linux-kernel - Re: [PATCH] clocksource: dw_apb_timer

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Date:	Fri, 30 Oct 2015 13:42:01 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	linux-arm-kernel@...ts.infradead.org
Cc:	Jisheng Zhang <jszhang@...vell.com>,
	Daniel Lezcano <daniel.lezcano@...aro.org>, tglx@...utronix.de,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] clocksource: dw_apb_timer_of: support timer-based delay

On Friday 30 October 2015 19:09:29 Jisheng Zhang wrote:
> > > diff --git a/drivers/clocksource/Kconfig b/drivers/clocksource/Kconfig
> > > index a7726db..7b081805 100644
> > > --- a/drivers/clocksource/Kconfig
> > > +++ b/drivers/clocksource/Kconfig
> > > @@ -29,6 +29,16 @@ config DW_APB_TIMER_OF
> > >     select DW_APB_TIMER
> > >     select CLKSRC_OF
> > >
> > > +config DW_APB_TIMER_BASED_DELAY
> > > +   bool "DW APB timer based delay"
> > > +   depends on ARM && DW_APB_TIMER_OF
> > > +   default n
> > > +   help
> > > +     This option enables support for using the DW APB timer to
> > > +     implement timer-based delay. It is useful for skiping the
> > > +     delay loop calibration at boot on some platforms. And the
> > > +     udelay() will be unaffected by CPU frequency changes.
> > > +  
> > 
> > Why do you want it to be optional ?
> > 
> 
> Because in some platforms which has arm arch timer, this dw apb timer
> delay isn't needed, the arch timer is better. So we want it be optional
> so that the platforms which need this feature select it manually when config
> the kernel.
> 

This is not ideal from an overall maintenance perspective. We want to
be able to have a kernel with all drivers enabled that gives us the
best behavior on all platforms.

The current behavior appears to be that we override all previous
registrations as long as the new one is higher resolution. Is that
the case here? I.e. does the arch timer have a lower resultion than
the dw-apb timer but have some other advantages?

	Arnd
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