Remove set_latency_target(), add lock_memory_to_ram()

set_latency_target() attempts to set keep "low latency" by
- writing a specific latency to /dev/cpu_dma_latency
- setting manually the minimum CPU processor frequency to be the maximum

There is no functionality to undo this after stopping the softmodem(s),
and most users are probably not even aware that OAI does this. It is
generally preferable to set this beforehand using a governor or by
disabling sleep states (as outlined in the tutorial, and in a follow-up
commit in the performance tuning docs).

The previous mlockall() call, to lock memory to RAM, is retained in a
new function. There were additional mlockall() calls, which have been
replaced with lock_memory_to_ram(), where necessary.

common/utils/system.c

@@ -326,37 +326,10 @@ void thread_top_init(char *thread_name,
   }
 }
 
-
-// Block CPU C-states deep sleep
-void set_latency_target(void) {
-  int ret;
-  static int latency_target_fd=-1;
-  uint32_t latency_target_value=2; // in microseconds
-  if (latency_target_fd == -1) {
-    if ( (latency_target_fd = open("/dev/cpu_dma_latency", O_RDWR)) != -1 ) {
-      ret = write(latency_target_fd, &latency_target_value, sizeof(latency_target_value));
-      if (ret == 0) {
-	printf("# error setting cpu_dma_latency to %u!: %s\n", latency_target_value, strerror(errno));
-	close(latency_target_fd);
-	latency_target_fd=-1;
-	return;
-      }
-    }
-  }
-  if (latency_target_fd != -1) 
-    LOG_I(HW,"# /dev/cpu_dma_latency set to %u us\n", latency_target_value);
-  else
-    LOG_E(HW,"Can't set /dev/cpu_dma_latency to %u us\n", latency_target_value);
-
-  // Set CPU frequency to it's maximum
-  int system_ret = system("for d in /sys/devices/system/cpu/cpu[0-9]*; do cat $d/cpufreq/cpuinfo_max_freq > $d/cpufreq/scaling_min_freq; done");
-  if (system_ret == -1) {
-    LOG_E(HW, "Can't set cpu frequency: [%d]  %s\n", errno, strerror(errno));
-    return;
-  }
-  if (!((WIFEXITED(system_ret)) && (WEXITSTATUS(system_ret) == 0))) {
-    LOG_E(HW, "Can't set cpu frequency\n");
-  }
-  mlockall(MCL_CURRENT | MCL_FUTURE);
-
+/* \brief lock memory to RAM to avoid delays */
+void lock_memory_to_ram(void)
+{
+  int rc = mlockall(MCL_CURRENT | MCL_FUTURE);
+  if (rc != 0)
+    LOG_W(UTIL, "mlockall() failed: %d, %s\n", errno, strerror(errno));
 }