linux-kernel - Re: [PATCH v3 2/2] driver: clocksource: Add nxp system counter timer driver support

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Date:   Sat, 6 Apr 2019 00:12:16 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Jacky Bai <ping.bai@....com>
cc:     "daniel.lezcano@...aro.org" <daniel.lezcano@...aro.org>,
        "robh+dt@...nel.org" <robh+dt@...nel.org>,
        "shawnguo@...nel.org" <shawnguo@...nel.org>,
        "mark.rutland@....com" <mark.rutland@....com>,
        Aisheng Dong <aisheng.dong@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        dl-linux-imx <linux-imx@....com>
Subject: Re: [PATCH v3 2/2] driver: clocksource: Add nxp system counter timer
 driver support

On Wed, 3 Apr 2019, Jacky Bai wrote:

> From: Bai Ping <ping.bai@....com>
> 
> The system counter (sys_ctr) is a programmable system counter
> which provides a shared time base to the Cortex A15, A7, A53 etc cores.
> It is intended for use in applications where the counter is always
> powered on and supports multiple, unrelated clocks. The sys_ctr hardware
> supports:
>  - 56-bit counter width (roll-over time greater than 40 years)
>  - compare frame(64-bit compare value) contains programmable interrupt
>    generation

I hope that's a <= compare and not a == ....

> +static void sysctr_timer_enable(bool enable)
> +{
> +	u32 val;
> +
> +	val = readl(sys_ctr_base + CMPCR);
> +	val &= ~SYS_CTR_EN;
> +	if (enable)
> +		val |= SYS_CTR_EN;
> +
> +	writel(val, sys_ctr_base + CMPCR);

This read is really just overhead. Why aren't you caching the control
register value? It's not a self modifying register and I don't see
concurrency here either.

> +}
> +
> +static void sysctr_irq_acknowledge(void)
> +{
> +	/*
> +	 * clear the enable bit(EN =0) will clear
> +	 * the status bit(ISTAT = 0), then the interrupt
> +	 * signal will be negated(acknowledged).
> +	 */
> +	sysctr_timer_enable(false);
> +}
> +
> +static inline u64 sysctr_read_counter(void)
> +{
> +	u32 cnt_hi, tmp_hi, cnt_lo;
> +
> +	do {
> +		cnt_hi = readl_relaxed(sys_ctr_base + CNTCV_HI);
> +		cnt_lo = readl_relaxed(sys_ctr_base + CNTCV_LO);
> +		tmp_hi = readl_relaxed(sys_ctr_base + CNTCV_HI);
> +	} while (tmp_hi != cnt_hi);

When will hardware people finally get it? Is it so damned hard to make the
readout do:

	lo = read_lo()		-> internally latches HI in hardware
	hi = read_hi()		-> reads the latched value

It's not rocket science, but it would spare these horrible read loops. But
sure, performance happens in whitepapers and marketing slides ....

> +
> +	return  ((u64) cnt_hi << 32) | cnt_lo;
> +}
> +
> +static int sysctr_set_next_event(unsigned long delta,
> +				 struct clock_event_device *evt)
> +{
> +	u32 cmp_hi, cmp_lo;
> +	u64 next;
> +
> +	sysctr_timer_enable(false);
> +
> +	next = sysctr_read_counter();
> +
> +	next += delta;
> +
> +	cmp_hi = (next >> 32) & 0x00fffff;
> +	cmp_lo = next & 0xffffffff;
> +
> +	writel_relaxed(cmp_hi, sys_ctr_base + CMPCV_HI);
> +	writel_relaxed(cmp_lo, sys_ctr_base + CMPCV_LO);

Please document that this is a <= comparator. If that's not true then this
function is broken for small deltas and delays between read_counter() and
enable.

> +
> +	sysctr_timer_enable(true);
> +
> +	return 0;
> +}
> +
> +static int sysctr_set_state_oneshot(struct clock_event_device *evt)
> +{
> +	sysctr_timer_enable(true);

That's wrong. Why do you want to enable the timer here? When the state is
set to one shot then the next operation is set_next_event() but before that
nothing should ever come out of the timer.

Thanks,

	tglx