From 538e1c5eb146db2334db5ee5d934d1d187cac1fa Mon Sep 17 00:00:00 2001
From: Bryan Christianson <bryan@whatroute.net>
Date: Mon, 31 Aug 2020 19:56:45 +1200
Subject: [PATCH] sys_timex: add workaround for broken ntp_adjtime() on macOS

On macOS 11.0 (Big Sur) beta, ntp_adjtime() incorrectly returns
timex.freq as an unsigned number. This patch is a workaround for the bug
and should be removed when Apple fix the problem (assuming they will).
---
 sys_timex.c | 23 +++++++++++++++++++++--
 1 file changed, 21 insertions(+), 2 deletions(-)

diff --git a/sys_timex.c b/sys_timex.c
index 0a6b438..9a8504e 100644
--- a/sys_timex.c
+++ b/sys_timex.c
@@ -68,6 +68,25 @@ static int sys_tai_offset;
 
 /* ================================================== */
 
+static double
+convert_timex_frequency(const struct timex *txc)
+{
+  double freq_ppm;
+
+  freq_ppm = txc->freq / FREQ_SCALE;
+
+#ifdef MACOSX
+  /* Temporary workaround for Apple bug treating freq as unsigned number */
+  if (freq_ppm > 32767) {
+    freq_ppm -= 65536;
+  }
+#endif
+
+  return -freq_ppm;
+}
+
+/* ================================================== */
+
 static double
 read_frequency(void)
 {
@@ -77,7 +96,7 @@ read_frequency(void)
 
   SYS_Timex_Adjust(&txc, 0);
 
-  return txc.freq / -FREQ_SCALE;
+  return convert_timex_frequency(&txc);
 }
 
 /* ================================================== */
@@ -92,7 +111,7 @@ set_frequency(double freq_ppm)
 
   SYS_Timex_Adjust(&txc, 0);
 
-  return txc.freq / -FREQ_SCALE;
+  return convert_timex_frequency(&txc);
 }
 
 /* ================================================== */