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Writing a Simple OS Kernel — Part 7, Serial Port Driver

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Last time we got IPC working, and used it to build a simple path-resolution module. This time we’re going to write a fully-functional serial port driver.

Generic Interrupts

When we got interrupts working, we sort of hacked the kernel part in to support the timer, which is all we were interested in at the time. Let’s make that a bit more generic. We’re going to need one more syscall, since interrupts have to be handled by the kernel. The simplest one we can implement just allows a process to wait for an interrupt to fire on a particular line:

void interrupt_wait(int intr);

.global interrupt_wait
interrupt_wait:
	push {r7}
	mov r7, #
	svc 0
	bx lr

# TASK_WAIT_INTR 3

case 0x5: /* interrupt_wait */
	/* Enable interrupt */
	*(PIC + VIC_INTENABLE) = tasks[current_task][2+0];
	/* Block task waiting for interrupt to happen */
	tasks[current_task][-1] = TASK_WAIT_INTR;
	break;

We are already enabling the timer interrupt when the kernel starts. Here, we enable the interrupt that the process is asking to wait on, since if it is not enabled the task will block forever, so this seems like a useful place to do it.

We also want to unblock tasks when the interrupt fires. Get rid of the -4 case that we have hardcoded for the timer and put this in:

default: /* Catch all interrupts */
	if((int)tasks[current_task][2+7] < 0) {
		unsigned int intr = (1 < < -tasks[current_task][2+7]);

		if(intr == PIC_TIMER01) {
			/* Never disable timer. We need it for pre-emption */
			if(*(TIMER0 + TIMER_MIS)) { /* Timer0 went off */
				*(TIMER0 + TIMER_INTCLR) = 1; /* Clear interrupt */
			}
		} else {
			/* Disable interrupt, interrupt_wait re-enables */
			*(PIC + VIC_INTENCLEAR) = intr;
		}
		/* Unblock any waiting tasks
			XXX: nondeterministic unblock order
		*/
		for(i = 0; i < task_count; i++) {
			if(tasks[i][-1] == TASK_WAIT_INTR && tasks[i][2+0] == intr) {
				tasks[i][-1] = TASK_READY;
			}
		}
	}

You’ll notice we also need a new magic number for versatilepb.h:

# VIC_INTENCLEAR 0x5 /* 0x14 bytes */

Code for this section on Github.

Output Driver

Let’s use this new ability to implement half of our driver: output. This will allow us to write to a specific fifo for output to the serial port instead of using bwputs.

A refresher of serial port related magic numbers:

/* http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0224i/Bbabegge.html */
# UART0 ((volatile unsigned int*)0x101f1000)
/* http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.ddi0183g/I18381.html */
# UARTFR 0x06 /* 0x18 bytes */
# UARTIMSC 0x0E /* 0x38 bytes */
# UARTFR_TXFF 0x20
# UARTIMSC_TXIM 0x20

# PIC_UART0 0x1000

Let’s make a generic driver that accepts parameters telling it what serial port to drive:

void serialout(volatile unsigned int* uart, unsigned int intr) {
	int fd;
	char c;

We need to create the fifo that processes send bytes on:

	mkfifo("/dev/tty0/out", 0);
	fd = open("/dev/tty0/out", 0);

We also need to tell the UART to trigger an interrupt when it becomes ready to send a byte:

	*(uart + UARTIMSC) |= UARTIMSC_TXIM;

And finally a loop to read bytes and send them to the UART:

	while(1) {
		read(fd, &c, 1);
		interrupt_wait(intr);
		*uart = c;
	}

This seems pretty reasonable, but there is a problem. The UART triggers an interrupt when it becomes ready to send a byte. If it is already ready (such as at boot time), the interrupt will not trigger. So we try this:

	while(1) {
		read(fd, &c, 1);
		*uart = c;
		interrupt_wait(intr);
	}

This is closer, but in both of these versions there is a race condition where the UART might not actually be ready anymore by the time we process the interrupt (depending on what other tasks are doing), so let’s add a check for that:

	while(1) {
		read(fd, &c, 1);
		if(!(*(uart + UARTFR) & UARTFR_TXFF)) {
			*uart = c;
		}
		interrupt_wait(intr);
	}

