rust-raspberrypi-OS-tutorials/03_hacky_hello_world

Tutorial 03 - Hacky Hello World

tl;dr

• Introducing global println!() macros to enable "printf debugging" at the earliest.
• To keep tutorial length reasonable, printing functions for now "abuse" a QEMU property that lets us use the Raspberry's UART without setting it up properly.
• Using the real hardware UART is enabled step-by-step in following tutorials.

Notable additions

• src/console.rs introduces interface Traits for console commands and global access to the kernel's console through console::console().
• src/bsp/raspberrypi/console.rs implements the interface for QEMU's emulated UART.
• The panic handler makes use of the new println!() to display user error messages.
• There is a new Makefile target, make test, intended for automated testing. It boots the compiled kernel in QEMU, and checks for an expected output string produced by the kernel.
  • In this tutorial, it checks for the string Stopping here, which is emitted by the panic!() at the end of main.rs.

Test it

QEMU is no longer running in assembly mode. It will from now on show the output of the console.

$ make qemu
[...]

Hello from Rust!
Kernel panic!

Panic location:
      File 'src/main.rs', line 126, column 5

Stopping here.

Diff to previous

diff -uNr 02_runtime_init/Cargo.toml 03_hacky_hello_world/Cargo.toml
--- 02_runtime_init/Cargo.toml
+++ 03_hacky_hello_world/Cargo.toml
@@ -1,6 +1,6 @@
 [package]
 name = "mingo"
-version = "0.2.0"
+version = "0.3.0"
 authors = ["Andre Richter <andre.o.richter@gmail.com>"]
 edition = "2021"


diff -uNr 02_runtime_init/Makefile 03_hacky_hello_world/Makefile
--- 02_runtime_init/Makefile
+++ 03_hacky_hello_world/Makefile
@@ -25,7 +25,7 @@
     KERNEL_BIN        = kernel8.img
     QEMU_BINARY       = qemu-system-aarch64
     QEMU_MACHINE_TYPE = raspi3
-    QEMU_RELEASE_ARGS = -d in_asm -display none
+    QEMU_RELEASE_ARGS = -serial stdio -display none
     OBJDUMP_BINARY    = aarch64-none-elf-objdump
     NM_BINARY         = aarch64-none-elf-nm
     READELF_BINARY    = aarch64-none-elf-readelf
@@ -36,7 +36,7 @@
     KERNEL_BIN        = kernel8.img
     QEMU_BINARY       = qemu-system-aarch64
     QEMU_MACHINE_TYPE =
-    QEMU_RELEASE_ARGS = -d in_asm -display none
+    QEMU_RELEASE_ARGS = -serial stdio -display none
     OBJDUMP_BINARY    = aarch64-none-elf-objdump
     NM_BINARY         = aarch64-none-elf-nm
     READELF_BINARY    = aarch64-none-elf-readelf
@@ -86,17 +86,20 @@
     --strip-all            \
     -O binary

-EXEC_QEMU = $(QEMU_BINARY) -M $(QEMU_MACHINE_TYPE)
+EXEC_QEMU          = $(QEMU_BINARY) -M $(QEMU_MACHINE_TYPE)
+EXEC_TEST_DISPATCH = ruby ../common/tests/dispatch.rb

 ##------------------------------------------------------------------------------
 ## Dockerization
 ##------------------------------------------------------------------------------
-DOCKER_CMD          = docker run -t --rm -v $(shell pwd):/work/tutorial -w /work/tutorial
-DOCKER_CMD_INTERACT = $(DOCKER_CMD) -i
+DOCKER_CMD            = docker run -t --rm -v $(shell pwd):/work/tutorial -w /work/tutorial
+DOCKER_CMD_INTERACT   = $(DOCKER_CMD) -i
+DOCKER_ARG_DIR_COMMON = -v $(shell pwd)/../common:/work/common

 # DOCKER_IMAGE defined in include file (see top of this file).
 DOCKER_QEMU  = $(DOCKER_CMD_INTERACT) $(DOCKER_IMAGE)
 DOCKER_TOOLS = $(DOCKER_CMD) $(DOCKER_IMAGE)
+DOCKER_TEST  = $(DOCKER_CMD) $(DOCKER_ARG_DIR_COMMON) $(DOCKER_IMAGE)



@@ -191,3 +194,27 @@
 	$(call color_header, "Launching nm")
 	@$(DOCKER_TOOLS) $(NM_BINARY) --demangle --print-size $(KERNEL_ELF) | sort | rustfilt

