linux-kernel - Re: [PATCH v9 4/9] clocksource/drivers/arm_arch

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Date:	Wed, 27 Jul 2016 11:33:42 +0800
From:	Jisheng Zhang <jszhang@...vell.com>
To:	Timur Tabi <timur@...eaurora.org>
CC:	Will Deacon <will.deacon@....com>, Fu Wei <fu.wei@...aro.org>,
	"Linaro ACPI Mailman List" <linaro-acpi@...ts.linaro.org>,
	Catalin Marinas <catalin.marinas@....com>,
	<rruigrok@...eaurora.org>, Wim Van Sebroeck <wim@...ana.be>,
	<wei@...hat.com>, Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
	Al Stone <al.stone@...aro.org>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
	Guenter Roeck <linux@...ck-us.net>,
	Len Brown <lenb@...nel.org>, <harba@...eaurora.org>,
	<linux-watchdog@...r.kernel.org>, Arnd Bergmann <arnd@...db.de>,
	"Marc Zyngier" <marc.zyngier@....com>,
	Jon Masters <jcm@...hat.com>,
	"Christopher Covington" <cov@...eaurora.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	<linux-arm-kernel@...ts.infradead.org>,
	G Gregory <graeme.gregory@...aro.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Leo Duran <leo.duran@....com>,
	Hanjun Guo <hanjun.guo@...aro.org>,
	"Suravee Suthikulpanit" <Suravee.Suthikulpanit@....com>,
	Sudeep Holla <sudeep.holla@....com>
Subject: Re: [PATCH v9 4/9] clocksource/drivers/arm_arch_timer: use readq to
 get 64-bit CNTVCT

+1

On Tue, 26 Jul 2016 09:11:49 -0500 Timur Tabi  wrote:

> Will Deacon wrote:
> > The kernel really needs to support both of those platforms :/
> >
> > For the memory-mapped counter registers, the architecture says:
> >
> >    `If the implementation supports 64-bit atomic accesses, then the
> >     CNTV_CVAL register must be accessible as an atomic 64-bit value.'
> >
> > which is borderline tautological. If we take the generous reading that
> > this means AArch64 CPUs can use readq (and I'm not completely
> > comfortable with that assertion, particularly as you say that it breaks
> > the model), then you still need to use readq_relaxed here to avoid a
> > DSB. Furthermore, what are you going to do for AArch32? readq doesn't
> > exist over there, and if you use the generic implementation then it's
> > not atomic. In which case, we end up with the current code, as well as a
> > readq_relaxed guarded by a questionable #ifdef that is known to break a
> > supported platform for an unknown performance improvement. Hardly a big
> > win.  
> 
> I know Fu dropped this patch, and I don't want to kick a dead horse, but 
> I was wondering if it would be okay to do this:
> 
> static u64 arch_counter_get_cntvct_mem(void)
> {
> #ifdef readq_relaxed
> 	return readq_relaxed(arch_counter_base + CNTVCT_LO);
> #else
> 	u32 vct_lo, vct_hi, tmp_hi;
> 
> 	do {
> 		vct_hi = readl_relaxed(arch_counter_base + CNTVCT_HI);
> 		vct_lo = readl_relaxed(arch_counter_base + CNTVCT_LO);
> 		tmp_hi = readl_relaxed(arch_counter_base + CNTVCT_HI);
> 	} while (vct_hi != tmp_hi);
> 
> 	return ((u64) vct_hi << 32) | vct_lo;
> #endif
> }
> 
> readq and readq_relaxed are defined in arch/arm64/include/asm/io.h.  Why 
> would the function exist if AArch64 CPUs can't use it?

+1

I measured the performance on berlin arm64 platforms:

compared with original version, using readq_relaxed could reduce
time of arch_counter_get_cntvct_mem() by about 42%!

Thanks,
Jisheng