Now, however, if the UART is not ready, we will skip bytes. So we only want to read a new byte if we actually wrote the current one:

	int doread = 1;
	while(1) {
		if(doread) read(fd, &c, 1);
		doread = 0;
		if(!(*(uart + UARTFR) & UARTFR_TXFF)) {
			*uart = c;
			doread = 1;
		}
		interrupt_wait(intr);
	}

To test this out, let’s set up a new first:

void first(void) {
	int fd;

	if(!fork()) pathserver();
	if(!fork()) serialout(UART0, PIC_UART0);

	fd = open("/dev/tty0/out", 0);
	write(fd, "woo\n", sizeof("woo\n"));
	write(fd, "thar\n", sizeof("thar\n"));
	while(1);
}

Our current pre-emption speed will make this a bit of a pain to run, so you can set the clock value in main to something lower.

We can now print to the serial port by writing to a fifo!

Code for this section on Github.

Input Driver

Some new magic numbers:

# UARTICR 0x11 /* 0x44 bytes */
# UARTFR_RXFE 0x10
# UARTIMSC_RXIM 0x10
# UARTICR_RXIC 0x10
# UARTICR_TXIC 0x20

The input driver will be another process that waits on the same interrupt. Unfortunately, because this one interrupt actually can signal one of several things, if two of them are true, and different processes work on processing them, the interrupt will fire constantly and prevent any work from getting done. We solve this by clearing the more specific interrupt in the drivers where we handle them. For example, in serialout:

*(uart + UARTICR) = UARTICR_TXIC;

Next, start with some setup:

void serialin(volatile unsigned int* uart, unsigned int intr) {
	int fd;
	char c;
	mkfifo("/dev/tty0/in", 0);
	fd = open("/dev/tty0/in", 0);

	/* enable RX interrupt on UART */
	*(uart + UARTIMSC) |= UARTIMSC_RXIM;

	while(1) {

We want to wait on the interrupt, and then immidiately clear the specific interrupt we handle, just like in the output driver:

	interrupt_wait(intr);
	*(uart + UARTICR) = UARTICR_RXIC;

And we want to make sure, not only that no race condition has changed the UART status, but that the interrupt was meant for us at all:

	if(!(*(uart + UARTFR) & UARTFR_RXFE)) {

And then just read the byte and put it in our fifo:

	c = *uart;
	write(fd, &c, 1);

This input server has a bit of a bug. If the fifo is ever full, it will block, and maybe miss input bytes from the serial port. Our processes are likely to be way faster than any serial port, but still, in a real kernel you would want to account for this by splitting the driver into two processes: one that waits on the interrupt, and one that buffers the bytes internally.

Code for this section on Github.

Echo Application

To tie all this up, we write a simple application that will echo back every character you type:

void echo(void) {
	int fdout, fdin;
	char c;
	fdout = open("/dev/tty0/out", 0);
	fdin = open("/dev/tty0/in", 0);

	while(1) {
		read(fdin, &c, 1);
		write(fdout, &c, 1);
	}
}

Code for this section on Github.

We’re done!

We now have a working serial port driver that respects the hardware by using interrupts instead of burning CPU time looping on flag checks. Remember, any interactios with the UART will affect these drivers, so using bwputs may have strange side effects now.

To be honest, this is about as far as I expected to get when I originally started this series. I could write a clock driver, to allow for sleep calls and similar, but you should be able to see how that might work. One big topic I have avoided thus far is enabling the MMU and doing virtual memory / memory protection. I may try to figure out how that works on this harware, but it will require quite a bit of doing. Suggestions for others things you would like to see in this series go in the comments!

Code so far on Github.