+
+
+##--------------------------------------------------------------------------------------------------
+## Testing targets
+##--------------------------------------------------------------------------------------------------
+.PHONY: test test_boot
+
+ifeq ($(QEMU_MACHINE_TYPE),) # QEMU is not supported for the board.
+
+test_boot test:
+	$(call color_header, "$(QEMU_MISSING_STRING)")
+
+else # QEMU is supported.
+
+##------------------------------------------------------------------------------
+## Run boot test
+##------------------------------------------------------------------------------
+test_boot: $(KERNEL_BIN)
+	$(call color_header, "Boot test - $(BSP)")
+	@$(DOCKER_TEST) $(EXEC_TEST_DISPATCH) $(EXEC_QEMU) $(QEMU_RELEASE_ARGS) -kernel $(KERNEL_BIN)
+
+test: test_boot
+
+endif

diff -uNr 02_runtime_init/src/bsp/raspberrypi/console.rs 03_hacky_hello_world/src/bsp/raspberrypi/console.rs
--- 02_runtime_init/src/bsp/raspberrypi/console.rs
+++ 03_hacky_hello_world/src/bsp/raspberrypi/console.rs
@@ -0,0 +1,47 @@
+// SPDX-License-Identifier: MIT OR Apache-2.0
+//
+// Copyright (c) 2018-2022 Andre Richter <andre.o.richter@gmail.com>
+
+//! BSP console facilities.
+
+use crate::console;
+use core::fmt;
+
+//--------------------------------------------------------------------------------------------------
+// Private Definitions
+//--------------------------------------------------------------------------------------------------
+
+/// A mystical, magical device for generating QEMU output out of the void.
+struct QEMUOutput;
+
+//--------------------------------------------------------------------------------------------------
+// Private Code
+//--------------------------------------------------------------------------------------------------
+
+/// Implementing `core::fmt::Write` enables usage of the `format_args!` macros, which in turn are
+/// used to implement the `kernel`'s `print!` and `println!` macros. By implementing `write_str()`,
+/// we get `write_fmt()` automatically.
+///
+/// See [`src/print.rs`].
+///
+/// [`src/print.rs`]: ../../print/index.html
+impl fmt::Write for QEMUOutput {
+    fn write_str(&mut self, s: &str) -> fmt::Result {
+        for c in s.chars() {
+            unsafe {
+                core::ptr::write_volatile(0x3F20_1000 as *mut u8, c as u8);
+            }
+        }
+
+        Ok(())
+    }
+}
+
+//--------------------------------------------------------------------------------------------------
+// Public Code
+//--------------------------------------------------------------------------------------------------
+
+/// Return a reference to the console.
+pub fn console() -> impl console::interface::Write {
+    QEMUOutput {}
+}

diff -uNr 02_runtime_init/src/bsp/raspberrypi.rs 03_hacky_hello_world/src/bsp/raspberrypi.rs
--- 02_runtime_init/src/bsp/raspberrypi.rs
+++ 03_hacky_hello_world/src/bsp/raspberrypi.rs
@@ -4,4 +4,5 @@

 //! Top-level BSP file for the Raspberry Pi 3 and 4.

+pub mod console;
 pub mod cpu;

diff -uNr 02_runtime_init/src/console.rs 03_hacky_hello_world/src/console.rs
--- 02_runtime_init/src/console.rs
+++ 03_hacky_hello_world/src/console.rs
@@ -0,0 +1,32 @@
+// SPDX-License-Identifier: MIT OR Apache-2.0
+//
+// Copyright (c) 2018-2022 Andre Richter <andre.o.richter@gmail.com>
+
+//! System console.
+
+use crate::bsp;
+
+//--------------------------------------------------------------------------------------------------
+// Public Definitions
+//--------------------------------------------------------------------------------------------------
+
+/// Console interfaces.
+pub mod interface {
+    /// Console write functions.
+    ///
+    /// `core::fmt::Write` is exactly what we need for now. Re-export it here because
+    /// implementing `console::Write` gives a better hint to the reader about the
+    /// intention.
+    pub use core::fmt::Write;
+}
+
+//--------------------------------------------------------------------------------------------------
+// Public Code
+//--------------------------------------------------------------------------------------------------
+
+/// Return a reference to the console.
+///
+/// This is the global console used by all printing macros.
+pub fn console() -> impl interface::Write {
+    bsp::console::console()
+}

diff -uNr 02_runtime_init/src/main.rs 03_hacky_hello_world/src/main.rs
--- 02_runtime_init/src/main.rs
+++ 03_hacky_hello_world/src/main.rs
@@ -107,12 +107,16 @@
 //! 2. Once finished with architectural setup, the arch code calls `kernel_init()`.

