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Message-Id: <20231028.182723.123878459003900402.fujita.tomonori@gmail.com> Date: Sat, 28 Oct 2023 18:27:23 +0900 (JST) From: FUJITA Tomonori <fujita.tomonori@...il.com> To: benno.lossin@...ton.me Cc: boqun.feng@...il.com, fujita.tomonori@...il.com, netdev@...r.kernel.org, rust-for-linux@...r.kernel.org, andrew@...n.ch, tmgross@...ch.edu, miguel.ojeda.sandonis@...il.com, wedsonaf@...il.com Subject: Re: [PATCH net-next v7 1/5] rust: core abstractions for network PHY drivers On Fri, 27 Oct 2023 21:19:38 +0000 Benno Lossin <benno.lossin@...ton.me> wrote: > On 10/27/23 21:59, Boqun Feng wrote: >> On Thu, Oct 26, 2023 at 09:10:46AM +0900, FUJITA Tomonori wrote: >> [...] >>> + /// Gets the current link state. >>> + /// >>> + /// It returns true if the link is up. >>> + pub fn is_link_up(&self) -> bool { >>> + const LINK_IS_UP: u32 = 1; >>> + // SAFETY: `phydev` is pointing to a valid object by the type invariant of `Self`. >>> + let phydev = unsafe { *self.0.get() }; >> >> Tomo, FWIW, the above line means *copying* the content pointed by >> `self.0.get()` into `phydev`, i.e. `phydev` is the semantically a copy >> of the `phy_device` instead of an alias. In C code, it means you did: > > Good catch. `phy_device` is rather large (did not look at the exact > size) and this will not be optimized on debug builds, so it could lead > to stackoverflows. > >> struct phy_device phydev = *ptr; >> >> Sure, both compilers can figure this out, therefore no extra copy is >> done, but still it's better to avoid this copy semantics by doing: >> >> let phydev = unsafe { &*self.0.get() }; > > We need to be careful here, since doing this creates a reference > `&bindings::phy_device` which asserts that it is immutable. That is not > the case, since the C side might change it at any point (this is the > reason we wrap things in `Opaque`, since that allows mutatation even > through sharde references). You meant that the C code might modify it independently anytime, not the C code called the Rust abstractions might modify it, right? > I did not notice this before, but this means we cannot use the `link` > function from bindgen, since that takes `&self`. We would need a > function that takes `*const Self` instead. Implementing functions to access to a bitfield looks tricky so we need to add such feature to bindgen or we add getters to the C side?
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