kernel/

debug.rs

// Licensed under the Apache License, Version 2.0 or the MIT License.
// SPDX-License-Identifier: Apache-2.0 OR MIT
// Copyright Tock Contributors 2022.

//! Support for in-kernel debugging.
//!
//! For printing, this module uses an internal buffer to write the strings into.
//! If you are writing and the buffer fills up, you can make the size of
//! `output_buffer` larger.
//!
//! Before debug interfaces can be used, the board file must assign them
//! hardware:
//!
//! ```ignore
//! let debug_gpios = static_init!(
//!     [&'static dyn kernel::hil::gpio::Pin; 2],
//!     [
//!         &sam4l::gpio::PA[13],
//!         &sam4l::gpio::PA[15],
//!     ]
//! );
//! kernel::debug::initialize_debug_gpio::<
//!     <ChipHw as kernel::platform::chip::Chip>::ThreadIdProvider,
//! >();
//! kernel::debug::assign_gpios(debug_gpios);
//!
//! components::debug_writer::DebugWriterComponent::new(
//!     uart_mux,
//!     create_capability!(kernel::capabilities::SetDebugWriterCapability)
//! )
//! .finalize(components::debug_writer_component_static!());
//! ```
//!
//! An alternative to using the default `DebugWriterComponent`, which defaults
//! to sending over UART, is to implement a custom [`DebugWriter`] that can be
//! used for other types of output. For example a simple "endless" FIFO with
//! fixed "push" address:
//!
//! ```ignore
//! use kernel::debug::DebugWriter;
//!
//! pub struct SyncDebugWriter;
//!
//! impl DebugWriter for SyncDebugWriter {
//!    fn write(&self, buf: &[u8], _overflow: &[u8]) -> usize {
//!        let out_reg = 0x4000 as *mut u8; // Replace with the actual address of the FIFO
//!        for c in buf.iter() {
//!            unsafe { out_reg.write_volatile(*c) };
//!        }
//!        buf.len()
//!    }
//!
//!    fn available_len(&self) -> usize {
//!        usize::MAX
//!    }
//!
//!    fn to_write_len(&self) -> usize {
//!        0
//!    }
//!
//!    fn publish(&self) -> usize {
//!        0
//!    }
//!
//!    fn flush(&self, _writer: &mut dyn IoWrite) { }
//! }
//! ```
//! And instantiate it in the main board file:
//!
//! ```ignore
//! let debug_writer = static_init!(
//!     utils::SyncDebugWriter,
//!     utils::SyncDebugWriter
//! );
//!
//! kernel::debug::initialize_debug_writer_wrapper::<
//!     <ChipHw as kernel::platform::chip::Chip>::ThreadIdProvider
//! >();
//!
//! kernel::debug::set_debug_writer_wrapper(
//!     debug_writer,
//!     create_capability!(kernel::capabilities::SetDebugWriterCapability)
//! );
//! ```
//!
//! Example
//! -------
//!
//! ```no_run
//! # use kernel::{debug, debug_gpio, debug_verbose};
//! # fn main() {
//! # let i = 42;
//! debug!("Yes the code gets here with value {}", i);
//! debug_verbose!("got here"); // Includes message count, file, and line.
//!
//! debug_gpio!(0, toggle); // Toggles the first debug GPIO.
//!
//! # }
//! ```
//!
//! ```text
//! Yes the code gets here with value 42
//! TOCK_DEBUG(0): /tock/capsules/src/sensys.rs:24: got here
//! ```

use core::cell::Cell;
use core::fmt::{Arguments, Write, write};
use core::panic::PanicInfo;
use core::str;

use crate::capabilities::SetDebugWriterCapability;
use crate::hil;
use crate::platform::chip::Chip;
use crate::platform::chip::PanicWriter;
use crate::platform::chip::ThreadIdProvider;
use crate::process::ProcessPrinter;
use crate::process::ProcessSlot;
use crate::processbuffer::ReadableProcessSlice;
use crate::utilities::binary_write::BinaryToWriteWrapper;
use crate::utilities::cells::MapCell;
use crate::utilities::cells::NumericCellExt;
use crate::utilities::io_write::IoWrite;
use crate::utilities::single_thread_value::SingleThreadValue;

