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XV6 Device Driver

Why we have a Driver?
A driver is the code in an operating system that manages a particular device: 1. It tells the device hardware to perform operations,. 2. Configures the device to generate interrupts when done. 3. Handles the resulting interrupts,. 4. Interacts with processes that may be waiting for I/O from the device.
Initialization
Config Hardware
Xv6’s main calls consoleinit (kernel/console.c:189) to initialize the UART hardware, and to configure the UART hardware to generate input interrupts (kernel/uart.c:34).
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;
}
Create console device
In init.c, create a console device:
mknod(“console”, 1, 1);
open(“console”, O_RDWR);
The file path is console, type is T_DEVICE.
mknod implementation.
uint64
sys_mknod(void)
{
  struct inode *ip;
  char path[MAXPATH];
  int major, minor;
​
  begin_op(ROOTDEV);
  if((argstr(0, path, MAXPATH)) < 0 ||
     argint(1, &major) < 0 ||
     argint(2, &minor) < 0 ||
     (ip = create(path, T_DEVICE, major, minor)) == 0){
    end_op(ROOTDEV);
    return -1;
  }
At this point, we have a console device. A file descriptor 0 points to this device.
How to read or write from device
Let’s look at fileread
int
fileread(struct file *f, uint64 addr, int n)
{
  int r = 0;
​
  if(f->readable == 0)
    return -1;
​
  if(f->type == FD_PIPE){
    r = piperead(f->pipe, addr, n);
  } else if(f->type == FD_DEVICE){
    if(f->major < 0 || f->major >= NDEV || !devsw[f->major].read)
      return -1;
    r = devsw[f->major].read(f, 1, addr, n);
If the file type is DEVICE, we call the special read function to perform read operation. Note: The special read is set up in consoleinit.
consoleread sleeps and wait for a line of inputs, then copy to user space. Details see flow section.
Flow
Every input char generates an interrupt
When the user types a character, the UART hardware asks the RISC-V to raise an interrupt.
Trap handler determines interrupt source
xv6’s trap handling code calls devintr (kernel/trap.c:177). devintr looks at the RISC-V scause register to discover that the interrupt is from an external device. Then it asks a hardware unit called the PLIC [1] to tell it which device interrupted (kernel/trap.c:186). If it was the UART, devintrcalls uartintr.
Accumulate inputs and Wake up
Read one char and hand over
uartintr (kernel/uart.c:84) reads any waiting input characters from the UART hardware and hands them to consoleintr.
// trap.c calls here when the uart interrupts.
void
uartintr(void)
{
  while(1){
    int c = uartgetc();
    if(c == -1)
      break;
    consoleintr(c);
  }
}
Accumulate inputs
The job of consoleintr is to accumulate input characters in cons.buf until a whole line arrives.
Wake up when get a whole line
When a newline arrives, consoleintr wakes up a waiting consoleread (if there is one).
Copy to user space
Once woken, consoleread will observe a full line in cons.buf, copy it to user space and return to user space.
Question:
How does open(“console", O_RDWR) opens a file with DEVICE type?
Research: It seems this console string is defined somewhere. I tried to change to other string, and it does not work. Using this console string to open file, the file inode returns a file with type is DEVICE.
Answer:
OS create a console device using system call mknod. Any read or write with console is interacting with the UART hardware.

Hardware, Device, Assembly - Previous

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