Driver

Now let’s use the new Registers struct in our driver.

// Copyright 2023 Google LLC
// SPDX-License-Identifier: Apache-2.0

/// Driver for a PL011 UART.
#[derive(Debug)]
pub struct Uart {
    registers: *mut Registers,
}

impl Uart {
    /// Constructs a new instance of the UART driver for a PL011 device with the
    /// given set of registers.
    ///
    /// # Safety
    ///
    /// The given pointer must point to the 8 MMIO control registers of a PL011
    /// device, which must be mapped into the address space of the process as
    /// device memory and not have any other aliases.
    pub unsafe fn new(registers: *mut Registers) -> Self {
        Self { registers }
    }

    /// Writes a single byte to the UART.
    pub fn write_byte(&mut self, byte: u8) {
        // Wait until there is room in the TX buffer.
        while self.read_flag_register().contains(Flags::TXFF) {}

        // SAFETY: We know that self.registers points to the control registers
        // of a PL011 device which is appropriately mapped.
        unsafe {
            // Write to the TX buffer.
            (&raw mut (*self.registers).dr).write_volatile(byte.into());
        }

        // Wait until the UART is no longer busy.
        while self.read_flag_register().contains(Flags::BUSY) {}
    }

    /// Reads and returns a pending byte, or `None` if nothing has been
    /// received.
    pub fn read_byte(&mut self) -> Option<u8> {
        if self.read_flag_register().contains(Flags::RXFE) {
            None
        } else {
            // SAFETY: We know that self.registers points to the control
            // registers of a PL011 device which is appropriately mapped.
            let data = unsafe { (&raw const (*self.registers).dr).read_volatile() };
            // TODO: Check for error conditions in bits 8-11.
            Some(data as u8)
        }
    }

    fn read_flag_register(&self) -> Flags {
        // SAFETY: We know that self.registers points to the control registers
        // of a PL011 device which is appropriately mapped.
        unsafe { (&raw const (*self.registers).fr).read_volatile() }
    }
}