///////////////////////////////////////////////////////////////////
// panic! support routines

/// Resources needed by the main panic routines.
pub struct PanicResources<C: Chip + 'static, PP: ProcessPrinter + 'static> {
    /// The array of process slots.
    pub processes: MapCell<&'static [ProcessSlot]>,
    /// The board-specific chip object.
    pub chip: MapCell<&'static C>,
    /// The tool for printing process details.
    pub printer: MapCell<&'static PP>,
}

impl<C: Chip, PP: ProcessPrinter> PanicResources<C, PP> {
    /// Create a new [`PanicResources`] with nothing stored.
    pub const fn new() -> Self {
        Self {
            processes: MapCell::empty(),
            chip: MapCell::empty(),
            printer: MapCell::empty(),
        }
    }
}

/// Tock panic routine, without the infinite LED-blinking loop.
///
/// This is useful for boards which do not feature LEDs to blink or want to
/// implement their own behavior. This method returns after performing the panic
/// dump.
///
/// After this method returns, the system is no longer in a well-defined state.
/// Care must be taken on how one interacts with the system once this function
/// returns.
///
/// Because this requires a [`PanicInfo`] reference, this can only be called
/// from a panic context.
pub fn panic_print<PW: PanicWriter, C: Chip, PP: ProcessPrinter>(
    writer_config: PW::Config,
    panic_info: &PanicInfo,
    nop: &dyn Fn(),
    panic_resources: Option<&PanicResources<C, PP>>,
) {
    // Create the synchronous writer we can use to output the panic message.
    let mut writer = PW::create_panic_writer(writer_config, panic_info);

    panic_begin(nop);

    // Flush debug buffer if needed
    flush(&mut writer);

    // Display general information about the kernel.
    panic_banner(&mut writer, panic_info);

    if let Some(pr) = panic_resources {
        let chip = pr.chip.take();

        // SAFETY: This may only be called during a panic, and we are guaranteed
        // to be in a panic when running this function.
        unsafe {
            panic_cpu_state(chip, &mut writer);
        }

        chip.map(|c| {
            use crate::platform::mpu::MPU;
            // Some systems may enforce memory protection regions for the kernel,
            // making application memory inaccessible. However, printing process
            // information will attempt to access memory. If we are provided a chip
            // reference, attempt to disable userspace memory protection first.
            //
            // SAFETY: This is safe because we are in a panic handler and we
            // will never run processes again. We do not guarantee we will
            // re-enable the MPU, but that is ok because in a panic we do not
            // run processes.
            unsafe { c.mpu().disable_app_mpu() }
        });
        if let Some(p) = pr.processes.take() {
            if let Some(process_printer) = pr.printer.take() {
                if p.iter().filter(|p| p.get().is_some()).count() > 0 {
                    // SAFETY: We are guaranteed to be in a panic context.
                    unsafe {
                        panic_process_info(p, process_printer, &mut writer);
                    }
                } else {
                    let _ = writer.write_str("\r\nNo loaded processes.\r\n");
                }
            } else {
                let _ = writer.write_str("\r\nProcess Printer is not available.\r\n");
            }
        } else {
            let _ = writer.write_str("\r\nProcesses List is not available.\r\n");
        }
    } else {
        let _ = writer.write_str("\r\nPanic Resources are not available.\r\n");
    }
}

