linux-kernel - Re: [RFC PATCH v2 09/14] m68k: hp300: Remove hp300

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Date:   Tue, 20 Nov 2018 10:09:13 +0100
From:   Kars de Jong <jongk@...ux-m68k.org>
To:     fthain@...egraphics.com.au
Cc:     geert@...ux-m68k.org, philb@....org, schwab@...ux-m68k.org,
        arnd@...db.de, schivers@....com.au, tglx@...utronix.de,
        daniel.lezcano@...aro.org, schmitzmic@...il.com,
        john.stultz@...aro.org, linus.walleij@...aro.org,
        linux-m68k@...ts.linux-m68k.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v2 09/14] m68k: hp300: Remove hp300_gettimeoffset()

Op ma 19 nov. 2018 om 02:10 schreef Finn Thain <fthain@...egraphics.com.au>:
>
> hp300_gettimeoffset() never checks the timer interrupt flag and will
> fail to notice when the timer counter gets reloaded. That means the
> clock could jump backwards.
>
> Remove this code and leave this platform on the 'jiffies' clocksource.
> Note that this amounts to a regression in clock precision. However,
> adopting the 'jiffies' clocksource does resolve the monotonicity issue.
>
> Signed-off-by: Finn Thain <fthain@...egraphics.com.au>
> ---
> hp300_gettimeoffset() cannot be used in a clocksource conversion
> unless it can be made monotonic. I can't fix this without knowing the
> details of the timer implementation, such as the relationship between
> the timer count and the interrupt flag.

I don't really like this regression...

According to NetBSD sources, there are 3 timers in the chip
(originally an MC6840 PTM). Timer 1 is used as the system timer, timer
3 runs at the same rate and is unused on Linux (on NetBSD it is used
as the statistics/profiling timer), and timer 3 is connected to timer
2 so you can make a 32-bit timer out of the two timers together (also
unused on Linux).

Timers 1 counts down at 25 MHz. The interrupt flag is set when the
counter reaches 0 after which it is automatically reloaded and starts
counting down again.

> ---
>  arch/m68k/hp300/time.c | 19 -------------------
>  1 file changed, 19 deletions(-)