diag: serial canary characters at kernel init checkpoints

src/arch/x86_shared/start.rs

@@ -84,6 +84,15 @@ extern "C" fn kstart() {
 /// The entry to Rust, all things must be initialized
 unsafe extern "C" fn start(args_ptr: *const KernelArgs, stack_end: usize) -> ! {
     unsafe {
+        // EARLY CANARY: write 'R' to COM1 before any kernel init.
+        // This proves the serial hardware works and the kernel reached Rust entry.
+        // If this character appears but "Redox OS starting..." does not,
+        // the hang is in args_ptr.read(), serial::init(), or graphical_debug::init().
+        #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
+        {
+            core::arch::asm!("out dx, al", in("dx") 0x3F8u16, in("al") b'R', options(nostack, preserves_flags));
+        }
+
         let bootstrap = {
             let args = args_ptr.read();
 
@@ -93,15 +102,31 @@ unsafe extern "C" fn start(args_ptr: *const KernelArgs, stack_end: usize) -> ! {
             // Set up graphical debug
             graphical_debug::init(args.env());
 
+            // SECOND CANARY: write 'S' to COM1 after serial init.
+            // If 'R' appears but 'S' does not, the hang is in serial::init() or graphical_debug::init().
+            #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
+            {
+                core::arch::asm!("out dx, al", in("dx") 0x3F8u16, in("al") b'S', options(nostack, preserves_flags));
+            }
+
             info!("Redox OS starting...");
             args.print();
 
+            #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
+            { core::arch::asm!("out dx, al", in("dx") 0x3F8u16, in("al") b'1', options(nostack, preserves_flags)); }
+
             // Set up GDT
             gdt::init_bsp(stack_end);
 
+            #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
+            { core::arch::asm!("out dx, al", in("dx") 0x3F8u16, in("al") b'2', options(nostack, preserves_flags)); }
+
             // Set up IDT
             idt::init_bsp();
 
+            #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
+            { core::arch::asm!("out dx, al", in("dx") 0x3F8u16, in("al") b'3', options(nostack, preserves_flags)); }
+
             // Initialize RMM
             #[cfg(target_arch = "x86")]
             let mut bump_allocator =
@@ -109,6 +134,9 @@ unsafe extern "C" fn start(args_ptr: *const KernelArgs, stack_end: usize) -> ! {
             #[cfg(target_arch = "x86_64")]
             let mut bump_allocator = crate::startup::memory::init(&args, Some(0x100000), None);
 
+            #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
+            { core::arch::asm!("out dx, al", in("dx") 0x3F8u16, in("al") b'4', options(nostack, preserves_flags)); }
+
             // Initialize paging
             paging::init();
 
@@ -135,6 +163,9 @@ unsafe extern "C" fn start(args_ptr: *const KernelArgs, stack_end: usize) -> ! {
             // Activate memory logging
             crate::log::init();
 
+            #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
+            { core::arch::asm!("out dx, al", in("dx") 0x3F8u16, in("al") b'6', options(nostack, preserves_flags)); }
+
             // Initialize miscellaneous processor features
             #[cfg(target_arch = "x86_64")]
             crate::arch::misc::init(LogicalCpuId::BSP);
@@ -142,6 +173,9 @@ unsafe extern "C" fn start(args_ptr: *const KernelArgs, stack_end: usize) -> ! {
             // Initialize devices
             device::init();
 
+            #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
+            { core::arch::asm!("out dx, al", in("dx") 0x3F8u16, in("al") b'7', options(nostack, preserves_flags)); }
+
             // Read ACPI tables, starts APs
             if cfg!(feature = "acpi") {
                 crate::acpi::init_after_mem(args.acpi_rsdp());