/// Tock default panic routine.
///
/// **NOTE:** The supplied `writer` must be synchronous.
///
/// This will print a detailed debugging message and then loop forever while
/// blinking an LED in a recognizable pattern.
///
/// Because this requires a [`PanicInfo`] reference, this can only be called
/// from a panic context.
pub fn panic<L: hil::led::Led, PW: PanicWriter, C: Chip, PP: ProcessPrinter>(
    leds: &mut [&L],
    writer_config: PW::Config,
    panic_info: &PanicInfo,
    nop: &dyn Fn(),
    panic_resources: Option<&PanicResources<C, PP>>,
) -> ! {
    // Call `panic_print` first which will print out the panic information and
    // return
    panic_print::<PW, C, PP>(writer_config, panic_info, nop, panic_resources);

    // The system is no longer in a well-defined state, we cannot
    // allow this function to return
    //
    // Forever blink LEDs in an infinite loop
    panic_blink_forever(leds)
}

/// Tock panic routine, without the infinite LED-blinking loop.
///
/// This is useful for boards which do not feature LEDs to blink or want to
/// implement their own behavior. This method returns after performing the panic
/// dump.
///
/// After this method returns, the system is no longer in a well-defined state.
/// Care must be taken on how one interacts with the system once this function
/// returns.
///
/// **NOTE:** The supplied `writer` must be synchronous.
///
/// # Safety
///
/// - This must ONLY be called during a panic.
pub unsafe fn panic_print_old<W: Write + IoWrite, C: Chip, PP: ProcessPrinter>(
    writer: &mut W,
    panic_info: &PanicInfo,
    nop: &dyn Fn(),
    panic_resources: Option<&PanicResources<C, PP>>,
) {
    // SAFETY: This has the same safety reasoning as `panic_print()`. This
    // implementation is deprecated. When all callers are updated it will be
    // removed.
    unsafe {
        panic_begin(nop);
        // Flush debug buffer if needed
        flush(writer);
        panic_banner(writer, panic_info);

        panic_resources.map(|pr| {
            let chip = pr.chip.take();
            panic_cpu_state(chip, writer);

            chip.map(|c| {
                // Some systems may enforce memory protection regions for the kernel,
                // making application memory inaccessible. However, printing process
                // information will attempt to access memory. If we are provided a chip
                // reference, attempt to disable userspace memory protection first:
                use crate::platform::mpu::MPU;
                c.mpu().disable_app_mpu()
            });
            pr.processes.take().map(|p| {
                if let Some(process_printer) = pr.printer.take() {
                    panic_process_info(p, process_printer, writer);
                }
            });
        });
    }
}

/// Tock default panic routine.
///
/// **NOTE:** The supplied `writer` must be synchronous.
///
/// This will print a detailed debugging message and then loop forever while
/// blinking an LED in a recognizable pattern.
///
/// # Safety
///
/// - This must ONLY be called during a panic.
pub unsafe fn panic_old<L: hil::led::Led, W: Write + IoWrite, C: Chip, PP: ProcessPrinter>(
    leds: &mut [&L],
    writer: &mut W,
    panic_info: &PanicInfo,
    nop: &dyn Fn(),
    panic_resources: Option<&PanicResources<C, PP>>,
) -> ! {
    // Call `panic_print` first which will print out the panic information and
    // return.
    //
    // SAFETY: The requirements match this function.
    unsafe {
        panic_print_old(writer, panic_info, nop, panic_resources);
    }

    // The system is no longer in a well-defined state, we cannot
    // allow this function to return
    //
    // Forever blink LEDs in an infinite loop
    panic_blink_forever(leds)
}

/// Generic panic entry.
///
/// This opaque method should always be called at the beginning of a board's
/// panic method to allow hooks for any core kernel cleanups that may be
/// appropriate.
pub fn panic_begin(nop: &dyn Fn()) {
    // Let any outstanding uart DMA's finish
    for _ in 0..200000 {
        nop();
    }
}

