linux-kernel - Re: [PATCHv3/RESEND 4/4] clocksource: arch_timer: Add support for memory mapped timers

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Date:	Sat, 22 Jun 2013 01:13:46 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Stephen Boyd <sboyd@...eaurora.org>
cc:	Daniel Lezcano <daniel.lezcano@...aro.org>,
	linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	John Stultz <john.stultz@...aro.org>,
	Mark Rutland <mark.rutland@....com>,
	Marc Zyngier <Marc.Zyngier@....com>,
	Rob Herring <robherring2@...il.com>
Subject: Re: [PATCHv3/RESEND 4/4] clocksource: arch_timer: Add support for
 memory mapped timers

On Fri, 21 Jun 2013, Stephen Boyd wrote:
> On 06/21/13 14:18, Thomas Gleixner wrote:
> > Something in my little brain yells: function pointer
> >
> > You can't be serious about hacking nested if/else/switch constructs
> > into a hot path.
> >
> > Why not making your cpu data:
> >
> > struct arch_timer {
> >        struct clock_event_device evt;
> >        ....
> >        void (*write_ctrl)(val, timer);
> >        void (*write_tval)(val, timer);
> >        ....
> > }
> >
> > and get rid of all that conditionals?
> 
> It sounds like that's undesirable according to the comment above
> arch_timer_reg_write(). It seems that all this code was written under
> the assumption that the compiler is good enough to optimize all the code
> paths and only generate the code that is necessary. So far this seems to
> be working and the hotpath is optimized for each type of access.

That might be true for kernels which are optimized for a specific
target, but this will fall flat for any multi-platform kernel.

Thanks,
	
	tglx
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