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: <20150225111910.GX8656@n2100.arm.linux.org.uk>
Date:	Wed, 25 Feb 2015 11:19:11 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Peter Griffin <peter.griffin@...aro.org>
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

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.

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