/// Lightweight prints about the current panic and kernel version.
///
/// **NOTE:** The supplied `writer` must be synchronous.
///
/// Because this requires a [`PanicInfo`] reference, this can only be called
/// from a panic context.
pub fn panic_banner<W: Write>(writer: &mut W, panic_info: &PanicInfo) {
    // Expand `PanicInfo` manually rather than using its `Display`
    // implementation. The `Display` implementation inserts bare LFs
    // between the location line and the message body, rather than a
    // CRLF.
    if let Some(location) = panic_info.location() {
        let _ = writer.write_fmt(format_args!(
            "\r\npanicked at {}:{}:{}:\r\n{}\r\n",
            location.file(),
            location.line(),
            location.column(),
            panic_info.message(),
        ));
    } else {
        let _ = writer.write_fmt(format_args!("\r\n{}\r\n", panic_info.message()));
    }

    // Print version of the kernel
    if crate::KERNEL_PRERELEASE_VERSION != 0 {
        let _ = writer.write_fmt(format_args!(
            "\tKernel version {}.{}.{}-dev{}\r\n",
            crate::KERNEL_MAJOR_VERSION,
            crate::KERNEL_MINOR_VERSION,
            crate::KERNEL_PATCH_VERSION,
            crate::KERNEL_PRERELEASE_VERSION,
        ));
    } else {
        let _ = writer.write_fmt(format_args!(
            "\tKernel version {}.{}.{}\r\n",
            crate::KERNEL_MAJOR_VERSION,
            crate::KERNEL_MINOR_VERSION,
            crate::KERNEL_PATCH_VERSION,
        ));
    }
}

/// Print current machine (CPU) state.
///
/// **NOTE:** The supplied `writer` must be synchronous.
///
/// # Safety
///
/// This may only be called during a panic.
pub unsafe fn panic_cpu_state<W: Write, C: Chip>(chip: Option<&'static C>, writer: &mut W) {
    // SAFETY: The function-level safety doc requires this only be called during
    // a panic, matching the requirement for `print_state()`.
    unsafe {
        C::print_state(chip, writer);
    }
}

/// More detailed prints about all processes.
///
/// **NOTE:** The supplied `writer` must be synchronous.
///
/// # Safety
///
/// This must only be called from a panic context.
pub unsafe fn panic_process_info<PP: ProcessPrinter, W: Write>(
    processes: &'static [ProcessSlot],
    process_printer: &'static PP,
    writer: &mut W,
) {
    // print data about each process
    let _ = writer.write_fmt(format_args!("\r\n---| App Status |---\r\n"));
    for slot in processes {
        slot.proc.get().map(|process| {
            // Print the memory map and basic process info.
            //
            // Because we are using a synchronous printer we do not need to
            // worry about looping on the print function.
            process_printer.print_overview(process, &mut BinaryToWriteWrapper::new(writer), None);
            // Print all of the process details.
            process.print_full_process(writer);
        });
    }
}

/// Blinks a recognizable pattern forever.
///
/// The LED will blink "sporadically" in a somewhat irregular pattern. This
/// should look different from a traditional blinking LED which typically blinks
/// with a consistent duty cycle. The panic blinking sequence is intentionally
/// unusual to make it easier to tell when a panic has occurred.
///
/// If a multi-color LED is used for the panic pattern, it is advised to turn
/// off other LEDs before calling this method.
///
/// Generally, boards should blink red during panic if possible, otherwise
/// choose the 'first' or most prominent LED. Some boards may find it
/// appropriate to blink multiple LEDs (e.g. one on the top and one on the
/// bottom), thus this method accepts an array, however most will only need one.
pub fn panic_blink_forever<L: hil::led::Led>(leds: &mut [&L]) -> ! {
    for led in leds.iter_mut() {
        led.init();
    }
    loop {
        for _ in 0..1000000 {
            for led in leds.iter_mut() {
                led.on();
            }
        }
        for _ in 0..100000 {
            for led in leds.iter_mut() {
                led.off();
            }
        }
        for _ in 0..1000000 {
            for led in leds.iter_mut() {
                led.on();
            }
        }
        for _ in 0..500000 {
            for led in leds.iter_mut() {
                led.off();
            }
        }
    }
}

// panic! support routines
///////////////////////////////////////////////////////////////////