 #![feature(asm_const)]
+#![feature(format_args_nl)]
+#![feature(panic_info_message)]
 #![no_main]
 #![no_std]

 mod bsp;
+mod console;
 mod cpu;
 mod panic_wait;
+mod print;

 /// Early init code.
 ///
@@ -120,5 +124,7 @@
 ///
 /// - Only a single core must be active and running this function.
 unsafe fn kernel_init() -> ! {
-    panic!()
+    println!("Hello from Rust!");
+
+    panic!("Stopping here.")
 }

diff -uNr 02_runtime_init/src/panic_wait.rs 03_hacky_hello_world/src/panic_wait.rs
--- 02_runtime_init/src/panic_wait.rs
+++ 03_hacky_hello_world/src/panic_wait.rs
@@ -4,14 +4,61 @@

 //! A panic handler that infinitely waits.

-use crate::cpu;
+use crate::{cpu, println};
 use core::panic::PanicInfo;

 //--------------------------------------------------------------------------------------------------
 // Private Code
 //--------------------------------------------------------------------------------------------------

+/// Stop immediately if called a second time.
+///
+/// # Note
+///
+/// Using atomics here relieves us from needing to use `unsafe` for the static variable.
+///
+/// On `AArch64`, which is the only implemented architecture at the time of writing this,
+/// [`AtomicBool::load`] and [`AtomicBool::store`] are lowered to ordinary load and store
+/// instructions. They are therefore safe to use even with MMU + caching deactivated.
+///
+/// [`AtomicBool::load`]: core::sync::atomic::AtomicBool::load
+/// [`AtomicBool::store`]: core::sync::atomic::AtomicBool::store
+fn panic_prevent_reenter() {
+    use core::sync::atomic::{AtomicBool, Ordering};
+
+    #[cfg(not(target_arch = "aarch64"))]
+    compile_error!("Add the target_arch to above's check if the following code is safe to use");
+
+    static PANIC_IN_PROGRESS: AtomicBool = AtomicBool::new(false);
+
+    if !PANIC_IN_PROGRESS.load(Ordering::Relaxed) {
+        PANIC_IN_PROGRESS.store(true, Ordering::Relaxed);
+
+        return;
+    }
+
+    cpu::wait_forever()
+}
+
 #[panic_handler]
-fn panic(_info: &PanicInfo) -> ! {
+fn panic(info: &PanicInfo) -> ! {
+    // Protect against panic infinite loops if any of the following code panics itself.
+    panic_prevent_reenter();
+
+    let (location, line, column) = match info.location() {
+        Some(loc) => (loc.file(), loc.line(), loc.column()),
+        _ => ("???", 0, 0),
+    };
+
+    println!(
+        "Kernel panic!\n\n\
+        Panic location:\n      File '{}', line {}, column {}\n\n\
+        {}",
+        location,
+        line,
+        column,
+        info.message().unwrap_or(&format_args!("")),
+    );
+
     cpu::wait_forever()
 }

diff -uNr 02_runtime_init/src/print.rs 03_hacky_hello_world/src/print.rs
--- 02_runtime_init/src/print.rs
+++ 03_hacky_hello_world/src/print.rs
@@ -0,0 +1,38 @@
+// SPDX-License-Identifier: MIT OR Apache-2.0
+//
+// Copyright (c) 2018-2022 Andre Richter <andre.o.richter@gmail.com>
+
+//! Printing.
+
+use crate::console;
+use core::fmt;
+
+//--------------------------------------------------------------------------------------------------
+// Public Code
+//--------------------------------------------------------------------------------------------------
+
+#[doc(hidden)]
+pub fn _print(args: fmt::Arguments) {
+    use console::interface::Write;
+
+    console::console().write_fmt(args).unwrap();
+}
+
+/// Prints without a newline.
+///
+/// Carbon copy from <https://doc.rust-lang.org/src/std/macros.rs.html>
+#[macro_export]
+macro_rules! print {
+    ($($arg:tt)*) => ($crate::print::_print(format_args!($($arg)*)));
+}
+
+/// Prints with a newline.
+///
+/// Carbon copy from <https://doc.rust-lang.org/src/std/macros.rs.html>
+#[macro_export]
+macro_rules! println {
+    () => ($crate::print!("\n"));
+    ($($arg:tt)*) => ({
+        $crate::print::_print(format_args_nl!($($arg)*));
+    })
+}

diff -uNr 02_runtime_init/tests/boot_test_string.rb 03_hacky_hello_world/tests/boot_test_string.rb
--- 02_runtime_init/tests/boot_test_string.rb
+++ 03_hacky_hello_world/tests/boot_test_string.rb
@@ -0,0 +1,3 @@
+# frozen_string_literal: true
+
+EXPECTED_PRINT = 'Stopping here'