6.S081/6.828: xv6源码分析--中断和控制台

.globl timervec
.align 4
timervec:
        # start.c has set up the memory that mscratch points to:
        # scratch[0,8,16] : register save area.
        # scratch[24] : address of CLINT's MTIMECMP register.
        # scratch[32] : desired interval between interrupts.
        
        csrrw a0, mscratch, a0
        sd a1, 0(a0)
        sd a2, 8(a0)
        sd a3, 16(a0)

        # schedule the next timer interrupt
        # by adding interval to mtimecmp.
        ld a1, 24(a0) # CLINT_MTIMECMP(hart)
        ld a2, 32(a0) # interval
        ld a3, 0(a1)
        add a3, a3, a2
        sd a3, 0(a1)

        # raise a supervisor software interrupt.
        //触发software interrupt来处理timer interrupt，直接处理会打破关中断的限制。
		li a1, 2
        csrw sip, a1

        ld a3, 16(a0)
        ld a2, 8(a0)
        ld a1, 0(a0)
        csrrw a0, mscratch, a0

        mret

//
// 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 UART0 0x10000000L
//内存映射IO，这是设备寄存器的内存地址
#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.
  //关中断
  WriteReg(IER, 0x00);

  //设置波特率
  // 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);

  //设置字符长度为8bit
  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 == 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);
    } else {
      uart_tx_buf[uart_tx_w % UART_TX_BUF_SIZE] = c;
      uart_tx_w += 1;
      //非同步发送，如果发送寄存器满，就会return掉，下次响应中断。
      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.
  //interrupt使得CPU和设备可以并行和异步，如果不用interrupt的话，那么就必须同步发送数据，
  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.
      //当发送寄存器空闲并准备好接收新字符时，会主动中断，cpu然后响应并在devintr()中会执行。
      // 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 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);
}

//一旦THR或者RHR处理结束，就会触发一个中断来继续处理数据，此时
//usertrap/kerneltrap-->devintr-->uartintr来发送和接收数据。
int
devintr()
{
  uint64 scause = r_scause();

  if((scause & 0x8000000000000000L) &&
     (scause & 0xff) == 9){
    // this is a supervisor external interrupt, via PLIC.

    // irq indicates which device interrupted.
    int irq = plic_claim();

    if(irq == UART0_IRQ){
      uartintr();
    } else if(irq == VIRTIO0_IRQ){
      virtio_disk_intr();
    } else if(irq){
      printf("unexpected interrupt irq=%d\n", irq);
    }

    // the PLIC allows each device to raise at most one
    // interrupt at a time; tell the PLIC the device is
    // now allowed to interrupt again.
    if(irq)
      plic_complete(irq);

    return 1;
  } else if(scause == 0x8000000000000001L){
    // software interrupt from a machine-mode timer interrupt,
    // forwarded by timervec in kernelvec.S.

    if(cpuid() == 0){
      clockintr();
    }
    
    // acknowledge the software interrupt by clearing
    // the SSIP bit in sip.
    w_sip(r_sip() & ~2);

    return 2;
  } else {
    return 0;
  }
}

struct {
  struct spinlock lock;
  
  // input
#define INPUT_BUF 128
  char buf[INPUT_BUF];
  uint r;  // Read index
  uint w;  // Write index
  uint e;  // Edit index
} cons;

void
consoleinit(void)
{
  initlock(&cons.lock, "cons");

  uartinit();

  // connect read and write system calls
  // to consoleread and consolewrite.
  devsw[CONSOLE].read = consoleread;
  devsw[CONSOLE].write = consolewrite;
}

//初始化init进程
void
userinit(void)
{
  struct proc *p;

  p = allocproc();
  initproc = p;
  
  // allocate one user page and copy init's instructions
  // and data into it.
  //初始化init代码段和页表，第一次返回用户态后就会执行initcode，相当于exec("/init")
  uvminit(p->pagetable, initcode, sizeof(initcode));
  p->sz = PGSIZE;