///////////////////////////////////////////////////////////////////
// debug_gpio! support

/// Static variable that holds an array of debug GPIO references.
pub static DEBUG_GPIOS: SingleThreadValue<MapCell<&'static [&'static dyn hil::gpio::Pin]>> =
    SingleThreadValue::new();

/// Initialize the static debug gpio variable.
///
/// This ensures it can safely be used as a global variable.
#[cfg(target_has_atomic = "ptr")]
pub fn initialize_debug_gpio<P: ThreadIdProvider>() {
    DEBUG_GPIOS
        .bind_to_thread::<P>(MapCell::empty())
        .map_err(|_| ())
        .unwrap();
}

/// Initialize the static debug gpio variable.
///
/// This ensures it can safely be used as a global variable.
///
/// # Safety
///
/// Callers of this function must ensure that this function is never called
/// concurrently with other calls to [`initialize_debug_gpio_unsafe`].
pub unsafe fn initialize_debug_gpio_unsafe<P: ThreadIdProvider>() {
    // SAFETY: The caller ensures there is no concurrent call to
    // `DEBUG_GPIOS.bind_to_thread_unsafe()`.
    unsafe {
        DEBUG_GPIOS
            .bind_to_thread_unsafe::<P>(MapCell::empty())
            .map_err(|_| ())
            .unwrap();
    }
}

/// Map an array of GPIO pins to use for debugging.
pub fn assign_gpios(gpio: &'static [&'static dyn hil::gpio::Pin]) {
    DEBUG_GPIOS.get().map(|gpio_array_cell| {
        gpio_array_cell.replace(gpio);
    });
}

/// In-kernel gpio debugging that accepts any GPIO HIL method.
#[macro_export]
macro_rules! debug_gpio {
    ($i:tt, $method:ident $(,)?) => {{
        #[allow(unused_unsafe)]
        unsafe {
            $crate::debug::DEBUG_GPIOS.get().map(|debug_gpio_cell| {
                debug_gpio_cell.map(|debug_gpio_array| {
                    debug_gpio_array.get($i).map(|g| g.$method());
                });
            });
        }
    }};
}

///////////////////////////////////////////////////////////////////
// debug! and debug_verbose! support

/// A trait for writing debug output.
///
/// This can be used for example to implement asynchronous or synchronous
/// writers, and buffered or unbuffered writers. Various platforms may have
/// in-memory logs, memory mapped "endless" FIFOs, JTAG support for output,
/// etc.
pub trait DebugWriter {
    /// Write bytes to output with overflow notification.
    ///
    /// The `overflow` slice is used as a message to be appended to the end of
    /// the available buffer if it becomes full.
    fn write(&self, bytes: &[u8], overflow_message: &[u8]) -> usize;

    /// Available length of the internal buffer if limited.
    ///
    /// If the buffer can support a write of any size, it should lie and return
    /// `usize::MAX`.
    ///
    /// Across subsequent calls to this function, without invoking `write()` in
    /// between, this returned value may only increase, but never decrease.
    fn available_len(&self) -> usize;

    /// How many bytes are buffered and not yet written.
    fn to_write_len(&self) -> usize;

    /// Publish bytes from the internal buffer to the output.
    ///
    /// Returns how many bytes were written.
    fn publish(&self) -> usize;

    /// Flush any buffered bytes to the provided output writer.
    ///
    /// `flush()` should be used to write any buffered bytes to a new `writer`
    /// instead of the internal writer that `publish()` would use.
    fn flush(&self, writer: &mut dyn IoWrite);
}

/// Static variable that holds the kernel's reference to the debug tool.
///
/// This is needed so the `debug!()` macros have a reference to the object to
/// use.
static DEBUG_WRITER: SingleThreadValue<MapCell<&'static dyn DebugWriter>> =
    SingleThreadValue::new();

/// Static variable that holds how many times `debug!()` has been called.
///
/// This enables printing a verbose header message that enumerates independent
/// debug messages.
static DEBUG_WRITER_COUNT: SingleThreadValue<Cell<usize>> = SingleThreadValue::new();

