diff options
Diffstat (limited to 'arch/m68k/mac/via.c')
| -rw-r--r-- | arch/m68k/mac/via.c | 16 |
1 files changed, 16 insertions, 0 deletions
diff --git a/arch/m68k/mac/via.c b/arch/m68k/mac/via.c index 01e6b0e37f8d..9cb813eda4fd 100644 --- a/arch/m68k/mac/via.c +++ b/arch/m68k/mac/via.c @@ -621,6 +621,22 @@ static u64 mac_read_clk(struct clocksource *cs) * These problems are avoided by ignoring the low byte. Clock accuracy * is 256 times worse (error can reach 0.327 ms) but CPU overhead is * reduced by avoiding slow VIA register accesses. + * + * The VIA timer counter observably decrements to 0xFFFF before the + * counter reload interrupt gets raised. That complicates things a bit. + * + * State | vT1CH | VIA_TIMER_1_INT | inference drawn + * ------+------------+-----------------+----------------------------- + * i | FE thru 00 | false | counter is decrementing + * ii | FF | false | counter wrapped + * iii | FF | true | wrapped, interrupt raised + * iv | FF | false | wrapped, interrupt handled + * v | FE thru 00 | true | wrapped, interrupt unhandled + * + * State iv is never observed because handling the interrupt involves + * a 6522 register access and every access consumes a "phi 2" clock + * cycle. So 0xFF implies either state ii or state iii, depending on + * the value of the VIA_TIMER_1_INT bit. */ local_irq_save(flags); |