netdev - Re: [PATCH net-next v3 1/3] rust: core abstractions for network PHY drivers

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Message-Id: <20231012.154444.1868411153601666717.fujita.tomonori@gmail.com>
Date: Thu, 12 Oct 2023 15:44:44 +0900 (JST)
From: FUJITA Tomonori <fujita.tomonori@...il.com>
To: boqun.feng@...il.com
Cc: fujita.tomonori@...il.com, netdev@...r.kernel.org,
 rust-for-linux@...r.kernel.org, andrew@...n.ch,
 miguel.ojeda.sandonis@...il.com, greg@...ah.com, tmgross@...ch.edu
Subject: Re: [PATCH net-next v3 1/3] rust: core abstractions for network
 PHY drivers

On Wed, 11 Oct 2023 23:34:18 -0700
Boqun Feng <boqun.feng@...il.com> wrote:

> On Thu, Oct 12, 2023 at 02:58:24PM +0900, FUJITA Tomonori wrote:
>> On Wed, 11 Oct 2023 11:29:45 -0700
>> Boqun Feng <boqun.feng@...il.com> wrote:
>> 
>> > On Mon, Oct 09, 2023 at 10:39:10AM +0900, FUJITA Tomonori wrote:
>> > [...]
>> >> +impl Device {
>> >> +    /// Creates a new [`Device`] instance from a raw pointer.
>> >> +    ///
>> >> +    /// # Safety
>> >> +    ///
>> >> +    /// For the duration of the lifetime 'a, the pointer must be valid for writing and nobody else
>> >> +    /// may read or write to the `phy_device` object.
>> >> +    pub unsafe fn from_raw<'a>(ptr: *mut bindings::phy_device) -> &'a mut Self {
>> >> +        unsafe { &mut *ptr.cast() }
>> >> +    }
>> >> +
>> >> +    /// Gets the id of the PHY.
>> >> +    pub fn phy_id(&mut self) -> u32 {
>> > 
>> > This function doesn't modify the `self`, why does this need to be a
>> > `&mut self` function? Ditto for a few functions in this impl block.
>> > 
>> > It seems you used `&mut self` for all the functions, which looks like
>> > more design work is required here.
>> 
>> Ah, I can drop all the mut here.
> 
> It may not be that easy... IIUC, most of the functions in the `impl`
> block can only be called correctly with phydev->lock held. In other
> words, their usage requires exclusive accesses. We should somehow
> express this in the type system, otherwise someone may lose track on
> this requirement in the future (for example, calling any function
> without the lock held).
>
> A simple type trick comes to me is that
> 
> impl Device {
>     // rename `from_raw` into `assume_locked`
>     pub unsafe fn assume_locked<'a>(ptr: *mut bindings::phy_device) -> &'a LockedDevice {
> 	...
>     }
> }

Hmm, the concept of PHYLIB is that a driver never play with a
lock. From the perspective of PHYLIB, this abstraction is a PHY
driver. The abstraction should not touch the lock.

How can someone lose track on this requirement? The abstraction
creates a Device instance only inside the callbacks.