/// Initialize the static debug writer.
///
/// This ensures it can safely be used as a global variable.
#[cfg(target_has_atomic = "ptr")]
pub fn initialize_debug_writer_wrapper<P: ThreadIdProvider>() {
    DEBUG_WRITER
        .bind_to_thread::<P>(MapCell::empty())
        .map_err(|_| ())
        .unwrap();
    DEBUG_WRITER_COUNT
        .bind_to_thread::<P>(Cell::new(0))
        .map_err(|_| ())
        .unwrap();
}

/// Initialize the static debug writer.
///
/// This ensures it can safely be used as a global variable.
///
/// # Safety
///
/// Callers of this function must ensure that this function is never called
/// concurrently with other calls to [`initialize_debug_writer_wrapper_unsafe`].
pub unsafe fn initialize_debug_writer_wrapper_unsafe<P: ThreadIdProvider>() {
    // SAFETY: The caller ensures there is no concurrent call to
    // `DEBUG_WRITER.bind_to_thread_unsafe()` and
    // `DEBUG_WRITER_COUNT.bind_to_thread_unsafe()`.
    unsafe {
        DEBUG_WRITER
            .bind_to_thread_unsafe::<P>(MapCell::empty())
            .map_err(|_| ())
            .unwrap();
        DEBUG_WRITER_COUNT
            .bind_to_thread_unsafe::<P>(Cell::new(0))
            .map_err(|_| ())
            .unwrap();
    }
}

fn try_get_debug_writer<F, R>(closure: F) -> Option<R>
where
    F: FnOnce(&dyn DebugWriter) -> R,
{
    DEBUG_WRITER
        .get()
        .and_then(|dw| dw.map_or(None, |writer| Some(closure(*writer))))
}

/// Function used by board main.rs to set a reference to the writer.
pub fn set_debug_writer_wrapper<C: SetDebugWriterCapability>(
    debug_writer: &'static dyn DebugWriter,
    _cap: C,
) {
    DEBUG_WRITER.get().map(|dw| dw.replace(debug_writer));
}

impl Write for &dyn DebugWriter {
    fn write_str(&mut self, s: &str) -> Result<(), core::fmt::Error> {
        self.write(s.as_bytes(), b"");
        Ok(())
    }
}

/// Write a debug message without a trailing newline.
pub fn debug_print(args: Arguments) {
    try_get_debug_writer(|mut writer| {
        let _ = write(&mut writer, args);
        writer.publish();
    });
}

/// Write a debug message with a trailing newline.
pub fn debug_println(args: Arguments) {
    try_get_debug_writer(|mut writer| {
        let _ = write(&mut writer, args);
        let _ = writer.write_str("\r\n");
        writer.publish();
    });
}

/// Write a [`ReadableProcessSlice`] to the debug output.
///
/// # Errors
///
/// Will return `Err` if it is not possible to write any output.
pub fn debug_slice(slice: &ReadableProcessSlice) -> Result<usize, ()> {
    try_get_debug_writer(|writer| {
        let mut total = 0;
        for b in slice.iter() {
            let buf: [u8; 1] = [b.get(); 1];
            let count = writer.write(&buf, b"");
            if count > 0 {
                total += count;
            } else {
                break;
            }
        }
        writer.publish();
        total
    })
    .ok_or(())
}

/// Return how many bytes are remaining in the internal debug buffer.
pub fn debug_available_len() -> usize {
    try_get_debug_writer(|writer| writer.available_len()).unwrap_or(0)
}

fn write_header(
    writer: &mut &dyn DebugWriter,
    (file, line): &(&'static str, u32),
) -> Result<(), core::fmt::Error> {
    let count = DEBUG_WRITER_COUNT.get().map_or(0, |count| {
        count.increment();
        count.get()
    });

    writer.write_fmt(format_args!("TOCK_DEBUG({}): {}:{}: ", count, file, line))
}

/// Write a debug message with file and line information without a trailing
/// newline.
pub fn debug_verbose_print(args: Arguments, file_line: &(&'static str, u32)) {
    try_get_debug_writer(|mut writer| {
        let _ = write_header(&mut writer, file_line);
        let _ = write(&mut writer, args);
        writer.publish();
    });
}

