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 .
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).
It can be fixed by making the counter recieved from PIT an increasing
value so it can be passed to timer_adjust_bits():
0, 1, 2 and up to 65535 and then the counter is re-loaded with 0.
Fixes: eac11944019 ("Unify pmtimer_read() and pittimer_read() code.")
Reported-by: Philippe Mathieu-Daudé <philmd(a)redhat.com>
Signed-off-by: Roman Bolshakov <r.bolshakov(a)yadro.com>
Changes since v1:
- Simplified change of counter direction (Kevin)
src/hw/timer.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/src/hw/timer.c b/src/hw/timer.c
index 56bb289..b6f102e 100644
@@ -180,7 +180,7 @@ timer_read(void)
// 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);
// Return the TSC value that is 'msecs' time in the future.