first commit

kernel/uart.c

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+//
+// 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 IER_RX_ENABLE (1<<0)
+#define IER_TX_ENABLE (1<<1)
+#define FCR 2                 // FIFO control register
+#define FCR_FIFO_ENABLE (1<<0)
+#define FCR_FIFO_CLEAR (3<<1) // clear the content of the two FIFOs
+#define ISR 2                 // interrupt status register
+#define LCR 3                 // line control register
+#define LCR_EIGHT_BITS (3<<0)
+#define LCR_BAUD_LATCH (1<<7) // special mode to set baud rate
+#define LSR 5                 // line status register
+#define LSR_RX_READY (1<<0)   // input is waiting to be read from RHR
+#define LSR_TX_IDLE (1<<5)    // THR can accept another character to send
+
+#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];
+uint64 uart_tx_w; // write next to uart_tx_buf[uart_tx_w % UART_TX_BUF_SIZE]
+uint64 uart_tx_r; // read next from uart_tx_buf[uart_tx_r % UART_TX_BUF_SIZE]
+
+extern volatile int panicked; // from printf.c
+
+void uartstart();
+
+void
+uartinit(void)
+{
+  // disable interrupts.
+  WriteReg(IER, 0x00);
+
+  // special mode to set baud rate.
+  WriteReg(LCR, LCR_BAUD_LATCH);
+
+  // 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, LCR_EIGHT_BITS);
+
+  // reset and enable FIFOs.
+  WriteReg(FCR, FCR_FIFO_ENABLE | FCR_FIFO_CLEAR);
+
+  // enable transmit and receive interrupts.
+  WriteReg(IER, IER_TX_ENABLE | IER_RX_ENABLE);
+
+  initlock(&uart_tx_lock, "uart");
+}
+
+// add a character to the output buffer and tell the
+// UART to start sending if it isn't already.
+// blocks if the output buffer is full.
+// because it may block, it can't be called
+// from interrupts; it's only suitable for use
+// by write().
+void
+uartputc(int c)
+{
+  acquire(&uart_tx_lock);
+
+  if(panicked){
+    for(;;)
+      ;
+  }
+  while(uart_tx_w == uart_tx_r + UART_TX_BUF_SIZE){
+    // buffer is full.
+    // wait for uartstart() to open up space in the buffer.
+    sleep(&uart_tx_r, &uart_tx_lock);
+  }
+  uart_tx_buf[uart_tx_w % UART_TX_BUF_SIZE] = c;
+  uart_tx_w += 1;
+  uartstart();
+  release(&uart_tx_lock);
+}
+
+
+// alternate version of uartputc() that doesn't 
+// use interrupts, for use by kernel printf() and
+// to echo characters. it spins waiting for the uart's
+// output register to be empty.
+void
+uartputc_sync(int c)
+{
+  push_off();
+
+  if(panicked){
+    for(;;)
+      ;
+  }
+
+  // wait for Transmit Holding Empty to be set in LSR.
+  while((ReadReg(LSR) & LSR_TX_IDLE) == 0)
+    ;
+  WriteReg(THR, c);
+
+  pop_off();
+}
+
+// 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) & LSR_TX_IDLE) == 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_BUF_SIZE];
+    uart_tx_r += 1;
+    
+    // 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 devintr().
+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);
+}