cpudmalatency.c

/*
 * Copyright 2009 Johannes Berg <johannes@sipsolutions.net>
 * Copyright 2010 Luis R. Rodriguez <lrodriguez@atheros.com>
 *
 * Permission to use, copy, modify, and/or distribute this software for any
 * purpose with or without fee is hereby granted, provided that the above
 * copyright notice and this permission notice appear in all copies.
 * 
 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
 * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
 * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 *
 *
 * Compile simply with:
 *	cc -o cpudmalatency cpudmalatency.c	
 *
 * Some unpatched buggy BIOSes create excessive C-state transition latencies
 * which can affect DMA on some devices. Instead of patching each and every
 * Linux kernel driver to account for these inefficiencies - lets punt these
 * work arounds to userspace. Linux distributions will hopefully have a way
 * to automatically detect these buggy platforms and call this. Note that
 * this app will hold the file open.
 */

#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdint.h>
#include <stdlib.h>
#include <unistd.h>

int main(int argc, char **argv)
{
	int32_t v;
	int fd;

	if (argc != 2) {
		fprintf(stderr, "Usage: %s <latency [us]>\n", argv[0]);
		fprintf(stderr, "\n");
		fprintf(stderr, "	latency: the maximum tolerable CPU DMA you\n");
		fprintf(stderr, "	         are willing to put up with [in microseconds]\n");
		fprintf(stderr, "\n");
		fprintf(stderr, "This program will block until you hit Ctrl-C, at which point\n");
		fprintf(stderr, "the file descriptor is closed and the latency requirement is\n");
		fprintf(stderr, "unregistered again.\n");
		fprintf(stderr, "Hint: if you have an platform with a buggy BIOS that has\n");
		fprintf(stderr, "issues with excessive C-state transition latencies try value 55\n");
		return 2;
	}

	v = atoi(argv[1]);

	printf("setting latency to %d.%.6d seconds\n", v/1000000, v % 1000000);

	fd = open("/dev/cpu_dma_latency", O_WRONLY);
	if (fd < 0) {
		perror("open /dev/cpu_dma_latency");
		return 1;
	}
	if (write(fd, &v, sizeof(v)) != sizeof(v)) {
		perror("write to /dev/cpu_dma_latency");
		return 1;
	}

	while (1) sleep(10);

	return 0;
}