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

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Message-Id: <DA051LGPX0NX.20CQCK4V3B6PF@kernel.org>
Date: Mon, 19 May 2025 14:32:44 +0200
From: "Benno Lossin" <lossin@...nel.org>
To: "FUJITA Tomonori" <fujita.tomonori@...il.com>
Cc: <ansuelsmth@...il.com>, <andrew+netdev@...n.ch>, <davem@...emloft.net>,
 <edumazet@...gle.com>, <kuba@...nel.org>, <pabeni@...hat.com>,
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 <ojeda@...nel.org>, <alex.gaynor@...il.com>, <boqun.feng@...il.com>,
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 <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 Mon May 19, 2025 at 2:00 PM CEST, FUJITA Tomonori wrote:
> On Sat, 17 May 2025 21:02:51 +0200
> "Benno Lossin" <lossin@...nel.org> wrote:
>>>> 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)
>>         }
>>     }
>
> Ah, yes, this works well.
>
>
>>> 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
>
> Maybe a bool would be more appropriate here because the C's comment
> says:
>
> Return: 1 if the PHY device matches the driver, 0 otherwise.
>
>> function as well (though I wouldn't use the vtable for that).
>
> What would you use instead?

The concept that I sketched above:

    impl Device {
        fn genphy_match_phy_device<T: Driver>(&self) -> bool {
            self.phy_id() == T::PHY_DEVICE_ID.id
        }
    }

---
Cheers,
Benno