xv6-65oo2/kernel/uart.c

//
// low-level driver routines for 16550a UART.
//

#include "types.h"
#include "param.h"
#include "memlayout.h"
#include "riscv.h"
#include "spinlock.h"
#include "proc.h"
#include "defs.h"

// the UART control registers are memory-mapped
// at address UART0. this macro returns the
// address of one of the registers.
#define Reg(reg) ((volatile unsigned char *)(UART0 + reg))

// the UART control registers.
// some have different meanings for
// read vs write.
// see http://byterunner.com/16550.html
#define RHR 0 // receive holding register (for input bytes)
#define THR 0 // transmit holding register (for output bytes)
#define IER 1 // interrupt enable register
#define FCR 2 // FIFO control register
#define ISR 2 // interrupt status register
#define LCR 3 // line control register
#define LSR 5 // line status register

#define ReadReg(reg) (*(Reg(reg)))
#define WriteReg(reg, v) (*(Reg(reg)) = (v))

// the transmit output buffer.
struct spinlock uart_tx_lock;
#define UART_TX_BUF_SIZE 32
char uart_tx_buf[UART_TX_BUF_SIZE];
int uart_tx_w; // write next to uart_tx_buf[uart_tx_w++]
int uart_tx_r; // read next from uart_tx_buf[uar_tx_r++]

void uartstart();

void
uartinit(void)
{
  // disable interrupts.
  WriteReg(IER, 0x00);

  // special mode to set baud rate.
  WriteReg(LCR, 0x80);

  // LSB for baud rate of 38.4K.
  WriteReg(0, 0x03);

  // MSB for baud rate of 38.4K.
  WriteReg(1, 0x00);

  // leave set-baud mode,
  // and set word length to 8 bits, no parity.
  WriteReg(LCR, 0x03);

  // reset and enable FIFOs.
  WriteReg(FCR, 0x07);

  // enable transmit and receive interrupts.
  WriteReg(IER, 0x02 | 0x01);

  initlock(&uart_tx_lock, "uart");
}

// add a character to the output buffer and tell the
// UART to start sending if it isn't already.
//
// usually called from the top-half -- by a process
// calling write(). can also be called from a uart
// interrupt to echo a received character, or by printf
// or panic from anywhere in the kernel.
//
// the block argument controls what happens if the
// buffer is full. for write(), block is 1, and the
// process waits. for kernel printf's and echoed
// characters, block is 0, and the character is
// discarded; this is necessary since sleep() is
// not possible in interrupts.
void
uartputc(int c, int block)
{
  acquire(&uart_tx_lock);
  while(1){
    if(((uart_tx_w + 1) % UART_TX_BUF_SIZE) == uart_tx_r){
      // buffer is full.
      if(block){
        // wait for uartstart() to open up space in the buffer.
        sleep(&uart_tx_r, &uart_tx_lock);
      } else {
        // caller does not want us to wait.
        release(&uart_tx_lock);
        return;
      }
    } else {
      uart_tx_buf[uart_tx_w] = c;
      uart_tx_w = (uart_tx_w + 1) % UART_TX_BUF_SIZE;
      uartstart();
      release(&uart_tx_lock);
      return;
    }
  }
}

// if the UART is idle, and a character is waiting
// in the transmit buffer, send it.
// caller must hold uart_tx_lock.
// called from both the top- and bottom-half.
void
uartstart()
{
  while(1){
    if(uart_tx_w == uart_tx_r){
      // transmit buffer is empty.
      return;
    }
    
    if((ReadReg(LSR) & (1 << 5)) == 0){
      // the UART transmit holding register is full,
      // so we cannot give it another byte.
      // it will interrupt when it's ready for a new byte.
      return;
    }
    
    int c = uart_tx_buf[uart_tx_r];
    uart_tx_r = (uart_tx_r + 1) % UART_TX_BUF_SIZE;
    
    // maybe uartputc() is waiting for space in the buffer.
    wakeup(&uart_tx_r);
    
    WriteReg(THR, c);
  }
}

// read one input character from the UART.
// return -1 if none is waiting.
int
uartgetc(void)
{
  if(ReadReg(LSR) & 0x01){
    // input data is ready.
    return ReadReg(RHR);
  } else {
    return -1;
  }
}

// handle a uart interrupt, raised because input has
// arrived, or the uart is ready for more output, or
// both. called from trap.c.
void
uartintr(void)
{
  // read and process incoming characters.
  while(1){
    int c = uartgetc();
    if(c == -1)
      break;
    consoleintr(c);
  }

  // send buffered characters.
  acquire(&uart_tx_lock);
  uartstart();
  release(&uart_tx_lock);
}