[PATCH] timer: Handle decrements of PIT counter

[Roman Bolshakov]

There's a fallback to PIT if TSC is not present but it doesn't work
properly. It prevents boot from floppy on isapc and 486 cpu [1][2].

SeaBIOS configures PIT in Mode 2. PIT counter is decremented in the mode
but timer_adjust_bits() thinks that the counter overflows and increases
32-bit tick counter on each detected "overflow". Invalid overflow
detection results in 55ms time advance (1 / 18.2Hz) on each read from
PIT counter. So all timers expire much faster and 5-second floppy
timeout expires in 83 real microseconds (or just a bit longer).

Provide counter direction to timer_adjust_bits() and normalize the
counter to advance ticks in monotonically increasing TimerLast.

1. https://bugs.launchpad.net/seabios/+bug/1840719
2. https://lists.gnu.org/archive/html/qemu-devel/2019-08/msg03924.html

Fixes: eac11944019 ("Unify pmtimer_read() and pittimer_read() code.")
Reported-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Cc: Kevin O'Connor <kevin@koconnor.net>
Signed-off-by: Roman Bolshakov <r.bolshakov@yadro.com>
---
 src/hw/timer.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/src/hw/timer.c b/src/hw/timer.c
index 56bb289..2441402 100644
--- a/src/hw/timer.c
+++ b/src/hw/timer.c
@@ -156,10 +156,15 @@ u32 TimerLast VARLOW;
 
 // Add extra high bits to timers that have less than 32bits of precision.
 static u32
-timer_adjust_bits(u32 value, u32 validbits)
+timer_adjust_bits(u32 value, u32 validbits, u8 countup)
 {
     u32 last = GET_LOW(TimerLast);
-    value = (last & ~validbits) | (value & validbits);
+    u32 validvalue;
+    if (countup)
+        validvalue = value & validbits;
+    else
+        validvalue = validbits - (value & validbits);
+    value = (last & ~validbits) | validvalue;
     if (value < last)
         value += validbits + 1;
     SET_LOW(TimerLast, value);
@@ -176,11 +181,11 @@ timer_read(void)
         return rdtscll() >> GET_GLOBAL(ShiftTSC);
     if (CONFIG_PMTIMER && port != PORT_PIT_COUNTER0)
         // Read from PMTIMER
-        return timer_adjust_bits(inl(port), 0xffffff);
+        return timer_adjust_bits(inl(port), 0xffffff, 1);
     // Read from PIT.
     outb(PM_SEL_READBACK | PM_READ_VALUE | PM_READ_COUNTER0, PORT_PIT_MODE);
     u16 v = inb(PORT_PIT_COUNTER0) | (inb(PORT_PIT_COUNTER0) << 8);
-    return timer_adjust_bits(v, 0xffff);
+    return timer_adjust_bits(v, 0xffff, 0);
 }
 
 // Return the TSC value that is 'msecs' time in the future.
-- 
2.26.1