linux-kernel - Re: [TEGRA194_CPUFREQ Patch v2 2/3] cpufreq: Add Tegra194 cpufreq driver

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Message-ID: <67f9feb1-ba51-f1ac-b6eb-4587924d0748@gmail.com>
Date:   Sun, 5 Apr 2020 17:05:14 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Sumit Gupta <sumitg@...dia.com>, rjw@...ysocki.net,
        viresh.kumar@...aro.org, catalin.marinas@....com, will@...nel.org,
        thierry.reding@...il.com, jonathanh@...dia.com, talho@...dia.com,
        linux-pm@...r.kernel.org, linux-tegra@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Cc:     bbasu@...dia.com, mperttunen@...dia.com
Subject: Re: [TEGRA194_CPUFREQ Patch v2 2/3] cpufreq: Add Tegra194 cpufreq
 driver

04.04.2020 22:29, Sumit Gupta пишет:
...
> +static void tegra_read_counters(struct work_struct *work)
> +{
> +	struct read_counters_work *read_counters_work;
> +	struct tegra_cpu_ctr *c;
> +	u64 val;
> +
> +	/*
> +	 * ref_clk_counter(32 bit counter) runs on constant clk,
> +	 * pll_p(408MHz).
> +	 * It will take = 2 ^ 32 / 408 MHz to overflow ref clk counter
> +	 *              = 10526880 usec = 10.527 sec to overflow
> +	 *
> +	 * Like wise core_clk_counter(32 bit counter) runs on core clock.
> +	 * It's synchronized to crab_clk (cpu_crab_clk) which runs at
> +	 * freq of cluster. Assuming max cluster clock ~2000MHz,
> +	 * It will take = 2 ^ 32 / 2000 MHz to overflow core clk counter
> +	 *              = ~2.147 sec to overflow
> +	 */
> +	read_counters_work = container_of(work, struct read_counters_work,
> +					  work);
> +	c = &read_counters_work->c;
> +
> +	val = read_freq_feedback();
> +	c->last_refclk_cnt = lower_32_bits(val);
> +	c->last_coreclk_cnt = upper_32_bits(val);
> +	udelay(c->delay);
> +	val = read_freq_feedback();
> +	c->refclk_cnt = lower_32_bits(val);
> +	c->coreclk_cnt = upper_32_bits(val);
> +}
> +
> +/*
> + * Return instantaneous cpu speed
> + * Instantaneous freq is calculated as -
> + * -Takes sample on every query of getting the freq.
> + *	- Read core and ref clock counters;
> + *	- Delay for X us
> + *	- Read above cycle counters again
> + *	- Calculates freq by subtracting current and previous counters
> + *	  divided by the delay time or eqv. of ref_clk_counter in delta time
> + *	- Return Kcycles/second, freq in KHz
> + *
> + *	delta time period = x sec
> + *			  = delta ref_clk_counter / (408 * 10^6) sec
> + *	freq in Hz = cycles/sec
> + *		   = (delta cycles / x sec
> + *		   = (delta cycles * 408 * 10^6) / delta ref_clk_counter
> + *	in KHz	   = (delta cycles * 408 * 10^3) / delta ref_clk_counter
> + *
> + * @cpu - logical cpu whose freq to be updated


> + * Returns freq in KHz on success, 0 if cpu is offline

I don't see any checks in the code about whether CPU is offline.

Googling for "queue_work_on offline cpu" suggests that this function
should hang.

> + */
> +static unsigned int tegra194_get_speed_common(u32 cpu, u32 delay)
> +{
> +	struct read_counters_work read_counters_work;
> +	struct tegra_cpu_ctr c;
> +	u32 delta_refcnt;
> +	u32 delta_ccnt;
> +	u32 rate_mhz;
> +
> +	read_counters_work.c.cpu = cpu;
> +	read_counters_work.c.delay = delay;
> +	INIT_WORK_ONSTACK(&read_counters_work.work, tegra_read_counters);
> +	queue_work_on(cpu, read_counters_wq, &read_counters_work.work);
> +	flush_work(&read_counters_work.work);
> +	c = read_counters_work.c;
> +
> +	if (c.coreclk_cnt < c.last_coreclk_cnt)
> +		delta_ccnt = c.coreclk_cnt + (MAX_CNT - c.last_coreclk_cnt);
> +	else
> +		delta_ccnt = c.coreclk_cnt - c.last_coreclk_cnt;
> +	if (!delta_ccnt)
> +		return 0;
> +
> +	/* ref clock is 32 bits */
> +	if (c.refclk_cnt < c.last_refclk_cnt)
> +		delta_refcnt = c.refclk_cnt + (MAX_CNT - c.last_refclk_cnt);
> +	else
> +		delta_refcnt = c.refclk_cnt - c.last_refclk_cnt;
> +	if (!delta_refcnt) {
> +		pr_debug("cpufreq: %d is idle, delta_refcnt: 0\n", cpu);
> +		return 0;
> +	}
> +	rate_mhz = ((unsigned long)(delta_ccnt * REF_CLK_MHZ)) / delta_refcnt;
> +
> +	return (rate_mhz * KHZ); /* in KHz */
> +}