netdev - Re: [PATCH net-next v4 6/6] net: phy: add Applied Micro QT2025 PHY driver

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Message-ID: <0797f8e8-ea3c-413d-b782-84dd97919ea9@proton.me>
Date: Sun, 18 Aug 2024 16:16:50 +0000
From: Benno Lossin <benno.lossin@...ton.me>
To: Andrew Lunn <andrew@...n.ch>
Cc: FUJITA Tomonori <fujita.tomonori@...il.com>, netdev@...r.kernel.org, rust-for-linux@...r.kernel.org, tmgross@...ch.edu, miguel.ojeda.sandonis@...il.com, aliceryhl@...gle.com
Subject: Re: [PATCH net-next v4 6/6] net: phy: add Applied Micro QT2025 PHY driver

On 18.08.24 17:44, Andrew Lunn wrote:
> On Sat, Aug 17, 2024 at 09:34:13PM +0000, Benno Lossin wrote:
>> On 17.08.24 20:51, Andrew Lunn wrote:
>>>> +    fn read_status(dev: &mut phy::Device) -> Result<u16> {
>>>> +        dev.genphy_read_status::<C45>()
>>>> +    }
>>>
>>> Probably a dumb Rust question. Shouldn't this have a ? at the end? It
>>> can return a negative error code.
>>
>> `read_status` returns a `Result<u16>` and `Device::genphy_read_status`
>> also returns a `Result<u16>`. In the function body we just delegate to
>> the latter, so no `?` is needed. We just return the entire result.
>>
>> Here is the equivalent pseudo-C code:
>>
>>     int genphy_read_status(struct phy_device *dev);
>>
>>     int read_status(struct phy_device *dev)
>>     {
>>         return genphy_read_status(dev);
>>     }
>>
>> There you also don't need an if for the negative error code, since it's
>> just propagated.
> 
> O.K, it seems to work. But one of the things we try to think about in
> the kernel is avoiding future bugs. Say sometime in the future i
> extend it:
> 
>     fn read_status(dev: &mut phy::Device) -> Result<u16> {
>         dev.genphy_read_status::<C45>()
> 
>         dev.genphy_read_foo()
>     }
> 
> By forgetting to add the ? to dev.genphy_read_status, have i just
> introduced a bug? Could i have avoided that by always having the ?
> even when it is not needed?

The above code will not compile, since there is a missing `;` in the
second line. If you try to do it with the semicolon:

    fn read_status(dev: &mut phy::Device) -> Result<u16> {
        dev.genphy_read_status::<C45>();
 
        dev.genphy_read_foo()
    }

Then you get this error:

    error: unused `core::result::Result` that must be used
      --> drivers/net/phy/qt2025.rs:88:9
       |
    88 |         dev.genphy_read_status::<C45>();
       |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
       |
       = note: this `Result` may be an `Err` variant, which should be handled
       = note: `-D unused-must-use` implied by `-D warnings`
       = help: to override `-D warnings` add `#[allow(unused_must_use)]`
    help: use `let _ = ...` to ignore the resulting value
       |
    88 |         let _ = dev.genphy_read_status::<C45>();
       |         +++++++

If you want to use `?` regardless, you will have to do this:
     
     fn read_status(dev: &mut phy::Device) -> Result<u16> {
         Ok(dev.genphy_read_status::<C45>()?)
     }

In my opinion this does not add significant protection for the scenario
that you outlined and is a lot more verbose. But if you're not used to
Rust, this might be different, since the code below looks more wrong:

    fn read_status(dev: &mut phy::Device) -> Result<u16> {
        Ok(dev.genphy_read_status::<C45>()?);
        
        dev.genphy_read_foo()
    }

But I would keep it the way it currently is.

---
Cheers,
Benno