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

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Message-Id: <20231012.160246.2019423056896039320.fujita.tomonori@gmail.com>
Date: Thu, 12 Oct 2023 16:02:46 +0900 (JST)
From: FUJITA Tomonori <fujita.tomonori@...il.com>
To: fujita.tomonori@...il.com
Cc: boqun.feng@...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 Thu, 12 Oct 2023 15:44:44 +0900 (JST)
FUJITA Tomonori <fujita.tomonori@...il.com> wrote:

> 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.

Note `pub` isn't necessary here. I removed it.

No chance to misuse a Device instance as explained above, but if we
need to express the exclusive here, better to keep `mut`?