linux-kernel - Re: [PATCH V5 6/8] rust: Extend cpufreq bindings for driver registration

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Message-ID: <2a510219-4681-4915-9c27-c7ecdc6df133@arm.com>
Date: Thu, 15 Aug 2024 10:30:04 +0200
From: Pierre Gondois <pierre.gondois@....com>
To: Viresh Kumar <viresh.kumar@...aro.org>,
 "Rafael J. Wysocki" <rafael@...nel.org>,
 Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
 Danilo Krummrich <dakr@...hat.com>, Miguel Ojeda <ojeda@...nel.org>,
 Alex Gaynor <alex.gaynor@...il.com>,
 Wedson Almeida Filho <wedsonaf@...il.com>, Boqun Feng
 <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
 Björn Roy Baron <bjorn3_gh@...tonmail.com>,
 Benno Lossin <benno.lossin@...ton.me>,
 Andreas Hindborg <a.hindborg@...sung.com>, Alice Ryhl <aliceryhl@...gle.com>
Cc: linux-pm@...r.kernel.org, Vincent Guittot <vincent.guittot@...aro.org>,
 Stephen Boyd <sboyd@...nel.org>, Nishanth Menon <nm@...com>,
 rust-for-linux@...r.kernel.org,
 Manos Pitsidianakis <manos.pitsidianakis@...aro.org>,
 Erik Schilling <erik.schilling@...aro.org>,
 Alex Bennée <alex.bennee@...aro.org>,
 Joakim Bech <joakim.bech@...aro.org>, Rob Herring <robh@...nel.org>,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH V5 6/8] rust: Extend cpufreq bindings for driver
 registration

Hello Viresh,

On 7/30/24 12:27, Viresh Kumar wrote:
> This extends the cpufreq bindings with bindings for registering a
> driver.
> 
> Signed-off-by: Viresh Kumar <viresh.kumar@...aro.org>
> ---
>   rust/kernel/cpufreq.rs | 478 ++++++++++++++++++++++++++++++++++++++++-
>   1 file changed, 476 insertions(+), 2 deletions(-)
> 
> diff --git a/rust/kernel/cpufreq.rs b/rust/kernel/cpufreq.rs
> index d58bb0bbaad4..2631dbb4865f 100644
> --- a/rust/kernel/cpufreq.rs
> +++ b/rust/kernel/cpufreq.rs
> @@ -9,14 +9,17 @@
>   use crate::{
>       bindings, clk, cpumask,
>       device::Device,
> -    error::{code::*, from_err_ptr, to_result, Result, VTABLE_DEFAULT_ERROR},
> +    devres::Devres,
> +    error::{code::*, from_err_ptr, from_result, to_result, Result, VTABLE_DEFAULT_ERROR},
>       prelude::*,
>       types::ForeignOwnable,
>   };
>   
>   use core::{
> +    cell::UnsafeCell,
> +    marker::PhantomData,
>       pin::Pin,
> -    ptr::self,
> +    ptr::{self, addr_of_mut},
>   };
>   
>   use macros::vtable;
> @@ -563,3 +566,474 @@ fn register_em(_policy: &mut Policy) {
>           kernel::build_error(VTABLE_DEFAULT_ERROR)
>       }
>   }
> +
> +/// Registration of a cpufreq driver.
> +pub struct Registration<T: Driver> {
> +    drv: Box<UnsafeCell<bindings::cpufreq_driver>>,
> +    _p: PhantomData<T>,
> +}
> +
> +// SAFETY: `Registration` doesn't offer any methods or access to fields when shared between threads
> +// or CPUs, so it is safe to share it.
> +unsafe impl<T: Driver> Sync for Registration<T> {}
> +
> +// SAFETY: Registration with and unregistration from the cpufreq subsystem can happen from any thread.
> +// Additionally, `T::Data` (which is dropped during unregistration) is `Send`, so it is okay to move
> +// `Registration` to different threads.
> +#[allow(clippy::non_send_fields_in_send_ty)]
> +unsafe impl<T: Driver> Send for Registration<T> {}
> +
> +impl<T: Driver> Registration<T> {
> +    /// Registers a cpufreq driver with the rest of the kernel.
> +    pub fn new(name: &'static CStr, data: T::Data, flags: u16, boost: bool) -> Result<Self> {
> +        let mut drv = Box::new(
> +            UnsafeCell::new(bindings::cpufreq_driver::default()),
> +            GFP_KERNEL,
> +        )?;
> +        let drv_ref = drv.get_mut();
> +
> +        // Account for the trailing null character.
> +        let len = name.len() + 1;
> +        if len > drv_ref.name.len() {
> +            return Err(EINVAL);
> +        };
> +
> +        // SAFETY: `name` is a valid Cstr, and we are copying it to an array of equal or larger
> +        // size.
> +        let name = unsafe { &*(name.as_bytes_with_nul() as *const [u8] as *const [i8]) };
> +        drv_ref.name[..len].copy_from_slice(name);
> +
> +        drv_ref.boost_enabled = boost;
> +        drv_ref.flags = flags;
> +
> +        // Allocate an array of 3 pointers to be passed to the C code.
> +        let mut attr = Box::new([ptr::null_mut(); 3], GFP_KERNEL)?;
> +        let mut next = 0;
> +
> +        // SAFETY: The C code returns a valid pointer here, which is again passed to the C code in
> +        // an array.
> +        attr[next] =
> +            unsafe { addr_of_mut!(bindings::cpufreq_freq_attr_scaling_available_freqs) as *mut _ };
> +        next += 1;
> +
> +        if boost {
> +            // SAFETY: The C code returns a valid pointer here, which is again passed to the C code
> +            // in an array.
> +            attr[next] =
> +                unsafe { addr_of_mut!(bindings::cpufreq_freq_attr_scaling_boost_freqs) as *mut _ };
> +            next += 1;
> +        }
> +        attr[next] = ptr::null_mut();
> +
> +        // Pass the ownership of the memory block to the C code. This will be freed when
> +        // the [`Registration`] object goes out of scope.
> +        drv_ref.attr = Box::leak(attr) as *mut _;

I think it would be better to give the possibility to the cpufreq driver to pass the
attr array, as not all drivers might want these attributes,

Regards,
Pierre