Re: [PATCHv3 3/7] clk: mvebu: extend clk-cpu for dynamic frequency scaling

Thomas Petazzoni
2014-07-24T06:33:28Z

Hello,

(Not sure why this e-mail comes with me as the From:, but anyway.)

On Wed, 23 Jul 2014 16:53:58 -0700, Thomas Petazzoni wrote:

> +static int clk_cpu_set_rate(struct clk_hw *hwclk, unsigned long rate,
> + unsigned long parent_rate)
> +{
> + if (__clk_is_enabled(hwclk->clk))
> + return clk_cpu_on_set_rate(hwclk, rate, parent_rate);
> + else
> + return clk_cpu_off_set_rate(hwclk, rate, parent_rate);
>
> This is racy. You don't hold the clk_enable lock so it could be enable
> between the conditional check and executing clk_cpu_on_set_rate.

Right.

> How do you ensure that secondary CPU clocks are not enabled/disabled
> when changing rates?

In practice, this currently cannot happen: we enable the secondary CPU
clocks before starting the secondary CPUs, and we never ever disable or
re-enable again those clocks. So with the present code, I believe there
is no problem. Even when we do CPU hotplug, we don't turn off the CPU
clocks, simply because they cannot be turned off: the enable/disable
state is only used here as an indication so that the clock driver knows
which frequency change strategy it should apply.

But you're anyway right, I'll send a followup patch adding the lock.
Would that be OK for you?

Thanks,

Thomas
--
Thomas Petazzoni, CTO, Free Electrons
Embedded Linux, Kernel and Android engineering