Re: [net-next PATCH v10 7/7] rust: net::phy sync with match_phy_device C changes

["Benno Lossin" <lossin@kernel.org>]

On Sat May 17, 2025 at 3:13 PM CEST, FUJITA Tomonori wrote:
> On Sat, 17 May 2025 10:06:13 +0200
> "Benno Lossin" <lossin@kernel.org> wrote:
>>>> 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.
>>>
>>> Yeah.
>>>
>>> I don't want to delay this patchset due to Rust side changes so
>>> casting a pointer to bindings::phy_driver to DriverVTable is ok but
>>> the following signature doesn't look useful for Rust phy drivers:
>>>
>>> fn match_phy_device(_dev: &mut Device, _drv: &DriverVTable) -> bool
>>>
>>> struct DriverVTable is only used to create an array of
>>> bindings::phy_driver for C side, and it doesn't provide any
>>> information to the Rust driver.
>> 
>> Yeah, but we could add accessor functions that provide that information.
>
> Yes. I thought that implementation was one of the options as well but
> realized it makes sense because inside match_phy_device() callback, a
> driver might call a helper function that takes a pointer to
> bindings::phy_driver (please see below for details).
>
>
>> Although that doesn't really make sense at the moment, see below.
>>
>>> In match_phy_device(), for example, a device driver accesses to
>>> PHY_DEVICE_ID, which the Driver trait provides. I think we need to
>>> create an instance of the device driver's own type that implements the
>>> Driver trait and make it accessible.
>> 
>> I think that's wrong, nothing stops me from implementing `Driver` for an
>> empty enum and that can't be instantiated. The reason that one wants to
>> have this in C is because the same `match` function is used for
>> different drivers (or maybe devices? I'm not too familiar with the
>> terminology). In Rust, you must implement the match function for a
>> single PHY_DEVICE_ID only, so maybe we don't need to change the
>> signature at all?
>
> I'm not sure I understand the last sentence. The Rust PHY abstraction
> allows one module to support multiple drivers. So we can could the
> similar trick that the second patch in this patchset does.
>
> fn match_device_id(dev: &mut phy::Device, drv: &phy::DriverVTable) -> bool {
>     // do comparison workking for three drivers
> }

I wouldn't do it like this in Rust, instead this would be a "rustier"
function signature:

    fn match_device_id<T: Driver>(dev: &mut phy::Device) -> bool {
        // do the comparison with T::PHY_DEVICE_ID
        dev.id() == T::PHY_DEVICE_ID
    }

And then in the impls for Phy{A,B,C,D} do this:

    impl Driver for PhyA {
        fn match_phy_device(dev: &mut phy::Device) -> bool {
            match_device_id::<Self>(dev)
        }
    }

>
> #[vtable]
> impl Driver for PhyA {
>     ...
>     fn match_phy_device(dev: &mut phy::Device, drv: &phy::DriverVTable) -> bool {
>         match_device_id(dev, drv)
>     }
> }
>
> #[vtable]
> impl Driver for PhyB {
>     ...
>     fn match_phy_device(dev: &mut phy::Device, drv: &phy::DriverVTable) -> bool {
>         match_device_id(dev, drv)
>     }
> }
>
> #[vtable]
> impl Driver for PhyC {
>     ...
>     fn match_phy_device(dev: &mut phy::Device, drv: &phy::DriverVTable) -> bool {
>         match_device_id(dev, drv)
>     }
> }
>
>
> The other use case, as mentioned above, is when using the generic helper
> function inside match_phy_device() callback. For example, the 4th
> patch in this patchset adds genphy_match_phy_device():
>
> int genphy_match_phy_device(struct phy_device *phydev,
>                            const struct phy_driver *phydrv)
>
> We could add a wrapper for this function as phy::Device's method like
>
> impl Device {
>     ...
>     pub fn genphy_match_phy_device(&self, drv: &phy::DriverVTable) -> i32 

Not sure why this returns an `i32`, but we probably could have such a
function as well (though I wouldn't use the vtable for that).

---
Cheers,
Benno

> Then a driver could do something like 5th patch in the patchset:
>
> #[vtable]
> impl Driver for PhyD {
>     ...
>     fn match_phy_device(dev: &mut phy::Device, drv: &phy::DriverVTable) -> bool {
>        let val = dev.genphy_match_phy_device(drv);
>        ...
>     }
> }