linux-kernel - Re: [RFC PATCH v2 07/14] m68k: atari: Convert to clocksource API

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Date:   Tue, 20 Nov 2018 20:30:09 +1100 (AEDT)
From:   Finn Thain <fthain@...egraphics.com.au>
To:     Linus Walleij <linus.walleij@...aro.org>
cc:     Geert Uytterhoeven <geert@...ux-m68k.org>, schwab@...ux-m68k.org,
        Arnd Bergmann <arnd@...db.de>, schivers@....com.au,
        Thomas Gleixner <tglx@...utronix.de>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        schmitzmic@...il.com, John Stultz <john.stultz@...aro.org>,
        linux-m68k@...ts.linux-m68k.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH v2 07/14] m68k: atari: Convert to clocksource API

On Tue, 20 Nov 2018, Linus Walleij wrote:

> 
> Yes you already see the same as I see: this chip MK68901 has no less 
> than four timers. I bet the kernel is just using one of them, out of 
> habit.
> 
> By just setting another timer as free-running we get a classic and clean 
> Linux clocksource for the Atari.
> 

These are all 8-bit timers. Whereas the smallest clocksource mask I can 
find with grep is 24-bits.

You can divide the oscillator down to 12288 Hz giving a maximum period of 
20 ms. My concern would be that clocksource counter wrap could still go 
undetected given a little interrupt latency.

> This is however a very good start in untangling the mess (as is the 
> whole patch series).
> 

It should be exciting to see what happens when some of these changes get 
tested 8-) I've only seen results for Mac and Atari so far.

> As with the Amiga, this chip also has an RTC clock that should go to the 
> RTC subsystem, naturally.
> 

I think some Atari's have an MC146818, which is drivers/rtc/rtc-cmos.c,
arch/alpha/kernel/rtc.c etc.

-- 

> Yours,
> Linus Walleij
>