linux-kernel - Re: [PATCH 1/5] clocksource: st_lpc: Add LPC timer as a clocksource.

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Date:	Tue, 21 Apr 2015 11:56:06 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Peter Griffin <peter.griffin@...aro.org>
cc:	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	srinivas.kandagatla@...il.com, maxime.coquelin@...com,
	patrice.chotard@...com, daniel.lezcano@...aro.org,
	lee.jones@...aro.org, devicetree@...r.kernel.org,
	Ajit Pal Singh <ajitpal.singh@...com>
Subject: Re: [PATCH 1/5] clocksource: st_lpc: Add LPC timer as a
 clocksource.

On Fri, 17 Apr 2015, Peter Griffin wrote:
> +/* Low Power Timer */
> +#define LPC_LPT_LSB_OFF		0x400
> +#define LPC_LPT_MSB_OFF		0x404
> +#define LPC_LPT_START_OFF	0x408
> +
> +struct st_lpc {
> +	struct clk *clk;
> +	void __iomem *iomem_cs;
> +};
> +
> +static struct st_lpc *st_lpc;
> +
> +static u64 notrace st_lpc_counter_read(void)
> +{
> +	u64 counter;
> +	u32 lower;
> +	u32 upper, old_upper;
> +
> +	upper = readl_relaxed(st_lpc->iomem_cs + LPC_LPT_MSB_OFF);
> +	do {
> +		old_upper = upper;
> +		lower = readl_relaxed(st_lpc->iomem_cs + LPC_LPT_LSB_OFF);
> +		upper = readl_relaxed(st_lpc->iomem_cs + LPC_LPT_MSB_OFF);
> +	} while (upper != old_upper);
> +
> +	counter = upper;
> +	counter <<= 32;
> +	counter |= lower;
> +	return counter;


What's the point of this exercise? The kernel can handle 32bit
clocksources nicely. So why do you want to artificially expand them to
64bit by adding useless loops and hoops to a hotpath?

Thanks,

	tglx
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