Add rx ability to uart

Cargo.lock

@@ -2,6 +2,40 @@
 # It is not intended for manual editing.
 version = 3
 
+[[package]]
+name = "autocfg"
+version = "1.3.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "0c4b4d0bd25bd0b74681c0ad21497610ce1b7c91b1022cd21c80c6fbdd9476b0"
+
+[[package]]
+name = "lock_api"
+version = "0.4.12"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "07af8b9cdd281b7915f413fa73f29ebd5d55d0d3f0155584dade1ff18cea1b17"
+dependencies = [
+ "autocfg",
+ "scopeguard",
+]
+
 [[package]]
 name = "riscv-uefi"
 version = "0.1.0"
+dependencies = [
+ "spinning_top",
+]
+
+[[package]]
+name = "scopeguard"
+version = "1.2.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "94143f37725109f92c262ed2cf5e59bce7498c01bcc1502d7b9afe439a4e9f49"
+
+[[package]]
+name = "spinning_top"
+version = "0.3.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d96d2d1d716fb500937168cc09353ffdc7a012be8475ac7308e1bdf0e3923300"
+dependencies = [
+ "lock_api",
+]

Cargo.toml

@@ -4,3 +4,4 @@ version = "0.1.0"
 edition = "2021"
 
 [dependencies]
+spinning_top = "0.3.0"

src/main.rs

@@ -22,6 +22,19 @@ unsafe extern "C" fn _start() -> ! {
     // set the stack pointer
     "la sp, _init_stack_top",
 
+    "la sp, _init_stack_top",
+
+    // clear the BSS
+    "la t0, _bss_start",
+    "la t1, _bss_end",
+    "bgeu t0, t1, 2f",
+    "1:",
+    "sb zero, 0(t0)",
+    "addi t0, t0, 1",
+    "bne t0, t1, 1b",
+    "2:",
+    // BSS is clear!
+
     // "tail-call" to {entry} (call without saving a return address)
     "tail {entry}",
     entry = sym entry, // {entry} refers to the function [entry] below
@@ -30,13 +43,16 @@ unsafe extern "C" fn _start() -> ! {
 }
 
 extern "C" fn entry() -> ! {
-    // UNSAFE: correct address for QEMU virt device
-    let mut console = unsafe { uart::Device::new(0x1000_0000) };
-    for byte in "Hello, world!".bytes() {
-        console.put(byte);
-    }
+    println!("This should not print because the console is not initialised.");
+    unsafe { uart::init_console(0x1000_0000) };
+    println!("Hello, world!");
 
-    loop {}
+    loop {
+        let c = uart::CONSOLE.lock().as_mut().and_then(uart::Device::get);
+        if let Some(c) = c {
+            print!("{}", c as char);
+        }
+    }
 }
 
 #[panic_handler]

src/uart.rs

@@ -1,5 +1,9 @@
 //! This module provides access to the UART console.
 
+use spinning_top::Spinlock;
+
+pub static CONSOLE: Spinlock<Option<Device>> = Spinlock::new(None);
+
 /// Represents an initialised UART device.
 pub struct Device {
     base: usize,
@@ -29,4 +33,55 @@ impl Device {
             core::ptr::write_volatile(ptr, character);
         }
     }
+
+    /// Gets a character from the UART input.
+    pub fn get(&mut self) -> Option<u8> {
+        const READY: u8 = 0b1;
+
+        let ptr = self.base as *mut u8;
+        // SAFETY: Fine as long as base is correct.
+        let lsr = unsafe { ptr.offset(5) };
+        unsafe {
+            if core::ptr::read_volatile(lsr) & READY == READY {
+                Some(core::ptr::read_volatile(ptr))
+            } else {
+                None
+            }
+        }
+    }
+}
+
+impl core::fmt::Write for Device {
+    fn write_str(&mut self, s: &str) -> core::fmt::Result {
+        for c in s.bytes() {
+            self.put(c);
+        }
+        Ok(()) // there are never errors writing to UART :)
+    }
+}
+
+/// Initialise the UART debugging console.
+/// # Safety
+/// `base` must point to the base address of a UART device.
+pub unsafe fn init_console(base: usize) {
+    let mut console = CONSOLE.lock();
+    *console = Some(unsafe { Device::new(base) });
+}
+
+/// Prints a formatted string to the [CONSOLE].
+#[macro_export]
+macro_rules! print {
+    ($($arg:tt)*) => ({
+        use core::fmt::Write;
+        $crate::uart::CONSOLE.lock().as_mut().map(|writer| {
+            writer.write_fmt(format_args!($($arg)*)).unwrap()
+        });
+    });
+}
+
+/// println prints a formatted string to the [CONSOLE] with a trailing newline character.
+#[macro_export]
+macro_rules! println {
+    ($fmt:expr) => ($crate::print!(concat!($fmt, "\n")));
+    ($fmt:expr, $($arg:tt)*) => ($crate::print!(concat!($fmt, "\n"), $($arg)*));
 }