linux-kernel - Re: [PATCH] clocksource/drivers/tango-xtal: Replace code by clocksource_mmio

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Date:	Fri, 13 Nov 2015 10:26:42 -0500 (EST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Marc Gonzalez <marc_gonzalez@...madesigns.com>
cc:	Daniel Lezcano <daniel.lezcano@...aro.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux ARM <linux-arm-kernel@...ts.infradead.org>,
	Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH] clocksource/drivers/tango-xtal: Replace code by
 clocksource_mmio_init

On Fri, 13 Nov 2015, Marc Gonzalez wrote:
> On 13/11/2015 15:16, Daniel Lezcano wrote:
> > For example:
> > 
> > struct clockcommon {
> > 	u32 mult;
> > 	u32 shift;
> > 	int rating;
> > 	void __iomem *base;
> > 	char *name;
> > 	int irq;
> > };
> > 
> > struct clocksource {
> > 	struct clockcommon common; /* MUST be the first field */
> > 	cycle_t (*read)(struct clocksource *cs);
> > 	cycle_t mask;
> > 	...
> > };
> 
> According to my notes, commit 369db4c952 grouped hot-path data
> into a single cache line (hence ____cacheline_aligned).
> 
> (AFAIR, ARMv7 ARCH_MULTIPLATFORM assumes CACHE_LINE=64)
> 
> Not sure how to make the two concepts (common base struct and
> grouping hot data) play nicely, without wasting a lot of space
> on padding.

It wont play well. We are not going to change the layout of struct
clocksource because it will hurt the sane use cases for no reason.

Thanks,

	tglx
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