linux-kernel - Re: [STLinux Kernel] [PATCH v3 7/8] rtc: st: add new driver for ST's LPC RTC

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Message-ID: <20150225142806.GA2160@griffinp-ThinkPad-X1-Carbon-2nd>
Date:	Wed, 25 Feb 2015 14:28:06 +0000
From:	Peter Griffin <peter.griffin@...aro.org>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Lee Jones <lee.jones@...aro.org>, devicetree@...r.kernel.org,
	kernel@...inux.com, rtc-linux@...glegroups.com,
	linux-kernel@...r.kernel.org, wim@...ana.be, linux@...ck-us.net,
	linux-arm-kernel@...ts.infradead.org,
	linux-watchdog@...r.kernel.org
Subject: Re: [STLinux Kernel] [PATCH v3 7/8] rtc: st: add new driver for ST's
 LPC RTC

Hi Russell,

On Wed, 25 Feb 2015, Russell King - ARM Linux wrote:

> On Mon, Feb 23, 2015 at 10:12:23AM +0000, Peter Griffin wrote:
> > I think all the writel IO accessors in this driver can be replaced
> > with the *_relaxed variant. This will avoid the overhead of taking a
> > spinlock in the l2 outer cache part of writel.
> 
> You're really operating on old information.  With much improved L2 cache
> support code which went in a year or so ago, for the popular L2 cache
> controllers, we don't take a spinlock anymore - we just write to the
> register directly.
> 
> The spinlock is only present for L2C-220 controllers, which are rare.
> 
> Moreover, taking the spinlock is only expensive if you have things like
> lockdep enabled, otherwise it should be inline and will be fast.
> 
> However, using the _relaxed variants where we can get away with weaker
> ordering of the writes to the device is a good thing nevertheless.  I'm
> just pointing out that your reasoning above is wrong.

Thanks for pointing this out, I wasn't aware until now that the spinlock
had been removed.

regards

Peter.

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