linux-kernel - Re: [PATCH v6 4/5] drivers: clocksource: add support for ARM architected timer event stream

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Message-ID: <20130829153914.GA10707@darko.cambridge.arm.com>
Date:	Thu, 29 Aug 2013 16:39:14 +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>,
	Lorenzo Pieralisi <Lorenzo.Pieralisi@....com>,
	Will Deacon <Will.Deacon@....com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Daniel Lezcano <daniel.lezcano@...aro.org>
Subject: Re: [PATCH v6 4/5] drivers: clocksource: add support for ARM
 architected timer event stream

On Thu, Aug 29, 2013 at 03:55:41PM +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.
> 
> An event stream might be used:
> - To impose a time-out on a wfe polling loop.
> - To safeguard against any programming error in case an expected event
>   is not generated.
> - To implement wfe-based timeouts for userspace locking implementations.
> 
> This patch computes the event stream frequency aiming for a period
> of 100us between events. It uses ARM/ARM64 specific backends to configure
> and enable the event stream.
> 
> Cc: Lorenzo Pieralisi <lorenzo.pieralisi@....com>
> Cc: Catalin Marinas <catalin.marinas@....com>
> Signed-off-by: Will Deacon <will.deacon@....com>
> [sudeep: moving ARM/ARM64 changes into separate patches
>          and adding Kconfig option]
> Signed-off-by: Sudeep KarkadaNagesha <sudeep.karkadanagesha@....com>
> ---
>  drivers/clocksource/Kconfig          | 14 ++++++++++++++
>  drivers/clocksource/arm_arch_timer.c | 15 +++++++++++++++
>  include/clocksource/arm_arch_timer.h |  2 ++
>  3 files changed, 31 insertions(+)
> 
> diff --git a/drivers/clocksource/Kconfig b/drivers/clocksource/Kconfig
> index b7b9b04..49f1f6e 100644
> --- a/drivers/clocksource/Kconfig
> +++ b/drivers/clocksource/Kconfig
> @@ -74,6 +74,20 @@ config ARM_ARCH_TIMER
>  	bool
>  	select CLKSRC_OF if OF
>  
> +config ARM_ARCH_TIMER_EVTSTREAM
> +	bool "Support for ARM architected timer event stream generation"
> +	default y if ARM_ARCH_TIMER
> +	help
> +	  This option enables support for event stream generation based on
> +	  the ARM architected timer. It is used for waking up CPUs executing
> +	  the wfe instruction at a frequency represented as a power-of-2
> +	  divisor of the clock rate. An event stream might be mainly useful
> +	  for wfe-based timeouts for userspace locking implementations.

I would be a bit more assertive here, something like "the main use of
the event stream is wfe-based timeouts of userspace locking
implementations."

> +	  It might also be useful for imposing timeout on a wfe or even
> +	  safeguarding against any programming errors in case an expected

Maybe "imposting timeout on wfe to safeguard against ...". We already
mentioned wfe-based timeout above.

Otherwise, for the whole series:

Reviewed-by: Catalin Marinas <catalin.marinas@....com>
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