Playing with CPU PM Quality Of Service Interface (/dev/cpu_dma_latency)

main.c

#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <stdarg.h>
#include <string.h>

#include <unistd.h>
#include <fcntl.h>
#include <signal.h>
#include <sys/stat.h>
#include <errno.h>

static int32_t latency_target_value = 0;
static int latency_target_fd = -1;

void err_doit(int err, const char *fmt, va_list ap)
{
	if (err)
		fprintf(stderr, "%s\n", strerror(err));
	vfprintf(stderr, fmt, ap);
	return;
}

/* Print an error message, plus a message for err, and return */
void err_msg_n(int err, char *fmt, ...)
{
	va_list ap;
	va_start(ap, fmt);
	err_doit(err, fmt, ap);
	va_end(ap);
	return;
}

/* Latency trick
 * if the file /dev/cpu_dma_latency exists,
 * open it and write a zero into it. This will tell
 * the power management system not to transition to
 * a high cstate (in fact, the system acts like idle=poll)
 * When the fd to /dev/cpu_dma_latency is closed, the behavior
 * goes back to the system default.
 *
 * Documentation/power/pm_qos_interface.txt
 */
static void set_latency_target(void)
{
	struct stat s;
	int err;

	errno = 0;
	err = stat("/dev/cpu_dma_latency", &s);
	if (err == -1) {
		err_msg_n(errno, "WARN: stat /dev/cpu_dma_latency failed");
		return;
	}

	errno = 0;
	latency_target_fd = open("/dev/cpu_dma_latency", O_RDWR);
	if (latency_target_fd == -1) {
		err_msg_n(errno, "WARN: open /dev/cpu_dma_latency");
		return;
	}

	errno = 0;
	err = write(latency_target_fd, &latency_target_value, 4);
	if (err < 1) {
		err_msg_n(errno, "# error setting cpu_dma_latency to %d!", latency_target_value);
		close(latency_target_fd);
		return;
	}
	printf("# /dev/cpu_dma_latency set to %dus\n", latency_target_value);
}

void handle_sigint(int sig) {
    if (latency_target_fd >= 0) close(latency_target_fd);

    printf("\nCaught signal %d (Ctrl+C). Exiting...\n", sig);
    exit(0);
}

int main()
{
  signal(SIGINT, handle_sigint);
  set_latency_target();
  while (1){}
  return 0;
}

Makefile

# Compiler and flags
CC = gcc
CFLAGS = -Wall -Wno-nonnull -Wextra -Wno-sign-compare

# Target names
TARGET = qos
SRCS = main.c
OBJS = $(SRCS:.c=.o)

# Default target
all: $(TARGET)

# Link the executable
$(TARGET): $(OBJS)
	$(CC) -o $@ $^ $(CFLAGS)

# Compile .c files into .o
%.o: %.c
	$(CC) -c $< -o $@ $(CFLAGS)

# Clean up build artifacts
clean:
	rm -f $(OBJS) $(TARGET)

# Phony targets
.PHONY: all clean