Add JTAG helpers

.github/workflows/build.yml

@@ -119,6 +119,9 @@ jobs:
           "noserial",
           "qemu",
           "noserial,qemu",
+          "jtag",
+          "noserial,jtag",
+          # jtag and qemu together don't make much sense
         ]
     runs-on: ubuntu-latest
     timeout-minutes: 10

nucleus/Cargo.toml

@@ -17,6 +17,9 @@ maintenance = { status = "experimental" }
 
 [features]
 noserial = []
+# Enable JTAG debugging of kernel - enable jtag helpers and
+# block waiting for JTAG probe attach at the start of kernel main.
+jtag = []
 # Build for running under QEMU with semihosting, so various halt/reboot options would for example quit QEMU instead.
 qemu = ["qemu-exit"]
 

nucleus/src/arch/aarch64/jtag.rs

@@ -0,0 +1,19 @@
+//! JTAG helper functions.
+
+use cortex_a::asm;
+
+#[no_mangle]
+static mut WAIT_FLAG: bool = true;
+
+/// Wait for debugger to attach.
+/// Then in gdb issue `> set var *(&WAIT_FLAG) = 0`
+/// from inside this function's frame to contiue running.
+pub fn wait_debugger() {
+    use core::ptr::{read_volatile, write_volatile};
+
+    while unsafe { read_volatile(&WAIT_FLAG) } {
+        asm::nop();
+    }
+    // Reset the flag so that next jtag::wait_debugger() would block again.
+    unsafe { write_volatile(&mut WAIT_FLAG, true) }
+}

nucleus/src/arch/aarch64/mod.rs

@@ -8,6 +8,8 @@
 use cortex_a::asm;
 
 mod boot;
+#[cfg(feature = "jtag")]
+pub mod jtag;
 pub mod memory;
 pub mod traps;
 

nucleus/src/main.rs

@@ -111,6 +111,9 @@ fn init_uart_serial() {
 /// `arch` crate is responsible for calling it.
 #[inline]
 pub fn kmain() -> ! {
+    #[cfg(feature = "jtag")]
+    jtag::wait_debugger();
+
     #[cfg(not(feature = "noserial"))]
     init_uart_serial();