dev: Add support for disabling ticking and the divider in MC146818

Some Linux versions disable updates (regB.set = 1) to prevent the chip
from updating its internal state while the OS is updating it. Support
for this was already there, this patch merely disables the check in
writeReg that prevented it from being enabled. The patch also includes
support for disabling the divider, which is used to control when clock
updates should start after setting the internal RTC state.

These changes are required to boot most vanilla Linux distributions
that update the RTC settings at boot.

1 files changed, 16 insertions, 6 deletions

diff --git a/src/dev/mc146818.cc b/src/dev/mc146818.cc
index 276c98581..abacd8742 100644
--- a/src/dev/mc146818.cc
+++ b/src/dev/mc146818.cc
@@ -144,7 +144,8 @@ MC146818::writeData(const uint8_t addr, const uint8_t data)
               // The "update in progress" bit is read only.
               stat_regA.uip = old_rega;
 
-              if (stat_regA.dv != RTCA_DV_32768HZ) {
+              if (!rega_dv_disabled(stat_regA) &&
+                  stat_regA.dv != RTCA_DV_32768HZ) {
                   inform("RTC: Unimplemented divider configuration: %i\n",
                         stat_regA.dv);
                   panic_unsupported = true;
@@ -155,14 +156,21 @@ MC146818::writeData(const uint8_t addr, const uint8_t data)
                         stat_regA.rs);
                   panic_unsupported = true;
               }
+
+              if (rega_dv_disabled(stat_regA)) {
+                  // The divider is disabled, make sure that we don't
+                  // schedule any ticks.
+                  if (tickEvent.scheduled())
+                      deschedule(tickEvent);
+              } else if (rega_dv_disabled(old_rega))  {
+                  // If the divider chain goes from reset to active, we
+                  // need to schedule a tick after precisely 0.5s.
+                  assert(!tickEvent.scheduled());
+                  schedule(tickEvent, curTick() + SimClock::Int::s / 2);
+              }
           } break;
           case RTC_STAT_REGB:
             stat_regB = data;
-            if (stat_regB.set) {
-                inform("RTC: Updating stopping not implemented.\n");
-                panic_unsupported = true;
-            }
-
             if (stat_regB.aie || stat_regB.uie) {
                 inform("RTC: Unimplemented interrupt configuration: %s %s\n",
                       stat_regB.aie ? "alarm" : "",
@@ -233,6 +241,8 @@ MC146818::readData(uint8_t addr)
 void
 MC146818::tickClock()
 {
+    assert(!rega_dv_disabled(stat_regA));
+
     if (stat_regB.set)
         return;
     time_t calTime = mkutctime(&curTime);