linux-kernel - Re: [PATCH v2 1/3] drivers: clocksource: configure event stream for ARM arch timer

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Message-ID: <20130723102334.GC3748@MacBook-Pro.local>
Date:	Tue, 23 Jul 2013 11:23:34 +0100
From:	Catalin Marinas <catalin.marinas@....com>
To:	Sudeep KarkadaNagesha <Sudeep.KarkadaNagesha@....com>
Cc:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Mark Rutland <Mark.Rutland@....com>,
	Lorenzo Pieralisi <Lorenzo.Pieralisi@....com>,
	Russell King <linux@....linux.org.uk>,
	Will Deacon <Will.Deacon@....com>,
	Mathieu Poirier <mathieu.poirier@...aro.org>
Subject: Re: [PATCH v2 1/3] drivers: clocksource: configure event stream for
 ARM arch timer

On Mon, Jul 22, 2013 at 12:21:20PM +0100, Sudeep KarkadaNagesha wrote:
> From: Will Deacon <will.deacon@....com>
> 
> The ARM architected timer can generate events (used for waking up
> CPUs executing the wfe instruction) at a frequency represented as a
> power-of-2 divisor of the clock rate.
> 
> This patch configures the event stream, aiming for a period of 100us
> between events. This can be used to implement wfe-based timeouts for
> userspace locking implementations.
...
> --- a/include/clocksource/arm_arch_timer.h
> +++ b/include/clocksource/arm_arch_timer.h
> @@ -29,6 +29,8 @@
>  #define ARCH_TIMER_PHYS_ACCESS		0
>  #define ARCH_TIMER_VIRT_ACCESS		1
>  
> +#define ARCH_TIMER_EVT_STREAM_FREQ	10000	/* 100us */

BTW, if user-space starts using this, it will become an ABI. Is this the
right frequency?

In addition, do we want to expose this via hwcap? Something like
HWCAP_EVSTR100US?

-- 
Catalin
--
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