WSL2-Linux-Kernel/Documentation/hpet.txt

		High Precision Event Timer Driver for Linux

The High Precision Event Timer (HPET) hardware is the future replacement for the 8254 and Real
Time Clock (RTC) periodic timer functionality.  Each HPET can have up two 32 timers.  It is possible
to configure the first two timers as legacy replacements for 8254 and RTC periodic.  A specification
done by INTEL and Microsoft can be found at http://www.intel.com/labs/platcomp/hpet/hpetspec.htm.

The driver supports detection of HPET driver allocation and initialization of the HPET before the
driver module_init routine is called.  This enables platform code which uses timer 0 or 1 as the
main timer to intercept HPET initialization.  An example of this initialization can be found in
arch/i386/kernel/time_hpet.c.

The driver provides two APIs which are very similar to the API found in the rtc.c driver.
There is a user space API and a kernel space API.  An example user space program is provided
below.

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <string.h>
#include <memory.h>
#include <malloc.h>
#include <time.h>
#include <ctype.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <signal.h>
#include <fcntl.h>
#include <errno.h>
#include <sys/time.h>
#include <linux/hpet.h>


extern void hpet_open_close(int, const char **);
extern void hpet_info(int, const char **);
extern void hpet_poll(int, const char **);
extern void hpet_fasync(int, const char **);
extern void hpet_read(int, const char **);

#include <sys/poll.h>
#include <sys/ioctl.h>
#include <signal.h>

struct hpet_command {
	char		*command;
	void		(*func)(int argc, const char ** argv);
} hpet_command[] = {
	{
		"open-close",
		hpet_open_close
	},
	{
		"info",
		hpet_info
	},
	{
		"poll",
		hpet_poll
	},
	{
		"fasync",
		hpet_fasync
	},
};

int
main(int argc, const char ** argv)
{
	int	i;

	argc--;
	argv++;

	if (!argc) {
		fprintf(stderr, "-hpet: requires command\n");
		return -1;
	}


	for (i = 0; i < (sizeof (hpet_command) / sizeof (hpet_command[0])); i++)
		if (!strcmp(argv[0], hpet_command[i].command)) {
			argc--;
			argv++;
			fprintf(stderr, "-hpet: executing %s\n",
				hpet_command[i].command);
			hpet_command[i].func(argc, argv);
			return 0;
		}

	fprintf(stderr, "do_hpet: command %s not implemented\n", argv[0]);

	return -1;
}

void
hpet_open_close(int argc, const char **argv)
{
	int	fd;

	if (argc != 1) {
		fprintf(stderr, "hpet_open_close: device-name\n");
		return;
	}

	fd = open(argv[0], O_RDONLY);
	if (fd < 0)
		fprintf(stderr, "hpet_open_close: open failed\n");
	else
		close(fd);

	return;
}

void
hpet_info(int argc, const char **argv)
{
}

void
hpet_poll(int argc, const char **argv)
{
	unsigned long		freq;
	int			iterations, i, fd;
	struct pollfd		pfd;
	struct hpet_info	info;
	struct timeval		stv, etv;
	struct timezone		tz;
	long			usec;

	if (argc != 3) {
		fprintf(stderr, "hpet_poll: device-name freq iterations\n");
		return;
	}

	freq = atoi(argv[1]);
	iterations = atoi(argv[2]);

	fd = open(argv[0], O_RDONLY);

	if (fd < 0) {
		fprintf(stderr, "hpet_poll: open of %s failed\n", argv[0]);
		return;
	}

	if (ioctl(fd, HPET_IRQFREQ, freq) < 0) {
		fprintf(stderr, "hpet_poll: HPET_IRQFREQ failed\n");
		goto out;
	}

	if (ioctl(fd, HPET_INFO, &info) < 0) {
		fprintf(stderr, "hpet_poll: failed to get info\n");
		goto out;
	}

	fprintf(stderr, "hpet_poll: info.hi_flags 0x%lx\n", info.hi_flags);

	if (info.hi_flags && (ioctl(fd, HPET_EPI, 0) < 0)) {
		fprintf(stderr, "hpet_poll: HPET_EPI failed\n");
		goto out;
	}

