From c8995932db2bad6fa093ac64dbaf7a3e8870eafa Mon Sep 17 00:00:00 2001 From: Finn Thain Date: Tue, 24 Jun 2025 14:32:48 +1000 Subject: [PATCH] m68k: mac: Improve clocksource driver commentary qemu-system-m68k -M q800 has an old bug that causes the kernel to occasionally complain about a soft lockup: watchdog: BUG: soft lockup - CPU#0 stuck for 5107s! There isn't any actual lockup. The via1 clocksource produced a large jump in jiffies, causing the watchdog to detect a stale timestamp. The 32-bit clocksource counter runs at 783360 Hz and its period is about 5482 seconds. Applying the "nanosecond" approximation used in get_timestamp() in kernel/watchdog.c then yields the duration reported in the log message above (always 5107 or 5108 in my tests): 0xffffffff / VIA_CLOCK_FREQ * 10**9 / 2**30 = 5106.209 seconds It is notoriously difficult to correctly emulate a MOS6522 VIA chip. So it seems wise to document the VIA clocksource driver better, especially those hardware behaviours which the kernel relies upon. Cc: Joshua Thompson Signed-off-by: Finn Thain Reviewed-by: Geert Uytterhoeven Link: https://lore.kernel.org/f7b4c02a1c8ed74ccceb5535d7e1e202deada8ce.1750739568.git.fthain@linux-m68k.org Signed-off-by: Geert Uytterhoeven --- arch/m68k/mac/via.c | 16 ++++++++++++++++ 1 file changed, 16 insertions(+) diff --git a/arch/m68k/mac/via.c b/arch/m68k/mac/via.c index 01e6b0e37f8dd..9cb813eda4fd0 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); -- 2.47.2