cpulatency/cpulatency.c

#include <errno.h>
#include <fcntl.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

static int32_t pm_qos_fd = -1;

int32_t
low_latency(
int32_t target
)
{
  uint32_t sot = sizeof ( target );
  uint32_t cwbytes = 0;

  if (sot <= 1)
  {
    fprintf(stderr, "Error: Invalid target size: %u bytes.\n", sot);
    return ( 1 );
  }

  if (pm_qos_fd >= 0)
  {
    fprintf(stderr, "Error: PM QoS fd: %d.\n", pm_qos_fd);
    return ( 1 );
  }

  pm_qos_fd = open("/dev/cpu_dma_latency", O_RDWR);

  if (pm_qos_fd < 0)
  {
    fprintf(
      stderr,
      "Error: Failed to open PM QoS device: %s.\n",
      strerror(errno));
    return ( 1 );
  }

  cwbytes = write(pm_qos_fd, &target, sot);

  if (cwbytes != sot)
  {
    fprintf(
      stderr,
      "Error: Only wrote %u out of %u bytes requested.\n",
      cwbytes,
      sot);
    return ( 1 );
  }

  fprintf(stdout, "CPU DMA latency limit set to %d μs\n", target);
  return ( 0 );
}

int
main(
int argc,
char *argv[]
)
{
  int32_t llr = 1;

  if (!( argc >= 2 ))
  {
    fprintf(
      stdout,
      "%s: Set PM QoS CPU DMA latency limit\n \
                        \rUsage: %s <latency limit in μs> (0 ⟷ %d)\n",
      argv[0],
      argv[0],
      INT32_MAX);
    return ( 1 );
  }

  uint32_t ditr;

  size_t dsiz = snprintf(NULL, 0, "%d", INT32_MAX);

  char *dstr = malloc(dsiz + 1);

  if (dstr == NULL || !dstr)
  {
    fprintf(stderr, "Error: malloc() failure.\n");
    exit(1);
  }

  size_t dmax = snprintf(dstr, dsiz + 1, "%d", INT32_MAX);

  free(dstr);

  size_t dlen = strnlen(argv[1], dmax);
  size_t dact = strlen(argv[1]);

  if (dact > dmax)
  {
    argv[1][0] = 0;
  }

  for (ditr = 0; ditr < (uint32_t)dlen; ++ditr)
  {
    if (argv[1][ditr] < 48 || argv[1][ditr] > 57)
    {
      fprintf(stderr, "Error: Value must be a non-negative integer.\n");
      return ( 1 );
    }
  }

  int64_t target = atoi(argv[1]);

  if ((int32_t)target < 0 || target > INT32_MAX)
  {
    fprintf(stderr, "Error: Value out of range (0 ⟷ %d)\n", INT32_MAX);
    return ( 1 );
  }

  llr = low_latency((int32_t)target);

  if (llr != 0)
  {
    exit(llr);
  }

  pause();

  fprintf(
    stderr,
    "Warning: pause() failed, falling back to sleep().\n");

  while (!sleep(999))
  {
    ;
  }

  fprintf(stderr, "Error: sleep() failed, aborting.\n");

  exit(1);

  return ( 2 );
}