Using It

Let’s write a small program using our driver to write to the serial console, and echo incoming bytes.

// Copyright 2025 Google LLC
// SPDX-License-Identifier: Apache-2.0

#![no_main]
#![no_std]

mod asm;
mod exceptions;
mod pl011;

use crate::pl011::Uart;
use core::fmt::Write;
use core::panic::PanicInfo;
use core::ptr::NonNull;
use log::error;
use safe_mmio::UniqueMmioPointer;
use smccc::Hvc;
use smccc::psci::system_off;

/// Base address of the primary PL011 UART.
const PL011_BASE_ADDRESS: NonNull<pl011::Registers> =
    NonNull::new(0x900_0000 as _).unwrap();

// SAFETY: There is no other global function of this name.
#[unsafe(no_mangle)]
extern "C" fn main(x0: u64, x1: u64, x2: u64, x3: u64) {
    // SAFETY: `PL011_BASE_ADDRESS` is the base address of a PL011 device, and
    // nothing else accesses that address range.
    let mut uart = Uart::new(unsafe { UniqueMmioPointer::new(PL011_BASE_ADDRESS) });

    writeln!(uart, "main({x0:#x}, {x1:#x}, {x2:#x}, {x3:#x})").unwrap();

    loop {
        if let Some(byte) = uart.read_byte() {
            uart.write_byte(byte);
            match byte {
                b'\r' => uart.write_byte(b'\n'),
                b'q' => break,
                _ => continue,
            }
        }
    }

    writeln!(uart, "\n\nBye!").unwrap();
    system_off::<Hvc>().unwrap();
}