Re: [PATCH V4 3/3] cpufreq: Tegra: implement intermediate frequency callbacks

Stephen Warren
2014-05-29T17:40:25Z

On 05/21/2014 02:59 AM, Viresh Kumar wrote:
> Tegra had always been switching to intermediate frequency (pll_p_clk) since
> ever. CPUFreq core has better support for handling notifications for these
> frequencies and so we can adapt Tegra's driver to it.
>
> Also do a WARN() if clk_set_parent() fails while moving back to pll_x as we
> should have atleast restored to earlier frequency on error.

This patch breaks Tegra. The reason is below.

> diff --git a/drivers/cpufreq/tegra-cpufreq.c b/drivers/cpufreq/tegra-cpufreq.c

> -static int tegra_cpu_clk_set_rate(unsigned long rate)
> +static unsigned int
> +tegra_get_intermediate(struct cpufreq_policy *policy, unsigned int index)

(BTW, can we please not put the return type on a separate line; it's
inconsistent with the rest of the code in this file)

> +{
> + unsigned int ifreq = clk_get_rate(pll_p_clk) / 1000;
> +
> + /*
> + * Don't switch to intermediate freq if:
> + * - we are already at it, i.e. policy->cur == ifreq
> + * - index corresponds to ifreq
> + */
> + if ((freq_table[index].frequency == ifreq) || (policy->cur == ifreq))
> + return 0;

If policy->cur == ifreq here, then tegra_target_intermediate() isn't
called by the cpufreq core, so ...

> +static int
> +tegra_target_intermediate(struct cpufreq_policy *policy, unsigned int index)
> {
> int ret;
>
> /*
> * Take an extra reference to the main pll so it doesn't turn
> * off when we move the cpu off of it
> */
> clk_prepare_enable(pll_x_clk);

... that reference isn't added...

> @@ -98,10 +96,23 @@ static int tegra_target(struct cpufreq_policy *policy, unsigned int index)
> else
> clk_set_rate(emc_clk, 100000000); /* emc 50Mhz */
>
> - ret = tegra_cpu_clk_set_rate(rate * 1000);
> + /* target freq == pll_p */
> + if (rate * 1000 == clk_get_rate(pll_p_clk)) {
> + ret = tegra_target_intermediate(policy, index);
> + goto disable_pll_x;
> + }

... and this code doesn't call it either, since we could be switching
from the pll_p rate to something faster ...

> +
> + ret = clk_set_rate(pll_x_clk, rate * 1000);
> + /* Restore to earlier frequency on error, i.e. pll_x */
> if (ret)
> - pr_err("cpu-tegra: Failed to set cpu frequency to %lu kHz\n",
> - rate);
> + pr_err("Failed to change pll_x to %lu\n", rate);
> +
> + ret = clk_set_parent(cpu_clk, pll_x_clk);
> + /* This shouldn't fail while changing or restoring */
> + WARN_ON(ret);
> +
> +disable_pll_x:
> + clk_disable_unprepare(pll_x_clk);

... so this turns off pll_x even though we're running from it.

It would be simpler if Tegra *always* used an intermediate frequency,
and hence the core *always* called tegra_target_intermediate().
Admittedly, this would result in tegra_target() sometimes (when
switching CPU clock rate to the pll_p rate) doing nothing other than
removing the extra reference on pll_x, but I think that the code would
be simpler to follow and more robust.