linux-kernel - Re: [PATCH 3/4] ARM: sched_clock: Add support for >32 bit sched

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Message-ID: <20130422104858.GB2572@mudshark.cambridge.arm.com>
Date:	Mon, 22 Apr 2013 11:48:58 +0100
From:	Will Deacon <will.deacon@....com>
To:	Stephen Boyd <sboyd@...eaurora.org>
Cc:	Rob Herring <robherring2@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-msm@...r.kernel.org" <linux-arm-msm@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Russell King <linux@....linux.org.uk>,
	"arm@...nel.org" <arm@...nel.org>,
	Catalin Marinas <Catalin.Marinas@....com>,
	John Stultz <john.stultz@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 3/4] ARM: sched_clock: Add support for >32 bit sched_clock

Hi Stephen,

On Sat, Apr 20, 2013 at 01:29:05AM +0100, Stephen Boyd wrote:
> The arm architected system counter has at least 56 bits of
> useable bits. Add support to ARM's sched_clock implementation for
> counters with more than 32 bits so we can avoid the complexity of
> dealing with wraparound on these devices while benefiting from
> the irqtime accounting and suspend/resume handling that the ARM
> sched_clock code already has.
> 
> Signed-off-by: Stephen Boyd <sboyd@...eaurora.org>
> ---
> 
> Maybe we need a union for the epoch_ns usage?
> 
>  arch/arm/include/asm/sched_clock.h |   2 +
>  arch/arm/kernel/sched_clock.c      | 101 +++++++++++++++++++++++++++----------
>  2 files changed, 77 insertions(+), 26 deletions(-)
> 
> diff --git a/arch/arm/include/asm/sched_clock.h b/arch/arm/include/asm/sched_clock.h
> index 3d520dd..7fcd2ee 100644
> --- a/arch/arm/include/asm/sched_clock.h
> +++ b/arch/arm/include/asm/sched_clock.h
> @@ -13,4 +13,6 @@ extern void setup_sched_clock(u32 (*read)(void), int bits, unsigned long rate);
>  
>  extern unsigned long long (*sched_clock_func)(void);
>  
> +extern void setup_sched_clock_64(u64 (*read)(void), int bits,
> +				 unsigned long rate);

Curious, but why do we need two setup_sched_clock functions, where the bits
are passed as a parameter? Can't we just do the right thing if the clock
claims to be more than 32 bits wide (and get rid of the BUG_ONs at the same
time)?

Will
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