  // prepare for the very first "return" from kernel to user.
  p->trapframe->epc = 0;      // user program counter
  p->trapframe->sp = PGSIZE;  // user stack pointer

  safestrcpy(p->name, "initcode", sizeof(p->name));
  p->cwd = namei("/");

  p->state = RUNNABLE;

  release(&p->lock);
}

// Load the user initcode into address 0 of pagetable,
// for the very first process.
// sz must be less than a page.
void
uvminit(pagetable_t pagetable, uchar *src, uint sz)
{
  char *mem;

  if(sz >= PGSIZE)
    panic("inituvm: more than a page");
  mem = kalloc();
  memset(mem, 0, PGSIZE);
  mappages(pagetable, 0, PGSIZE, (uint64)mem, PTE_W|PTE_R|PTE_X|PTE_U);
  memmove(mem, src, sz);
}
uchar initcode[] = {
  0x17, 0x05, 0x00, 0x00, 0x13, 0x05, 0x45, 0x02,
  0x97, 0x05, 0x00, 0x00, 0x93, 0x85, 0x35, 0x02,
  0x93, 0x08, 0x70, 0x00, 0x73, 0x00, 0x00, 0x00,
  0x93, 0x08, 0x20, 0x00, 0x73, 0x00, 0x00, 0x00,
  0xef, 0xf0, 0x9f, 0xff, 0x2f, 0x69, 0x6e, 0x69,
  0x74, 0x00, 0x00, 0x24, 0x00, 0x00, 0x00, 0x00,
  0x00, 0x00, 0x00, 0x00
};

char *argv[] = { "sh", 0 };

int
main(void)
{
  int pid, wpid;

  if(open("console", O_RDWR) < 0){
    //创建控制台设备
    mknod("console", CONSOLE, 0);
    open("console", O_RDWR);
  }
  //复制多个
  dup(0);  // stdout
  dup(0);  // stderr

  for(;;){
    printf("init: starting sh\n");
    pid = fork();
    if(pid < 0){
      printf("init: fork failed\n");
      exit(1);
    }
    if(pid == 0){
      //子进程是sh进程，也就是终端
      exec("sh", argv);
      //正常情况替换可执行文件后就切换到新执行流了，不会再回到这个位置
      printf("init: exec sh failed\n");
      exit(1);
    }

    for(;;){
      // this call to wait() returns if the shell exits,
      // or if a parentless process exits.
      wpid = wait((int *) 0);
      if(wpid == pid){
        // the shell exited; restart it.
        break;
      } else if(wpid < 0){
        printf("init: wait returned an error\n");
        exit(1);
      } else {
        // it was a parentless process; do nothing.
      }
    }
  }
}

int
main(void)
{
  static char buf[100];
  int fd;

  // Ensure that three file descriptors are open.
  while((fd = open("console", O_RDWR)) >= 0){
    if(fd >= 3){
      close(fd);
      break;
    }
  }

  // Read and run input commands.
  while(getcmd(buf, sizeof(buf)) >= 0){
    //如果是cd
    if(buf[0] == 'c' && buf[1] == 'd' && buf[2] == ' '){
      // Chdir must be called by the parent, not the child.
      buf[strlen(buf)-1] = 0;  // chop \n
      if(chdir(buf+3) < 0)
        fprintf(2, "cannot cd %s\n", buf+3);
      continue;
    }
    //fork进程执行命令
    if(fork1() == 0)
      runcmd(parsecmd(buf));
    wait(0);
  }
  exit(0);
}

int
consolewrite(int user_src, uint64 src, int n)
{
  int i;

  for(i = 0; i < n; i++){
    char c;
    //读入输出字符
    if(either_copyin(&c, user_src, src+i, 1) == -1)
      break;
    //写入uart_tx_buf，异步发送出去
    uartputc(c);
  }

  return i;
}

//
// user read()s from the console go here.
// copy (up to) a whole input line to dst.
// user_dist indicates whether dst is a user
// or kernel address.
//
int
consoleread(int user_dst, uint64 dst, int n)
{
  uint target;
  int c;
  char cbuf;

  target = n;
  acquire(&cons.lock);
  while(n > 0){
    // wait until interrupt handler has put some
    // input into cons.buffer.
    //buf为空就阻塞
    while(cons.r == cons.w){
      if(myproc()->killed){
        release(&cons.lock);
        return -1;
      }
      //键盘每输入一个字符就会唤醒一次
      sleep(&cons.r, &cons.lock);
    }
    //读取一个字符
    c = cons.buf[cons.r++ % INPUT_BUF];
    // ctrl + D，终止读取，从阻塞中唤醒
    if(c == C('D')){  // end-of-file
      if(n < target){
        // Save ^D for next time, to make sure
        // caller gets a 0-byte result.
        cons.r--;
      }
      break;
    }

    // copy the input byte to the user-space buffer.
    cbuf = c;
    if(either_copyout(user_dst, dst, &cbuf, 1) == -1)
      break;

    dst++;
    --n;
    //读取整行就退出
    if(c == '\n'){
      // a whole line has arrived, return to
      // the user-level read().
      break;
    }
  }
  release(&cons.lock);
  //读取的字符数
  return target - n;
}