Re: [PATCH] cpufreq: Register cpufreq drivers only after CPU devices are registered

[Amit Kucheria]

On Thu, Nov 14, 2019 at 9:06 AM Viresh Kumar <viresh.kumar@xxxxxxxxxx> wrote:
>
> The cpufreq core heavily depends on the availability of the struct
> device for CPUs and if they aren't available at the time cpufreq driver
> is registered, we will never succeed in making cpufreq work.
>
> This happens due to following sequence of events:
>
> - cpufreq_register_driver()
>   - subsys_interface_register()
>   - return 0; //successful registration of driver
>
> ... at a later point of time
>
> - register_cpu();
>   - device_register();
>     - bus_probe_device();
>       - sif->add_dev();
>         - cpufreq_add_dev();
>           - get_cpu_device(); //FAILS
>   - per_cpu(cpu_sys_devices, num) = &cpu->dev; //used by get_cpu_device()
>   - return 0; //CPU registered successfully
>
> Because the per-cpu variable cpu_sys_devices is set only after the CPU
> device is regsitered, cpufreq will never be able to get it when
> cpufreq_add_dev() is called.
>
> This patch avoids this failure by making sure device structure of at
> least CPU0 is available when the cpufreq driver is registered, else
> return -EPROBE_DEFER.
>
> Reported-by: Bjorn Andersson <bjorn.andersson@xxxxxxxxxx>
> Signed-off-by: Amit Kucheria <amit.kucheria@xxxxxxxxxx>
> Signed-off-by: Viresh Kumar <viresh.kumar@xxxxxxxxxx>

Tested-by: Amit Kucheria <amit.kucheria@xxxxxxxxxx>

> ---
> @Amit: I have added your sob without asking as you were involved in
> getting us to this patch, you did a lot of testing yesterday to find the
> root cause.

SoB for the SoB :-)

> @Rafael: This fixes the issues reported by Bjorn on Amit's series and so
> should land before Amit's series, if at all this is acceptable to you.
> Thanks.

My series was going to get merged through the thermal tree. Currently
it is hosted here[1] for linux-next testing. We could sign-off this
patch to the thermal tree or bring the series into the PM tree. Up to
Rafael and you.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/thermal/linux.git/log/?h=thermal/linux-next

>  drivers/cpufreq/cpufreq.c | 7 +++++++
>  1 file changed, 7 insertions(+)
>
> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> index 681c1b5f0a1a..05293b43e56d 100644
> --- a/drivers/cpufreq/cpufreq.c
> +++ b/drivers/cpufreq/cpufreq.c
> @@ -2641,6 +2641,13 @@ int cpufreq_register_driver(struct cpufreq_driver *driver_data)
>         if (cpufreq_disabled())
>                 return -ENODEV;
>
> +       /*
> +        * The cpufreq core depends heavily on the availability of device
> +        * structure, make sure they are available before proceeding further.
> +        */
> +       if (!get_cpu_device(0))
> +               return -EPROBE_DEFER;
> +
>         if (!driver_data || !driver_data->verify || !driver_data->init ||
>             !(driver_data->setpolicy || driver_data->target_index ||
>                     driver_data->target) ||
> --
> 2.21.0.rc0.269.g1a574e7a288b
>