[PATCH v2 1/7] cpufreq: cpufreq-cpu0: allow optional safe voltage during frequency transitions

[Mike Turquette]

Quoting Thomas Abraham (2014-01-18 04:10:51)
> From: Thomas Abraham <thomas.ab at samsung.com>
> 
> On some platforms such as the Samsung Exynos, changing the frequency
> of the CPU clock requires changing the frequency of the PLL that is
> supplying the CPU clock. To change the frequency of the PLL, the CPU
> clock is temporarily reparented to another parent clock.
> 
> The clock frequency of this temporary parent clock could be much higher
> than the clock frequency of the PLL at the time of reparenting. Due
> to the temporary increase in the CPU clock speed, the CPU (and any other
> components in the CPU clock domain such as dividers, mux, etc.) have to
> to be operated at a higher voltage level, called the safe voltage level.
> This patch adds optional support to temporarily switch to a safe voltage
> level during CPU frequency transitions.
> 
> Cc: Shawn Guo <shawn.guo at linaro.org>
> Signed-off-by: Thomas Abraham <thomas.ab at samsung.com>

I'm not a fan of this change. This corner case should be abstracted away
somehow. I had talked to Chander Kayshap previously about handling
voltage changes in clock notifier callbacks, which then renders any
voltage change as a trivial part of the clock rate transition. That
means that this "safe voltage" thing could be handled automagically
without any additional code in the CPUfreq driver.

There are two nice ways to do this with the clock framework. First is
explicit re-parenting with voltage scaling done in the clock rate-change
notifiers:

clk_set_parent(cpu_clk, temp_parent);
/* implicit voltage scaling to "safe voltage" happens above */
clk_set_rate(pll, some_rate);
clk_set_parent(cpu_clk, pll);
/* implicit voltage scaling to nominal OPP voltage happens above */

The above sequence would require a separate exnyos CPUfreq driver, due
to the added clk_set_parent logic.

The second way to do this is to abstract the clk re-muxing logic out
into the clk driver, which would allow cpufreq-cpu0 to be used for the
exynos chips.

I'm more a fan of explicitly listing the Exact Steps for the cpu opp
transition in a separate exynos-specific CPUfreq driver, but that's
probably an unpopular view.

