linux-kernel - Re: [RFC PATCH 01/13] arm: Fix mutual exclusion in arch

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Date:   Wed, 14 Nov 2018 08:55:37 +1100 (AEDT)
From:   Finn Thain <fthain@...egraphics.com.au>
To:     Russell King - ARM Linux <linux@...linux.org.uk>
cc:     Christoph Hellwig <hch@...radead.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Arnd Bergmann <arnd@...db.de>,
        Stephen N Chivers <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-arm-kernel@...ts.infradead.org
Subject: Re: [RFC PATCH 01/13] arm: Fix mutual exclusion in
 arch_gettimeoffset

On Tue, 13 Nov 2018, Russell King - ARM Linux wrote:

> On Tue, Nov 13, 2018 at 02:39:00PM +1100, Finn Thain wrote:
> > 
> > You could remove the old arch_gettimeoffset API without dropping any 
> > platforms.
> > 
> > If no-one converts a given platform to the clocksource API it would mean 
> > that the default 'jiffies' clocksource will get used on that platform.
> > 
> > Clock resolution and timer precision would be degraded, but that might not 
> > matter.
> > 
> > Anyway, if someone who has this hardware is willing to test a clocksource 
> > API conversion, they can let me know and I'll attempt that patch.
> 
> There's reasons why that's not appropriate - such as not having two
> separate timers in order to supply a clocksource and separate clock
> event.
> 
> Not all hardware is suited to the clocksource + clockevent idea.
> 

Sorry, I don't follow.

AFAIK, clocksources and clock event devices are orthogonal concepts. There 
are platforms with !ARCH_USES_GETTIMEOFFSET && !GENERIC_CLOCKEVENTS (and 
every other combination).

A clocksource read method just provides a cycle count, and in this sense 
arch_gettimeoffset() is equivalent to a clocksource.

If these two arm platforms have an existing clock event device which 
somehow precludes any new clocksources, why doesn't that also render 
arch_gettimeoffset() impossible?

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