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From:   Geert Uytterhoeven <geert@linux-m68k.org>
Date:   Wed, 21 Nov 2018 09:55:24 +0100
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Subject: Re: [RFC PATCH v2 09/14] m68k: hp300: Remove hp300_gettimeoffset()
To:     Finn Thain <fthain@telegraphics.com.au>
Cc:     Kars de Jong <jongk@linux-m68k.org>,
        Philip Blundell <philb@gnu.org>,
        Andreas Schwab <schwab@linux-m68k.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Stephen N Chivers <schivers@csc.com.au>,
        Thomas Gleixner <tglx@linutronix.de>,
        Daniel Lezcano <daniel.lezcano@linaro.org>,
        Michael Schmitz <schmitzmic@gmail.com>,
        John Stultz <john.stultz@linaro.org>,
        Linus Walleij <linus.walleij@linaro.org>,
        linux-m68k <linux-m68k@lists.linux-m68k.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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Hi Finn,

On Wed, Nov 21, 2018 at 9:41 AM Finn Thain <fthain@telegraphics.com.au> wrote:
> On Wed, 21 Nov 2018, Geert Uytterhoeven wrote:
> > On Wed, Nov 21, 2018 at 12:13 AM Finn Thain <fthain@telegraphics.com.au> wrote:
> > > On atari, the 68901 counts down to 0x01 and raises an interrupt. On
> > > mac, the 6522 counts down to 0xFFFF then raises an interrupt. No idea
> > > about amiga (Geert?) -- this has to be handled correctly to get a
> > > monotonic clocksource. I'll fix this in v3 (where the information is
> > > available).
> >
> > The docs state that the CIA generates on interrupt on underflow, so I
> > guess that's the same behavior as the 6522 VIA.
>
> Difficult to say. The sequence varies from one implementation to another.
> Let's ignore the MSB and LSB and pretend it's one register:
>
> MC68901: N, N-1, N-2, ..., 2, 1, N, N-1, N-2, ...
> MC6840:  N, N-1, N-2, ..., 2, 1, 0, N, N-1, N-2, ...
> SY6522:  N, N-1, N-2, ..., 2, 1, 0, 0xFFFF, N, N-1, N-2, ...
>
> Now the question is, when the timer asserts its interrupt, and the count
> register is fetched immediately, what value does it have?
>
> For the MC68901, you get 1. For the SY6522, you get 0xFFFF. For MC6840, as
> far as I can tell, you'd get 0.

I assume 0xFFFF. The 8520 CIA is almost identical to the 6526 CIA, as used in
the C64, which is a direct descendant of the 6522 VIA.

> > Unfortunately the 24-bit ("TOD") counters in the two CIAs run from HSYNC
> > resp. VSYNC, which depends on the video mode, and thus can't be used as
> > a monotonic clock source.
>
> Is that because of video mode changes? Could the clocksource be
> unregistered before the mode change and then re-registered at a different
> frequency afterwards?

On older Amigas (most plain 68000, unless expanded), video modes are fixed.
Some have a jumper to select the power supply tick instead of the VSYNC
signal, which is more accurate, but still runs at a low 50 or 60 Hz.
On anything more modern (A3000 and up), video modes are programmable.
In addition, blanking the screen according to VESA will disable sync signals,
thus stopping the clock, I assume.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-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