	if (ioctl(fd, HPET_IE_ON, 0) < 0) {
		fprintf(stderr, "hpet_poll, HPET_IE_ON failed\n");
		goto out;
	}

	pfd.fd = fd;
	pfd.events = POLLIN;

	for (i = 0; i < iterations; i++) {
		pfd.revents = 0;
		gettimeofday(&stv, &tz);
		if (poll(&pfd, 1, -1) < 0)
			fprintf(stderr, "hpet_poll: poll failed\n");
		else {
			long 	data;

			gettimeofday(&etv, &tz);
			usec = stv.tv_sec * 1000000 + stv.tv_usec;
			usec = (etv.tv_sec * 1000000 + etv.tv_usec) - usec;

			fprintf(stderr,
				"hpet_poll: expired time = 0x%lx\n", usec);

			fprintf(stderr, "hpet_poll: revents = 0x%x\n",
				pfd.revents);

			if (read(fd, &data, sizeof(data)) != sizeof(data)) {
				fprintf(stderr, "hpet_poll: read failed\n");
			}
			else
				fprintf(stderr, "hpet_poll: data 0x%lx\n",
					data);
		}
	}

out:
	close(fd);
	return;
}

static int hpet_sigio_count;

static void
hpet_sigio(int val)
{
	fprintf(stderr, "hpet_sigio: called\n");
	hpet_sigio_count++;
}

void
hpet_fasync(int argc, const char **argv)
{
	unsigned long		freq;
	int			iterations, i, fd, value;
	sig_t			oldsig;
	struct hpet_info	info;

	hpet_sigio_count = 0;
	fd = -1;

	if ((oldsig = signal(SIGIO, hpet_sigio)) == SIG_ERR) {
		fprintf(stderr, "hpet_fasync: failed to set signal handler\n");
		return;
	}

	if (argc != 3) {
		fprintf(stderr, "hpet_fasync: device-name freq iterations\n");
		goto out;
	}

	fd = open(argv[0], O_RDONLY);

	if (fd < 0) {
		fprintf(stderr, "hpet_fasync: failed to open %s\n", argv[0]);
		return;
	}


	if ((fcntl(fd, F_SETOWN, getpid()) == 1) ||
		((value = fcntl(fd, F_GETFL)) == 1) ||
		(fcntl(fd, F_SETFL, value | O_ASYNC) == 1)) {
		fprintf(stderr, "hpet_fasync: fcntl failed\n");
		goto out;
	}

	freq = atoi(argv[1]);
	iterations = atoi(argv[2]);

	if (ioctl(fd, HPET_IRQFREQ, freq) < 0) {
		fprintf(stderr, "hpet_fasync: HPET_IRQFREQ failed\n");
		goto out;
	}

	if (ioctl(fd, HPET_INFO, &info) < 0) {
		fprintf(stderr, "hpet_fasync: failed to get info\n");
		goto out;
	}

	fprintf(stderr, "hpet_fasync: info.hi_flags 0x%lx\n", info.hi_flags);

	if (info.hi_flags && (ioctl(fd, HPET_EPI, 0) < 0)) {
		fprintf(stderr, "hpet_fasync: HPET_EPI failed\n");
		goto out;
	}

	if (ioctl(fd, HPET_IE_ON, 0) < 0) {
		fprintf(stderr, "hpet_fasync, HPET_IE_ON failed\n");
		goto out;
	}

	for (i = 0; i < iterations; i++) {
		(void) pause();
		fprintf(stderr, "hpet_fasync: count = %d\n", hpet_sigio_count);
	}

out:
	signal(SIGIO, oldsig);

	if (fd >= 0)
		close(fd);

	return;
}

The kernel API has three interfaces exported from the driver:

	hpet_register(struct hpet_task *tp, int periodic)
	hpet_unregister(struct hpet_task *tp)
	hpet_control(struct hpet_task *tp, unsigned int cmd, unsigned long arg)

The kernel module using this interface fills in the ht_func and ht_data members of the
hpet_task structure before calling hpet_register.  hpet_control simply vectors to the hpet_ioctl
routine and has the same commands and respective arguments as the user API.  hpet_unregister
is used to terminate usage of the HPET timer reserved by hpet_register.