linux-kernel - Re: [net-next PATCH v10 7/7] rust: net::phy sync with match_phy

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Message-Id: <D9XV0Y22JHU5.3T51FVQONVERC@kernel.org>
Date: Fri, 16 May 2025 22:16:23 +0200
From: "Benno Lossin" <lossin@...nel.org>
To: "Christian Marangi" <ansuelsmth@...il.com>
Cc: "FUJITA Tomonori" <fujita.tomonori@...il.com>, <andrew+netdev@...n.ch>,
 <davem@...emloft.net>, <edumazet@...gle.com>, <kuba@...nel.org>,
 <pabeni@...hat.com>, <robh@...nel.org>, <krzk+dt@...nel.org>,
 <conor+dt@...nel.org>, <hkallweit1@...il.com>, <linux@...linux.org.uk>,
 <florian.fainelli@...adcom.com>, <bcm-kernel-feedback-list@...adcom.com>,
 <kabel@...nel.org>, <andrei.botila@....nxp.com>, <tmgross@...ch.edu>,
 <ojeda@...nel.org>, <alex.gaynor@...il.com>, <boqun.feng@...il.com>,
 <gary@...yguo.net>, <bjorn3_gh@...tonmail.com>, <benno.lossin@...ton.me>,
 <a.hindborg@...nel.org>, <aliceryhl@...gle.com>, <dakr@...nel.org>,
 <sd@...asysnail.net>, <michael@...sekall.de>, <daniel@...rotopia.org>,
 <netdev@...r.kernel.org>, <devicetree@...r.kernel.org>,
 <linux-kernel@...r.kernel.org>, <rust-for-linux@...r.kernel.org>
Subject: Re: [net-next PATCH v10 7/7] rust: net::phy sync with
 match_phy_device C changes

On Fri May 16, 2025 at 5:12 PM CEST, Christian Marangi wrote:
> On Fri, May 16, 2025 at 04:48:53PM +0200, Benno Lossin wrote:
>> On Fri May 16, 2025 at 2:30 PM CEST, FUJITA Tomonori wrote:
>> > On Thu, 15 May 2025 13:27:12 +0200
>> > Christian Marangi <ansuelsmth@...il.com> wrote:
>> >> @@ -574,6 +577,23 @@ pub const fn create_phy_driver<T: Driver>() -> DriverVTable {
>> >>  /// This trait is used to create a [`DriverVTable`].
>> >>  #[vtable]
>> >>  pub trait Driver {
>> >> +    /// # Safety
>> >> +    ///
>> >> +    /// For the duration of `'a`,
>> >> +    /// - the pointer must point at a valid `phy_driver`, and the caller
>> >> +    ///   must be in a context where all methods defined on this struct
>> >> +    ///   are safe to call.
>> >> +    unsafe fn from_raw<'a>(ptr: *const bindings::phy_driver) -> &'a Self
>> >> +    where
>> >> +        Self: Sized,
>> >> +    {
>> >> +        // CAST: `Self` is a `repr(transparent)` wrapper around `bindings::phy_driver`.
>> >> +        let ptr = ptr.cast::<Self>();
>> >> +        // SAFETY: by the function requirements the pointer is valid and we have unique access for
>> >> +        // the duration of `'a`.
>> >> +        unsafe { &*ptr }
>> >> +    }
>> >
>> > We might need to update the comment. phy_driver is const so I think
>> > that we can access to it any time.
>> 
>> Why is any type implementing `Driver` a transparent wrapper around
>> `bindings::phy_driver`?
>> 
>
> Is this referred to a problem with using from_raw or more of a general
> question on how the rust wrapper are done for phy code?

I looked at the `phy.rs` file again and now I'm pretty sure the above
code is wrong. `Self` can be implemented on any type (even types like
`Infallible` that do not have any valid bit patterns, since it's an
empty enum). The abstraction for `bindings::phy_driver` is
`DriverVTable` not an object of type `Self`, so you should cast to that
pointer instead.

>> >>      /// Defines certain other features this PHY supports.
>> >>      /// It is a combination of the flags in the [`flags`] module.
>> >>      const FLAGS: u32 = 0;
>> >> @@ -602,7 +622,7 @@ fn get_features(_dev: &mut Device) -> Result {
>> >>  
>> >>      /// Returns true if this is a suitable driver for the given phydev.
>> >>      /// If not implemented, matching is based on [`Driver::PHY_DEVICE_ID`].
>> >> -    fn match_phy_device(_dev: &Device) -> bool {
>> >> +    fn match_phy_device<T: Driver>(_dev: &mut Device, _drv: &T) -> bool {
>> >>          false
>> >>      }
>> >
>> > I think that it could be a bit simpler:
>> >
>> > fn match_phy_device(_dev: &mut Device, _drv: &Self) -> bool
>> >
>> > Or making it a trait method might be more idiomatic?
>> >
>> > fn match_phy_device(&self, _dev: &mut Device) -> bool
>> 
>> Yeah that would make most sense.
>>
>
> I think
>
> fn match_phy_device(_dev: &mut Device, _drv: &Self) -> bool
>
> more resemble the C parallel function so I think this suite the best,
> should make it easier to port if ever (am I wrong?)

I don't understand what you mean by "easier to port if ever". From a
Rust perspective, it makes much more sense to use the `&self` receiver,
since the driver is asked if it can take care of the device. If you want
to keep the order how it is in C that is also fine, but if I were to
write it, I'd use the receiver.

---
Cheers,
Benno