linux-kernel - Re: [PATCH v8 4/9] clocksource/drivers/arm_arch

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Date:	Mon, 25 Jul 2016 23:50:19 +0800
From:	Fu Wei <fu.wei@...aro.org>
To:	Will Deacon <will.deacon@....com>
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>,
	Timur Tabi <timur@...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 v8 4/9] clocksource/drivers/arm_arch_timer: use readq to
 get 64-bit CNTVCT

Hi Will,

On 25 July 2016 at 17:02, Will Deacon <will.deacon@....com> wrote:
> On Wed, Jul 20, 2016 at 02:17:59AM +0800, fu.wei@...aro.org wrote:
>> From: Fu Wei <fu.wei@...aro.org>
>>
>> This patch simplify arch_counter_get_cntvct_mem function by
>> using readq to get 64-bit CNTVCT value instead of readl_relaxed.
>>
>> Signed-off-by: Fu Wei <fu.wei@...aro.org>
>> ---
>>  drivers/clocksource/arm_arch_timer.c | 10 +---------
>>  1 file changed, 1 insertion(+), 9 deletions(-)
>>
>> diff --git a/drivers/clocksource/arm_arch_timer.c b/drivers/clocksource/arm_arch_timer.c
>> index e6fd42d..483d2f9 100644
>> --- a/drivers/clocksource/arm_arch_timer.c
>> +++ b/drivers/clocksource/arm_arch_timer.c
>> @@ -418,15 +418,7 @@ u32 arch_timer_get_rate(void)
>>
>>  static u64 arch_counter_get_cntvct_mem(void)
>>  {
>> -     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;
>> +     return readq(arch_counter_base + CNTVCT_LO);
>

Sorry, right after posting v9, I got your comment,

> What's the benefit of doing this? If you use readq here, how can we

benefit:
1. simplify the code
2. from arch/arm64/include/asm/io.h, I guess readq is more efficient

> guarantee that (a) the endpoint won't generate a SLVERR or similar and
> (b) that we get ?

I think so, according to arch/arm64/include/asm/io.h.
readq Implement by "LDR" and "LDAR", So I think It is an atomic read.

Please correct me, If I misunderstand something, thanks

>
> "If it ain't broke, don't fix it"
>
> Will



-- 
Best regards,

Fu Wei
Software Engineer
Red Hat