linux-kernel - Re: [PATCH] ARM: convert arm/arm64 arch timer to use CLKSRC

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Date:	Mon, 25 Mar 2013 17:26:33 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Mark Rutland <mark.rutland@....com>
Cc:	Rob Herring <robherring2@...il.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Kukjin Kim <kgene.kim@...sung.com>,
	Arnd Bergmann <arnd@...db.de>,
	"linux-sh@...r.kernel.org" <linux-sh@...r.kernel.org>,
	Tony Lindgren <tony@...mide.com>,
	Catalin Marinas <Catalin.Marinas@....com>,
	Magnus Damm <magnus.damm@...il.com>,
	"rob.herring@...xeda.com" <rob.herring@...xeda.com>,
	"linux-samsung-soc@...r.kernel.org" 
	<linux-samsung-soc@...r.kernel.org>,
	Simon Horman <horms@...ge.net.au>,
	John Stultz <john.stultz@...aro.org>,
	Will Deacon <Will.Deacon@....com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"linux-omap@...r.kernel.org" <linux-omap@...r.kernel.org>
Subject: Re: [PATCH] ARM: convert arm/arm64 arch timer to use CLKSRC_OF init

On Thu, Mar 21, 2013 at 11:06:47AM +0000, Mark Rutland wrote:
> On TC2 this series leads to using the vexpress 24MHz clock as the sched clock
> in preference to the architected timer:
> 
>   Architected local timer running at 24.00MHz (virt).
>   Switching to timer-based delay loop
>   Registered arch_counter_get_cntvct+0x0/0x14 as sched_clock source
>   sched_clock: 32 bits at 24MHz, resolution 41ns, wraps every 178956ms
>   Registered versatile_read_sched_clock+0x0/0x28 as sched_clock source
> 
> As they both have the same frequency, neither overrides the other, and
> whichever gets registered last is used as the sched_clock. As accesses
> to the architected timer are going to have a much lower overhead, this
> isn't very nice (and it could be better to use it even if it had a lower
> frequency).

I'll remind people that sched_clock() is supposed to be functional at
the point in the boot sequence where the call to sched_init() is called.
That is after setup_arch() and *before* time_init() is called.
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