linux-kernel - Re: [PATCH v3 02/10] clocksource:arm_global

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Date:	Fri, 21 Jun 2013 08:15:28 +0100
From:	Srinivas KANDAGATLA <srinivas.kandagatla@...com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	linux-arm-kernel@...ts.infradead.org,
	Rob Landley <rob@...dley.net>,
	Grant Likely <grant.likely@...aro.org>,
	Rob Herring <rob.herring@...xeda.com>,
	Russell King <linux@....linux.org.uk>,
	Stuart Menefy <stuart.menefy@...com>,
	John Stultz <john.stultz@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linus Walleij <linus.walleij@...aro.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jslaby@...e.cz>, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org, devicetree-discuss@...ts.ozlabs.org,
	kernel@...inux.com, linux-serial@...r.kernel.org,
	Mark Brown <broonie@...nel.org>,
	Stephen Gallimore <stephen.gallimore@...com>,
	Rob Herring <robherring2@...il.com>,
	Will Deacon <will.deacon@....com>
Subject: Re: [PATCH v3 02/10] clocksource:arm_global_timer: Add ARM global
 timer support.

On 20/06/13 20:02, Arnd Bergmann wrote:
> On Thursday 20 June 2013, Srinivas KANDAGATLA wrote:
>> +static u64 gt_counter_read(void)
>> +{
>> +       u64 counter;
>> +       u32 lower;
>> +       u32 upper, old_upper;
>> +
>> +       upper = __raw_readl(gt_base + GT_COUNTER1);
>> +       do {
>> +               old_upper = upper;
>> +               lower = __raw_readl(gt_base + GT_COUNTER0);
>> +               upper = __raw_readl(gt_base + GT_COUNTER1);
>> +       } while (upper != old_upper);
>> +
>> +       counter = upper;
>> +       counter <<= 32;
>> +       counter |= lower;
>> +       return counter;
>> +}
> 
> Please replace __raw_readl etc with the non-__raw variants
> throughout the code.
> 
> The __raw accessors are not endian safe and are not meant
> for device drivers. If you are worried about latency from
> extra barriers, use readl_relaxed().
Endian safe is good point. I will move to using readl/writel which does
both ordering and byteswapping.
And use relaxed versions in gt_counter_read to reduce the latency.

looks like smp_twd.c too needs this type of change.

Thanks,
srini

> 
> 	Arnd
> 

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