xv6-65oo2/kernel/uart.c
Matt Harvey b9359c3533 Corrects order of UART RX/TX interrupt enable bits
(per http://byterunner.com/16550.html and manually tested in qemu bare metal echo)
2020-10-03 16:35:47 -04:00

195 lines
4.6 KiB
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 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];
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++]
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(1){
if(((uart_tx_w + 1) % UART_TX_BUF_SIZE) == uart_tx_r){
// buffer is full.
// wait for uartstart() to open up space in the buffer.
sleep(&uart_tx_r, &uart_tx_lock);
} 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;
}
}
}
// 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_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);
}