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

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Date:   Sat, 24 Nov 2018 15:03:54 +1300
From:   Michael Schmitz <schmitzmic@...il.com>
To:     Andreas Schwab <schwab@...ux-m68k.org>,
        Linus Walleij <linus.walleij@...aro.org>
Cc:     fthain@...egraphics.com.au,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Arnd Bergmann <arnd@...db.de>, schivers@....com.au,
        Thomas Gleixner <tglx@...utronix.de>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        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

Am 20.11.2018 um 23:02 schrieb Andreas Schwab:
> On Nov 20 2018, Linus Walleij <linus.walleij@...aro.org> 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.
>
> Note that not all timers can be used freely.  Some of them are hardwired
> to generate the clock for the serial interfaces.

Timer A is used by the DMA sound driver - no workaround possible there.

Timer B is used by the framebuffer driver, but it's used only once to 
reprogram the screen base address at driver init. This one could 
potentially be used after framebuffer init to improve the clocksource 
accuracy.

Timer D is already used to generate timer interrupts used to poll the 
ROM port network card / USB adapters. This timer is initialized early in 
the boot process, which prevents using the MFP UART as serial console 
(something that I hadn't properly considered before). I'll send a patch 
for that. I'll also consider using timer B or timer C interrupts instead 
to poll ROM port hardware.

There are no serial drivers anymore that could use the MFP UART 
hardware, so that point is somewhat moot at present.

Cheers,

	Michael

> Andreas.
>