On Sat, Jun 13, 2020 at 02:19:12PM +0300, Roman Bolshakov wrote:
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).
Provide counter direction to timer_adjust_bits() and normalize the counter to advance ticks in monotonically increasing TimerLast.
Good catch. Could we fix it using the patch below instead though?
--- a/src/hw/timer.c +++ b/src/hw/timer.c @@ -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.