linux-kernel - Re: [PATCH v5 4/9] rust: device: Enable printing fwnode name and path

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Message-Id: <DA220Y73P1NR.192OYSQH3UD7A@buenzli.dev>
Date: Wed, 21 May 2025 20:36:10 +0200
From: "Remo Senekowitsch" <remo@...nzli.dev>
To: "Greg Kroah-Hartman" <gregkh@...uxfoundation.org>
Cc: "Rob Herring" <robh@...nel.org>, "Saravana Kannan"
 <saravanak@...gle.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>, "Rafael J. Wysocki" <rafael@...nel.org>,
 "Dirk Behme" <dirk.behme@...bosch.com>, <linux-kernel@...r.kernel.org>,
 <devicetree@...r.kernel.org>, <rust-for-linux@...r.kernel.org>
Subject: Re: [PATCH v5 4/9] rust: device: Enable printing fwnode name and
 path

On Wed May 21, 2025 at 6:58 PM CEST, Greg Kroah-Hartman wrote:
> On Wed, May 21, 2025 at 03:03:07PM +0200, Remo Senekowitsch wrote:
>> On Wed May 21, 2025 at 2:02 PM CEST, Greg Kroah-Hartman wrote:
>> > On Tue, May 20, 2025 at 10:00:19PM +0200, Remo Senekowitsch wrote:
>> >> Add two new public methods `display_name` and `display_path` to
>> >> `FwNode`. They can be used by driver authors for logging purposes. In
>> >> addition, they will be used by core property abstractions for automatic
>> >> logging, for example when a driver attempts to read a required but
>> >> missing property.
>> >> 
>> >> Signed-off-by: Remo Senekowitsch <remo@...nzli.dev>
>> >> ---
>> >>  rust/kernel/device/property.rs | 72 ++++++++++++++++++++++++++++++++++
>> >>  1 file changed, 72 insertions(+)
>> >> 
>> >> diff --git a/rust/kernel/device/property.rs b/rust/kernel/device/property.rs
>> >> index 70593343bd811..6ccc7947f9c31 100644
>> >> --- a/rust/kernel/device/property.rs
>> >> +++ b/rust/kernel/device/property.rs
>> >> @@ -32,6 +32,78 @@ pub(crate) fn as_raw(&self) -> *mut bindings::fwnode_handle {
>> >>          self.0.get()
>> >>      }
>> >>  
>> >> +    /// Returns an object that implements [`Display`](core::fmt::Display) for
>> >> +    /// printing the name of a node.
>> >> +    pub fn display_name(&self) -> impl core::fmt::Display + '_ {
>> >> +        struct FwNodeDisplayName<'a>(&'a FwNode);
>> >> +
>> >> +        impl core::fmt::Display for FwNodeDisplayName<'_> {
>> >> +            fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
>> >> +                // SAFETY: self is valid by its type invariant
>> >> +                let name = unsafe { bindings::fwnode_get_name(self.0.as_raw()) };
>> >> +                if name.is_null() {
>> >> +                    return Ok(());
>> >
>> > So if there is no name, you are returning Ok()?  Are you sure that's ok
>> > to do?  What will the result of the string look like then?
>> 
>> In that case we're not writing anything to the formatter, which is
>> equivalent to an empty string. `Ok(())` means that writing succeeded.
>> 
>> I assumed that a valid node would always have a name. And we're
>> guaranteed to have a valid node. So I assumed this case would never
>> happen and didn't think too hard about it. But even if a valid node has
>> not name, empty string is probably the correct thing, right?
>
> I don't know what this "name" is used for.  An empty string might not be
> what you want to use here, given that you could be naming something
> based on it, right?  fwnode_get_name() is used for many things,
> including the detection if a name is not present at all, and if not,
> then the code needs to clean up and abort.
>
> So what exactly are you going to be using this for?

Valid question... I'm not using it for anything. I was trying to match
the C API which has capabilities to print a fwnode's name and full path.
I think the format specifiers are %pfwP and %pfwf if I'm reading the
source correctly. It also looks to me like the C API for printing fwnode
names does the same thing - print nothing if the name is null. [1] [2]

[1] https://elixir.bootlin.com/linux/v6.14.7/source/lib/vsprintf.c#L2242
[2] https://elixir.bootlin.com/linux/v6.14.7/source/lib/vsprintf.c#L711