On Fri, Jun 26, 2020 at 09:06:58PM +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).
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.
Thanks. I committed this change.