/// Write a debug message with file and line information with a trailing
/// newline.
pub fn debug_verbose_println(args: Arguments, file_line: &(&'static str, u32)) {
    try_get_debug_writer(|mut writer| {
        let _ = write_header(&mut writer, file_line);
        let _ = write(&mut writer, args);
        let _ = writer.write_str("\r\n");
        writer.publish();
    });
}

/// In-kernel `println()` debugging.
#[macro_export]
macro_rules! debug {
    () => ({
        // Allow an empty debug!() to print the location when hit
        debug!("")
    });
    ($msg:expr $(,)?) => ({
        $crate::debug::debug_println(format_args!($msg));
    });
    ($fmt:expr, $($arg:tt)+) => ({
        $crate::debug::debug_println(format_args!($fmt, $($arg)+));
    });
}

/// In-kernel `println()` debugging that can take a process slice.
#[macro_export]
macro_rules! debug_process_slice {
    ($msg:expr $(,)?) => {{ $crate::debug::debug_slice($msg) }};
}

/// In-kernel `println()` debugging with filename and line numbers.
#[macro_export]
macro_rules! debug_verbose {
    () => ({
        // Allow an empty debug_verbose!() to print the location when hit
        debug_verbose!("")
    });
    ($msg:expr $(,)?) => ({
        $crate::debug::debug_verbose_println(format_args!($msg), {
            // TODO: Maybe make opposite choice of panic!, no `static`, more
            // runtime code for less static data
            static _FILE_LINE: (&'static str, u32) = (file!(), line!());
            &_FILE_LINE
        })
    });
    ($fmt:expr, $($arg:tt)+) => ({
        $crate::debug::debug_verbose_println(format_args!($fmt, $($arg)+), {
            static _FILE_LINE: (&'static str, u32) = (file!(), line!());
            &_FILE_LINE
        })
    });
}

/// Prints out the expression and its location, then returns it.
///
/// ```rust,ignore
/// let foo: u8 = debug_expr!(0xff);
/// // Prints [main.rs:2] 0xff = 255
/// ```
/// Taken straight from Rust `std::dbg`.
#[macro_export]
macro_rules! debug_expr {
    // NOTE: We cannot use `concat!` to make a static string as a format
    // argument of `eprintln!` because `file!` could contain a `{` or `$val`
    // expression could be a block (`{ .. }`), in which case the `eprintln!`
    // will be malformed.
    () => {
        $crate::debug!("[{}:{}]", file!(), line!())
    };
    ($val:expr $(,)?) => {
        // Use of `match` here is intentional because it affects the lifetimes
        // of temporaries - https://stackoverflow.com/a/48732525/1063961
        match $val {
            tmp => {
                $crate::debug!("[{}:{}] {} = {:#?}",
                    file!(), line!(), stringify!($val), &tmp);
                tmp
            }
        }
    };
    ($($val:expr),+ $(,)?) => {
        ($($crate::debug_expr!($val)),+,)
    };
}

/// Flush any stored messages to the output writer.
fn flush<W: Write + IoWrite>(writer: &mut W) {
    try_get_debug_writer(|debug_writer|{
        if debug_writer.to_write_len() > 0 {
            let _ = writer.write_str(
                    "\r\n---| Debug buffer not empty. Flushing. May repeat some of last message(s):\r\n",
                );
            debug_writer.flush(writer);
        }
    }).or_else(||{
        let _ = writer.write_str(
            "\r\n---| Global debug writer not registered.\
             \r\n     Call `set_debug_writer_wrapper` in board initialization.\r\n",
        );
        None
    });
}