linux-kernel - Re: [PATCH v1 02/20] clocksource: Add NPS400 timers driver

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Date:	Mon, 2 Nov 2015 16:33:52 +0530
From:	Vineet Gupta <Vineet.Gupta1@...opsys.com>
To:	Noam Camus <noamc@...hip.com>, <linux-snps-arc@...ts.infradead.org>
CC:	<linux-kernel@...r.kernel.org>, <talz@...hip.com>,
	<gilf@...hip.com>, <cmetcalf@...hip.com>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	"Rob Herring" <robh+dt@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"John Stultz" <john.stultz@...aro.org>
Subject: Re: [PATCH v1 02/20] clocksource: Add NPS400 timers driver

On Saturday 31 October 2015 06:45 PM, Noam Camus wrote:
> From: Noam Camus <noamc@...hip.com>
> 
> Add internal tick generator which is shared by all cores.
> Each cluster of cores view it through dedicated address.
> This is used for SMP system where all CPUs synced by same
> clock source.
> 
> Signed-off-by: Noam Camus <noamc@...hip.com>
> Cc: Daniel Lezcano <daniel.lezcano@...aro.org>
> Cc: Rob Herring <robh+dt@...nel.org>
> ---

[snip]

> +/*
> + * To get the value from the Global Timer Counter register proceed as follows:
> + * 1. Read the upper 32-bit timer counter register
> + * 2. Read the lower 32-bit timer counter register
> + * 3. Read the upper 32-bit timer counter register again. If the value is
> + *  different to the 32-bit upper value read previously, go back to step 2.
> + *  Otherwise the 64-bit timer counter value is correct.
> + */
> +static cycle_t nps_clksrc_read(struct clocksource *clksrc)
> +{
> +	u64 counter;
> +	u32 lower;
> +	u32 upper, old_upper;
> +	int cpu;
> +	int cluster;
> +	void *lower_p, *upper_p;
> +	unsigned long flags;
> +
> +	local_irq_save(flags);
> +	cpu = smp_processor_id();
> +	cluster = cpu >> NPS_CLUSTER_OFFSET;
> +	lower_p = (void *)nps_msu_reg_low_addr[cluster];
> +	upper_p = lower_p + 4;
> +	local_irq_restore(flags);
> +
> +	upper = ioread32be(upper_p);

Consider using the _relaxed macros even if your platform doesn't have specific IO
barriers.

> +	do {
> +		old_upper = upper;
> +		lower = ioread32be(lower_p);
> +		upper = ioread32be(upper_p);
> +	} while (upper != old_upper);
> +
> +	counter = upper;
> +	counter <<= 32;
> +	counter |= lower;

It is easier to read:

counter = (upper << 32) | lower;

> +	return (cycle_t)counter;
> +}
> +

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