Add support for ADJ_SETOFFSET mode

This adjtimex mode allows precise stepping of the system clock.

wrap_adjtimex.c

@@ -229,5 +229,53 @@ TMX_GetPLLOffsetLeft(long *offset)
   return result;
 }
 
+int
+TMX_TestStepOffset(void)
+{
+  struct timex txc;
+
+  /* Zero maxerror and check it's reset to a maximum after ADJ_SETOFFSET.
+     This seems to be the only way how to verify that the kernel really
+     supports the ADJ_SETOFFSET mode as it doesn't return an error on unknown
+     mode. */
+
+  txc.modes = ADJ_MAXERROR;
+  txc.maxerror = 0;
+  if (adjtimex(&txc) < 0 || txc.maxerror != 0)
+    return -1;
+
+  txc.modes = ADJ_SETOFFSET;
+  txc.time.tv_sec = 0;
+  txc.time.tv_usec = 0;
+  if (adjtimex(&txc) < 0 || txc.maxerror < 100000)
+    return -1;
+
+  return 0;
+}
+
+int
+TMX_ApplyStepOffset(double offset)
+{
+  struct timex txc;
+
+  txc.modes = ADJ_SETOFFSET;
+  if (offset >= 0) {
+    txc.time.tv_sec = offset;
+  } else {
+    txc.time.tv_sec = offset - 1;
+  }
+
+  /* ADJ_NANO changes the status even with ADJ_SETOFFSET, use it only when
+     STA_NANO is already enabled */
+  if (status & STA_PLL) {
+    txc.modes |= ADJ_NANO;
+    txc.time.tv_usec = 1e9 * (offset - txc.time.tv_sec);
+  } else {
+    txc.time.tv_usec = 1e6 * (offset - txc.time.tv_sec);
+  }
+
+  return adjtimex(&txc);
+}
+
 #endif