Re: [RFC PATCH V3 8/8] cpufreq: Add Rust based cpufreq-dt driver

[Danilo Krummrich <dakr@redhat.com>]

Hi Viresh,

On 7/3/24 09:14, Viresh Kumar wrote:
> This commit adds a Rust based cpufreq-dt driver, which covers most of
> the functionality of the existing C based driver. Only a handful of
> things are left, like fetching platform data from cpufreq-dt-platdev.c.
> 
> This is tested with the help of QEMU for now and switching of
> frequencies work as expected.
> 
> Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
> ---
>   drivers/cpufreq/Kconfig        |  12 ++
>   drivers/cpufreq/Makefile       |   1 +
>   drivers/cpufreq/rcpufreq_dt.rs | 225 +++++++++++++++++++++++++++++++++
>   3 files changed, 238 insertions(+)
>   create mode 100644 drivers/cpufreq/rcpufreq_dt.rs
> 
> diff --git a/drivers/cpufreq/rcpufreq_dt.rs b/drivers/cpufreq/rcpufreq_dt.rs
> new file mode 100644
> index 000000000000..652458e7a3b9
> --- /dev/null
> +++ b/drivers/cpufreq/rcpufreq_dt.rs

<snip>

> +
> +type DeviceData = Box<cpufreq::Registration<CPUFreqDTDriver>>;
> +
> +impl platform::Driver for CPUFreqDTDriver {
> +    type Data = Arc<DeviceData>;
> +
> +    define_of_id_table! {(), [
> +        (of::DeviceId(b_str!("operating-points-v2")), None),
> +    ]}
> +
> +    fn probe(_dev: &mut platform::Device, _id_info: Option<&Self::IdInfo>) -> Result<Self::Data> {
> +        let drv = Arc::new(
> +            cpufreq::Registration::<CPUFreqDTDriver>::register(
> +                c_str!("cpufreq-dt"),
> +                (),
> +                cpufreq::flags::NEED_INITIAL_FREQ_CHECK | cpufreq::flags::IS_COOLING_DEV,
> +                true,
> +            )?,
> +            GFP_KERNEL,
> +        )?;

Putting the `cpufreq::Registration` into `Arc<DeviceData>` is unsafe from a
lifetime point of view. Nothing prevents this `Arc` to out-live the
`platform::Driver`.

Instead, you should wrap `cpufreq::Registration` into `Devres`. See
`drm::drv::Registration` for an example [1].

[1] https://gitlab.freedesktop.org/drm/nova/-/blob/nova-next/rust/kernel/drm/drv.rs?ref_type=heads#L173

> +
> +        pr_info!("CPUFreq DT driver registered\n");
> +
> +        Ok(drv)
> +    }
> +}
> +
> +module_platform_driver! {
> +    type: CPUFreqDTDriver,
> +    name: "cpufreq_dt",
> +    author: "Viresh Kumar <viresh.kumar@linaro.org>",
> +    description: "Generic CPUFreq DT driver",
> +    license: "GPL v2",
> +}