aarch64: implement mini UART for bcm2837

crate/bcm2837/src/asm.rs

@@ -2,9 +2,9 @@
 
 /// delay for some clocks
 #[inline]
-pub unsafe fn delay(clock: u32) {
+pub unsafe fn delay(_clock: u32) {
     #[cfg(target_arch = "aarch64")]
     asm!("mov x1, x0; 1: subs x1, x1, #1; bne 1b;"
-        :: "{x0}"(clock)
+        :: "{x0}"(_clock)
         :: "volatile");
 }

crate/bcm2837/src/mini_uart.rs

@@ -1 +1,117 @@
 use super::IO_BASE;
+use gpio::{Function, Gpio};
+use volatile::{ReadOnly, Volatile};
+
+/// The base address for the `MU` registers.
+const MU_REG_BASE: usize = IO_BASE + 0x215040;
+
+/// The `AUXENB` register from page 9 of the BCM2837 documentation.
+const AUX_ENABLES: *mut Volatile<u8> = (IO_BASE + 0x215004) as *mut Volatile<u8>;
+
+/// Enum representing bit fields of the `AUX_MU_LSR_REG` register.
+#[repr(u8)]
+enum LsrStatus {
+    DataReady = 1,
+    TxAvailable = 1 << 5,
+}
+
+#[repr(C)]
+#[allow(non_snake_case)]
+struct Registers {
+    AUX_MU_IO_REG: Volatile<u8>,
+    __r0: [u8; 3],
+    AUX_MU_IER_REG: Volatile<u8>,
+    __r1: [u8; 3],
+    AUX_MU_IIR_REG: Volatile<u8>,
+    __r2: [u8; 3],
+    AUX_MU_LCR_REG: Volatile<u8>,
+    __r3: [u8; 3],
+    AUX_MU_MCR_REG: Volatile<u8>,
+    __r4: [u8; 3],
+    AUX_MU_LSR_REG: ReadOnly<u8>,
+    __r5: [u8; 3],
+    AUX_MU_MSR_REG: ReadOnly<u8>,
+    __r6: [u8; 3],
+    AUX_MU_SCRATCH: Volatile<u8>,
+    __r7: [u8; 3],
+    AUX_MU_CNTL_REG: Volatile<u8>,
+    __r8: [u8; 3],
+    AUX_MU_STAT_REG: ReadOnly<u32>,
+    AUX_MU_BAUD_REG: Volatile<u16>,
+}
+
+/// The Raspberry Pi's "mini UART".
+pub struct MiniUart {
+    registers: &'static mut Registers,
+    timeout: Option<u32>,
+}
+
+impl MiniUart {
+    /// Initializes the mini UART by enabling it as an auxiliary peripheral,
+    /// setting the data size to 8 bits, setting the BAUD rate to ~115200 (baud
+    /// divider of 270), setting GPIO pins 14 and 15 to alternative function 5
+    /// (TXD1/RDXD1), and finally enabling the UART transmitter and receiver.
+    ///
+    /// By default, reads will never time out. To set a read timeout, use
+    /// `set_read_timeout()`.
+    pub fn new() -> MiniUart {
+        let registers = unsafe {
+            // Enable the mini UART as an auxiliary device.
+            (*AUX_ENABLES).write(1);
+            &mut *(MU_REG_BASE as *mut Registers)
+        };
+
+        Gpio::new(14).into_alt(Function::Alt5).set_gpio_pd(0);
+        Gpio::new(15).into_alt(Function::Alt5).set_gpio_pd(0);
+
+        registers.AUX_MU_CNTL_REG.write(0); // Disable auto flow control and disable receiver and transmitter (for now)
+        registers.AUX_MU_IER_REG.write(0); // Disable receive and transmit interrupts
+        registers.AUX_MU_LCR_REG.write(3); // Enable 8 bit mode
+        registers.AUX_MU_MCR_REG.write(0); // Set RTS line to be always high
+        registers.AUX_MU_BAUD_REG.write(270); // Set baud rate to 115200
+
+        registers.AUX_MU_CNTL_REG.write(3); // Finally, enable transmitter and receiver
+
+        MiniUart {
+            registers: registers,
+            timeout: None,
+        }
+    }
+
+    /// Set the read timeout to `milliseconds` milliseconds.
+    pub fn set_read_timeout(&mut self, milliseconds: u32) {
+        self.timeout = Some(milliseconds)
+    }
+
+    /// Write the byte `byte`. This method blocks until there is space available
+    /// in the output FIFO.
+    pub fn write_byte(&mut self, byte: u8) {
+        while self.registers.AUX_MU_LSR_REG.read() & (LsrStatus::TxAvailable as u8) == 0 {}
+        self.registers.AUX_MU_IO_REG.write(byte);
+    }
+
+    /// Returns `true` if there is at least one byte ready to be read. If this
+    /// method returns `true`, a subsequent call to `read_byte` is guaranteed to
+    /// return immediately. This method does not block.
+    pub fn has_byte(&self) -> bool {
+        self.registers.AUX_MU_LSR_REG.read() & (LsrStatus::DataReady as u8) != 0
+    }
+
+    /// Blocks until there is a byte ready to read. If a read timeout is set,
+    /// this method blocks for at most that amount of time. Otherwise, this
+    /// method blocks indefinitely until there is a byte to read.
+    ///
+    /// Returns `Ok(())` if a byte is ready to read. Returns `Err(())` if the
+    /// timeout expired while waiting for a byte to be ready. If this method
+    /// returns `Ok(())`, a subsequent call to `read_byte` is guaranteed to
+    /// return immediately.
+    pub fn wait_for_byte(&self) -> Result<(), ()> {
+        unimplemented!()
+    }
+
+    /// Reads a byte. Blocks indefinitely until a byte is ready to be read.
+    pub fn read_byte(&mut self) -> u8 {
+        while !self.has_byte() {}
+        self.registers.AUX_MU_IO_REG.read()
+    }
+}