Add support for 16550 serial controller

Signed-off-by: Harry Chen <i@harrychen.xyz>

kernel/src/arch/mipsel/io.rs

@@ -1,6 +1,7 @@
 use core::fmt::{Write, Result, Arguments};
+use core::ptr::{read_volatile, write_volatile};
 
-struct SerialPort;
+struct SerialPort {};
 
 impl Write for SerialPort {
     fn write_str(&mut self, s: &str) -> Result {
@@ -17,26 +18,40 @@ impl Write for SerialPort {
     }
 }
 
+fn write<T>(addr: usize, content: T) {
+    let cell = (addr) as *mut T;
+    write_volatile(cell, content);
+}
+
+fn read<T>(addr: usize) -> T {
+    let cell = (addr) as *const T;
+    read_volatile(cell);
+}
+
+/// non-blocking version of putchar()
 fn putchar(c: u8) {
-    // TODO: output to uart
+    write(COM1 + COM_TX, c);
 }
 
+/// blocking version of getchar()
 pub fn getchar() -> char {
-    // TODO: get char from uart
-    let c = 0 as u8;
-
+    loop {
+        if (read::<u8>(COM1 + COM_LSR) & COM_LSR_DATA) == 0 {
+            break;
+        }
+    }
+    let c = read::<u8>(COM1 + COM_RX);
     match c {
         255 => '\0',   // null
         c => c as char,
     }
 }
 
+/// non-blocking version of getchar()
 pub fn getchar_option() -> Option<char> {
-    // TODO: get char from uart
-    let c = 0 as u8;
-    match c {
-        -1 => None,
-        c => Some(c as u8 as char),
+    match read::<u8>(COM1 + COM_LSR) & COM_LSR_DATA {
+        0 => None,
+        else => Some(read::<u8>(COM1 + COM_RX) as u8 as char),
     }
 }
 
@@ -44,5 +59,42 @@ pub fn putfmt(fmt: Arguments) {
     SerialPort.write_fmt(fmt).unwrap();
 }
 
-const TXDATA: *mut u32 = 0x38000000 as *mut u32;
-const RXDATA: *mut u32 = 0x38000004 as *mut u32;
+pub fn init(serial_base_addr: usize) {
+    COM1 = serial_base_addr;
+    // Turn off the FIFO
+    write(COM1 + COM_FCR, 0 as u8);
+    // Set speed; requires DLAB latch
+    write(COM1 + COM_LCR, COM_LCR_DLAB);
+    write(COM1 + COM_DLL, (115200 / 9600) as u8);
+    write(COM1 + COM_DLM, 0 as u8);
+
+    // 8 data bits, 1 stop bit, parity off; turn off DLAB latch
+    write(COM1 + COM_LCR, COM_LCR_WLEN8 & !COM_LCR_DLAB);
+
+    // No modem controls
+    write(COM1 + COM_MCR, 0 as u8);
+    // Enable rcv interrupts
+    write(COM1 + COM_IER, COM_IER_RDI);
+}
+
+static mut COM1: usize = 0;
+
+const COM_RX        :usize = 0;   // In:  Receive buffer (DLAB=0)
+const COM_TX        :usize = 0;   // Out: Transmit buffer (DLAB=0)
+const COM_DLL       :usize = 0;   // Out: Divisor Latch Low (DLAB=1)
+const COM_DLM       :usize = 1;   // Out: Divisor Latch High (DLAB=1)
+const COM_IER       :usize = 1;   // Out: Interrupt Enable Register
+const COM_IER_RDI   :u8 = 0x01;   // Enable receiver data interrupt
+const COM_IIR       :usize = 2;   // In:  Interrupt ID Register
+const COM_FCR       :usize = 2;   // Out: FIFO Control Register
+const COM_LCR       :usize = 3;   // Out: Line Control Register
+const COM_LCR_DLAB  :u8 = 0x80;   // Divisor latch access bit
+const COM_LCR_WLEN8 :u8 = 0x03;   // Wordlength: 8 bits
+const COM_MCR       :usize = 4;   // Out: Modem Control Register
+const COM_MCR_RTS   :u8 = 0x02;   // RTS complement
+const COM_MCR_DTR   :u8 = 0x01;   // DTR complement
+const COM_MCR_OUT2  :u8 = 0x08;   // Out2 complement
+const COM_LSR       :usize = 5;   // In:  Line Status Register
+const COM_LSR_DATA  :u8 = 0x01;   // Data available
+const COM_LSR_TXRDY :u8 = 0x20;   // Transmit buffer avail
+const COM_LSR_TSRE  :u8 = 0x40;   // Transmitter off

kernel/src/arch/mipsel/mod.rs

@@ -38,6 +38,8 @@ pub extern fn rust_main() -> ! {
 
     unsafe { memory::clear_bss(); }
 
+    crate::io::init();
+
     println!("Hello MIPS 32 from CPU {}, dtb @ {:#x}", cpu_id, dtb_start);
 
     crate::logging::init();