Regards,
Mike

> ---
>  .../devicetree/bindings/cpufreq/cpufreq-cpu0.txt   |    7 ++++
>  drivers/cpufreq/cpufreq-cpu0.c                     |   37 +++++++++++++++++--
>  2 files changed, 40 insertions(+), 4 deletions(-)
> 
> diff --git a/Documentation/devicetree/bindings/cpufreq/cpufreq-cpu0.txt b/Documentation/devicetree/bindings/cpufreq/cpufreq-cpu0.txt
> index f055515..37453ab 100644
> --- a/Documentation/devicetree/bindings/cpufreq/cpufreq-cpu0.txt
> +++ b/Documentation/devicetree/bindings/cpufreq/cpufreq-cpu0.txt
> @@ -19,6 +19,12 @@ Optional properties:
>  - cooling-min-level:
>  - cooling-max-level:
>       Please refer to Documentation/devicetree/bindings/thermal/thermal.txt.
> +- safe-opp: Certain platforms require that during a opp transition,
> +  a system should not go below a particular opp level. For such systems,
> +  this property specifies the minimum opp to be maintained during the
> +  opp transitions. The safe-opp value is a tuple with first element
> +  representing the safe frequency and the second element representing the
> +  safe voltage.
>  
>  Examples:
>  
> @@ -36,6 +42,7 @@ cpus {
>                         396000  950000
>                         198000  850000
>                 >;
> +               safe-opp = <396000 950000>
>                 clock-latency = <61036>; /* two CLK32 periods */
>                 #cooling-cells = <2>;
>                 cooling-min-level = <0>;
> diff --git a/drivers/cpufreq/cpufreq-cpu0.c b/drivers/cpufreq/cpufreq-cpu0.c
> index 0c12ffc..075d3d1 100644
> --- a/drivers/cpufreq/cpufreq-cpu0.c
> +++ b/drivers/cpufreq/cpufreq-cpu0.c
> @@ -27,6 +27,8 @@
>  
>  static unsigned int transition_latency;
>  static unsigned int voltage_tolerance; /* in percentage */
> +static unsigned long safe_frequency;
> +static unsigned long safe_voltage;
>  
>  static struct device *cpu_dev;
>  static struct clk *cpu_clk;
> @@ -64,17 +66,30 @@ static int cpu0_set_target(struct cpufreq_policy *policy, unsigned int index)
>                 volt_old = regulator_get_voltage(cpu_reg);
>         }
>  
> -       pr_debug("%u MHz, %ld mV --> %u MHz, %ld mV\n",
> +       pr_debug("\n\n%u MHz, %ld mV --> %u MHz, %ld mV\n",
>                  old_freq / 1000, volt_old ? volt_old / 1000 : -1,
>                  new_freq / 1000, volt ? volt / 1000 : -1);
>  
>         /* scaling up?  scale voltage before frequency */
> -       if (!IS_ERR(cpu_reg) && new_freq > old_freq) {
> +       if (!IS_ERR(cpu_reg) && new_freq > old_freq &&
> +                               new_freq >= safe_frequency) {
>                 ret = regulator_set_voltage_tol(cpu_reg, volt, tol);
>                 if (ret) {
>                         pr_err("failed to scale voltage up: %d\n", ret);
>                         return ret;
>                 }
> +       } else if (!IS_ERR(cpu_reg) && old_freq < safe_frequency) {
> +               /*
> +                * the scaled up voltage level for the new_freq is lower
> +                * than the safe voltage level. so set safe_voltage
> +                * as the intermediate voltage level and revert it
> +                * back after the frequency has been changed.
> +                */
> +               ret = regulator_set_voltage_tol(cpu_reg, safe_voltage, tol);
> +               if (ret) {
> +                       pr_err("failed to set safe voltage: %d\n", ret);
> +                       return ret;
> +               }
>         }
>  
>         ret = clk_set_rate(cpu_clk, freq_exact);
> @@ -86,7 +101,8 @@ static int cpu0_set_target(struct cpufreq_policy *policy, unsigned int index)
>         }
>  
>         /* scaling down?  scale voltage after frequency */
> -       if (!IS_ERR(cpu_reg) && new_freq < old_freq) {
> +       if (!IS_ERR(cpu_reg) &&
> +                       (new_freq < old_freq || new_freq < safe_frequency)) {
>                 ret = regulator_set_voltage_tol(cpu_reg, volt, tol);
>                 if (ret) {
>                         pr_err("failed to scale voltage down: %d\n", ret);
> @@ -116,6 +132,8 @@ static struct cpufreq_driver cpu0_cpufreq_driver = {
>  
>  static int cpu0_cpufreq_probe(struct platform_device *pdev)
>  {
> +       const struct property *prop;
> +       struct dev_pm_opp *opp;
>         struct device_node *np;
>         int ret;
>  
> @@ -165,13 +183,24 @@ static int cpu0_cpufreq_probe(struct platform_device *pdev)
>                 goto out_put_node;
>         }
>  
> +       prop = of_find_property(np, "safe-opp", NULL);
> +       if (prop) {
> +               if (prop->value && (prop->length / sizeof(u32)) == 2) {
> +                       const __be32 *val;
> +                       val = prop->value;
> +                       safe_frequency = be32_to_cpup(val++);
> +                       safe_voltage = be32_to_cpup(val);
> +               } else {
> +                       pr_err("invalid safe-opp level specified\n");
> +               }
> +       }
> +
>         of_property_read_u32(np, "voltage-tolerance", &voltage_tolerance);
>  
>         if (of_property_read_u32(np, "clock-latency", &transition_latency))
>                 transition_latency = CPUFREQ_ETERNAL;
>  
>         if (!IS_ERR(cpu_reg)) {
> -               struct dev_pm_opp *opp;
>                 unsigned long min_uV, max_uV;
>                 int i;
>  
> -- 
> 1.6.6.rc2
>