Re: [PATCH] timer: Handle decrements of PIT counter

[Kevin O'Connor]

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 [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.

Good catch.  Could we fix it using the patch below instead though?

-Kevin


--- 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.