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

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Date:	Tue, 26 Jul 2016 09:11:49 -0500
From:	Timur Tabi <timur@...eaurora.org>
To:	Will Deacon <will.deacon@....com>, Fu Wei <fu.wei@...aro.org>
Cc:	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Len Brown <lenb@...nel.org>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Marc Zyngier <marc.zyngier@....com>,
	Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
	Sudeep Holla <sudeep.holla@....com>,
	Hanjun Guo <hanjun.guo@...aro.org>,
	linux-arm-kernel@...ts.infradead.org,
	Linaro ACPI Mailman List <linaro-acpi@...ts.linaro.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
	rruigrok@...eaurora.org, harba@...eaurora.org,
	Christopher Covington <cov@...eaurora.org>,
	G Gregory <graeme.gregory@...aro.org>,
	Al Stone <al.stone@...aro.org>, Jon Masters <jcm@...hat.com>,
	wei@...hat.com, Arnd Bergmann <arnd@...db.de>,
	Wim Van Sebroeck <wim@...ana.be>,
	Catalin Marinas <catalin.marinas@....com>,
	Suravee Suthikulpanit <Suravee.Suthikulpanit@....com>,
	Leo Duran <leo.duran@....com>,
	Guenter Roeck <linux@...ck-us.net>,
	linux-watchdog@...r.kernel.org
Subject: Re: [PATCH v9 4/9] clocksource/drivers/arm_arch_timer: use readq to
 get 64-bit CNTVCT

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?

Do we need something like ARCH_HAS_64BIT_ATOMIC_READ in order to decide 
whether readq is safe?

-- 
Sent by an employee of the Qualcomm Innovation Center, Inc.
The Qualcomm Innovation Center, Inc. is a member of the
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