linux-kernel - Re: [PATCH v6] clocksource:arm_global_timer: Add ARM global timer support.

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Date:	Tue, 25 Jun 2013 15:17:40 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Srinivas KANDAGATLA <srinivas.kandagatla@...com>
cc:	John Stultz <john.stultz@...aro.org>,
	Grant Likely <grant.likely@...aro.org>,
	Rob Herring <rob.herring@...xeda.com>,
	Rob Landley <rob@...dley.net>,
	devicetree-discuss@...ts.ozlabs.org, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Linus Walleij <linus.walleij@...aro.org>,
	Stuart Menefy <stuart.menefy@...com>,
	Arnd Bergmann <arnd@...db.de>,
	Rob Herring <robherring2@...il.com>,
	Will Deacon <will.deacon@....com>
Subject: Re: [PATCH v6] clocksource:arm_global_timer: Add ARM global timer
 support.

On Tue, 25 Jun 2013, Srinivas KANDAGATLA wrote:
> If its not too late can this patch be considered for 3.11 via clocksource tree?

Sure. No worry, though I noticed a little detail when reading through it
again. See below.

> +/**
> + * To ensure that updates to comparator value register do not set the
> + * Interrupt Status Register proceed as follows:
> + * 1. Clear the Comp Enable bit in the Timer Control Register.
> + * 2. Write the lower 32-bit Comparator Value Register.
> + * 3. Write the upper 32-bit Comparator Value Register.
> + * 4. Set the Comp Enable bit and, if necessary, the IRQ enable bit.
> + */
> +static void gt_compare_set(unsigned long delta, int periodic)
> +{
> +	u64 counter = gt_counter_read();
> +	unsigned long ctrl = readl(gt_base + GT_CONTROL);

Why are you reading the control register back over and over? All you
need to preserve is the GT_CONTROL_TIMER_ENABLE bit. So you can spare
that read out and just do

	ctrl = GT_CONTROL_TIMER_ENABLE;
	writel(ctrl, gt_base + GT_CONTROL);

> +static irqreturn_t gt_clockevent_interrupt(int irq, void *dev_id)
> +{
> +	struct clock_event_device *evt = dev_id;
> +
> +	if (readl_relaxed(gt_base + GT_INT_STATUS) &
> +				GT_INT_STATUS_EVENT_FLAG) {

If you negate the check and return IRQ_NONE, you save one indent level
for the real code.

> +		/**
> +		 * ERRATA 740657( Global Timer can send 2 interrupts for
> +		 * the same event in single-shot mode)
> +		 * Workaround:
> +		 *	Either disable single-shot mode.
> +		 *	Or
> +		 *	Modify the Interrupt Handler to avoid the
> +		 *	offending sequence. This is achieved by clearing
> +		 *	the Global Timer flag _after_ having incremented
> +		 *	the Comparator register	value to a higher value.
> +		 */
> +		if (!(readl_relaxed(gt_base + GT_CONTROL) &
> +						GT_CONTROL_AUTO_INC))

Again, no need to read from the hardware.

       if (evt->mode == CLOCK_EVT_MODE_ONESHOT)

tells you what you need to know.

Thanks,

	tglx
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