linux-kernel - Re: [PATCH V6 04/15] rust: device: Add few helpers

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Message-ID: <2025010734-march-cultivate-bd96@gregkh>
Date: Tue, 7 Jan 2025 12:56:08 +0100
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: Viresh Kumar <viresh.kumar@...aro.org>
Cc: "Rafael J. Wysocki" <rafael@...nel.org>,
	Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
	Danilo Krummrich <dakr@...hat.com>, Miguel Ojeda <ojeda@...nel.org>,
	Alex Gaynor <alex.gaynor@...il.com>,
	Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Benno Lossin <benno.lossin@...ton.me>,
	Andreas Hindborg <a.hindborg@...nel.org>,
	Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>,
	Danilo Krummrich <dakr@...nel.org>, linux-pm@...r.kernel.org,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Stephen Boyd <sboyd@...nel.org>, Nishanth Menon <nm@...com>,
	rust-for-linux@...r.kernel.org,
	Manos Pitsidianakis <manos.pitsidianakis@...aro.org>,
	Erik Schilling <erik.schilling@...aro.org>,
	Alex Bennée <alex.bennee@...aro.org>,
	Joakim Bech <joakim.bech@...aro.org>, Rob Herring <robh@...nel.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH V6 04/15] rust: device: Add few helpers

On Tue, Jan 07, 2025 at 04:51:37PM +0530, Viresh Kumar wrote:
> Add from_cpu() and property_present() helpers to the device bindings.

That says what, but not why.

Also those are two totally different things, I'm going to argue with you
about one of them...

> +    /// Creates a new ref-counted instance of device of a CPU.
> +    pub fn from_cpu(cpu: u32) -> Result<ARef<Self>> {

Why is this a reference counted device, yet the C structure is NOT
properly reference counted at all?  Are you _sure_ this is going to work
properly?

And really, we should fix up the C side to properly reference count all
of this.  Just read the comment in cpu_device_release() for a hint at
what needs to be done here.

> +        // SAFETY: It is safe to call `get_cpu_device()` for any CPU number.

For any number at all, no need to say "CPU" here, right?

> +        let ptr = unsafe { bindings::get_cpu_device(cpu) };
> +        if ptr.is_null() {
> +            return Err(ENODEV);
> +        }
> +
> +        // SAFETY: By the safety requirements, ptr is valid.
> +        Ok(unsafe { Device::get_device(ptr) })

So why is this device reference counted?  I get it that it should be,
but how does that play with the "real" device here?

> +    }
> +
>      /// Obtain the raw `struct device *`.
>      pub(crate) fn as_raw(&self) -> *mut bindings::device {
>          self.0.get()
> @@ -180,6 +195,12 @@ unsafe fn printk(&self, klevel: &[u8], msg: fmt::Arguments<'_>) {
>              )
>          };
>      }
> +
> +    /// Checks if property is present or not.
> +    pub fn property_present(&self, name: &CString) -> bool {
> +        // SAFETY: `name` is null-terminated. `self.as_raw` is valid because `self` is valid.
> +        unsafe { bindings::device_property_present(self.as_raw(), name.as_ptr() as *const _) }

is "self.as_raw()" a constant pointer too?

thanks,

greg k-h