linux-kernel - Re: [PATCH v2] m68k/mac: Improve clocksource driver commentary

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Message-ID: <CAMuHMdXmxV03tza_4M3+YJg9TT6yJKuvmNh9=JvdM7r-r-kR7A@mail.gmail.com>
Date: Sun, 6 Jul 2025 11:08:11 +0200
From: Geert Uytterhoeven <geert@...ux-m68k.org>
To: Finn Thain <fthain@...ux-m68k.org>
Cc: Joshua Thompson <funaho@...ai.org>, linux-m68k@...ts.linux-m68k.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] m68k/mac: Improve clocksource driver commentary

On Tue, 24 Jun 2025 at 06:36, Finn Thain <fthain@...ux-m68k.org> wrote:
> 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 <funaho@...ai.org>
> Signed-off-by: Finn Thain <fthain@...ux-m68k.org>
> ---
> Changed since v1:
>  - The letters A through E could be mistaken for hex digits so number the
>    states using roman numerals instead.

LGTM, so
Reviewed-by: Geert Uytterhoeven <geert@...ux-m68k.org>
i.e. will queue in the m68k tree for v6.17.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds