logger: Feature gate logging to serial output

Logging to serial outputs on all panics significantly increases code
size. We add two (on by default) features to disable serial logging:
  - Everywhere
  - Just in panics

Signed-off-by: Joe Richey <joerichey@google.com>

Cargo.toml

@@ -14,6 +14,15 @@ lto = true
 panic = "abort"
 lto = true
 
+[features]
+default = ["log-serial", "log-panic"]
+# Have the log! macro write to serial output. Disabling this significantly
+# reduces code size, but makes debugging essentially impossible
+log-serial = []
+# Log panics to serial output. Disabling this (without disabling log-serial)
+# gets you most of the code size reduction, without losing _all_ debugging.
+log-panic = ["log-serial"]
+
 [dependencies]
 cpuio = "*"
 spin = "0.4.9"

src/logger.rs

@@ -54,9 +54,9 @@ impl fmt::Write for Logger {
 macro_rules! log {
     ($($arg:tt)*) => {{
         use core::fmt::Write;
-        #[cfg(not(test))]
+        #[cfg(all(feature = "log-serial", not(test)))]
         writeln!(&mut crate::logger::LOGGER.lock(), $($arg)*).unwrap();
-        #[cfg(test)]
+        #[cfg(all(feature = "log-serial", test))]
         println!($($arg)*);
     }};
 }

src/main.rs

@@ -15,8 +15,8 @@
 #![feature(global_asm)]
 #![cfg_attr(not(test), no_std)]
 #![cfg_attr(not(test), no_main)]
-#![cfg_attr(test, allow(unused_imports))]
-#![cfg_attr(test, allow(dead_code))]
+#![cfg_attr(test, allow(unused_imports, dead_code))]
+#![cfg_attr(not(feature = "log-serial"), allow(unused_variables, unused_imports))]
 
 #[macro_use]
 mod logger;
@@ -45,12 +45,19 @@ extern "C" {
     fn halt_loop() -> !;
 }
 
-#[cfg_attr(not(test), panic_handler)]
+#[cfg(all(not(test), feature = "log-panic"))]
+#[panic_handler]
 fn panic(info: &PanicInfo) -> ! {
     log!("PANIC: {}", info);
     unsafe { halt_loop() }
 }
 
+#[cfg(all(not(test), not(feature = "log-panic")))]
+#[panic_handler]
+fn panic(_: &PanicInfo) -> ! {
+    loop {}
+}
+
 /// Setup page tables to provide an identity mapping over the full 4GiB range
 fn setup_pagetables() {
     type PageTable = [u64; 512];