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

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Message-ID: <alpine.LNX.2.21.1811241357040.317@nippy.intranet>
Date:   Sat, 24 Nov 2018 14:08:49 +1100 (AEDT)
From:   Finn Thain <fthain@...egraphics.com.au>
To:     Michael Schmitz <schmitzmic@...il.com>
cc:     Andreas Schwab <schwab@...ux-m68k.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        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

On Sat, 24 Nov 2018, Michael Schmitz wrote:

> 
> 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.
> 

I don't think it would make a good clocksource as MFP timers are all 8 
bits wide. It could be used as a clock event device in the course of a 
GENERIC_CLOCKEVENTS conversion (discussed elsewhere in this thread), 
though the longest timed interrupt inverval would be only 21 ms.

-- 

> 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.
> > 
>