buffer size. Default: 1024; can be customized by setting `DEFMT_RTT_BUFFER_SIZE`.
+/// Use a power of 2 for best performance.
+use crate::consts::BUF_SIZE;
+
 #[defmt::global_logger]
 struct Logger;
 
@@ -94,10 +100,6 @@ const MODE_BLOCK_IF_FULL: usize = 2;
 /// Don't block if the RTT buffer is full. Truncate data to output as much as fits.
 const MODE_NON_BLOCKING_TRIM: usize = 1;
 
-// TODO make configurable
-// NOTE use a power of 2 for best performance
-const SIZE: usize = 1024;
-
 // make sure we only get shared references to the header/channel (avoid UB)
 /// # Safety
 /// `Channel` API is not re-entrant; this handle should not be held from different execution
@@ -115,7 +117,7 @@ unsafe fn handle() -> &'static Channel {
  up_channel: Channel {
  name: NAME as *const _ as *const u8,
  buffer: unsafe { &mut BUFFER as *mut _ as *mut u8 },
- size: SIZE,
+ size: BUF_SIZE,
  write: AtomicUsize::new(0),
  read: AtomicUsize::new(0),
  flags: AtomicUsize::new(MODE_NON_BLOCKING_TRIM),
@@ -124,7 +126,7 @@ unsafe fn handle() -> &'static Channel {
 
  #[cfg_attr(target_os = "macos", link_section = ".uninit,defmt-rtt.BUFFER")]
  #[cfg_attr(not(target_os = "macos"), link_section = ".uninit.defmt-rtt.BUFFER")]
- static mut BUFFER: [u8; SIZE] = [0; SIZE];
+ static mut BUFFER: [u8; BUF_SIZE] = [0; BUF_SIZE];
 
  static NAME: &[u8] = b"defmt\0";