linux-kernel - Re: [PATCH v2 1/5] rust: Move property

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Message-Id: <D96IEJBTKCGK.J44WDQUEHXVV@buenzli.dev>
Date: Mon, 14 Apr 2025 18:40:54 +0200
From: "Remo Senekowitsch" <remo@...nzli.dev>
To: "Danilo Krummrich" <dakr@...nel.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>, "Greg
 Kroah-Hartman" <gregkh@...uxfoundation.org>, "Rafael J. Wysocki"
 <rafael@...nel.org>, <linux-kernel@...r.kernel.org>,
 <devicetree@...r.kernel.org>, <rust-for-linux@...r.kernel.org>
Subject: Re: [PATCH v2 1/5] rust: Move property_present to separate file

On Mon Apr 14, 2025 at 6:00 PM CEST, Danilo Krummrich wrote:
> On Mon, Apr 14, 2025 at 05:26:26PM +0200, Remo Senekowitsch wrote:
>> Not all property-related APIs can be exposed directly on a device.
>> For example, iterating over child nodes of a device will yield
>> fwnode_handle. Thus, in order to access properties on these child nodes,
>> duplicate the APIs on a fwnode as they are in C.
>
> What do you mean with "duplicate the APIs"?

The property related methods on `Device` and `FwNode` are mostly
the same. Same names, same function signatures. The methods on
`Device` simply call `self.fwnode()` and then the same method on
`FwNode`. I was asked to do this so driver authors don't have to write
`dev.fwnode().read_propert()` etc. every time they want to do something
on the device itself. This is the case here for `property_present` as
well as the methods added in the following patches.

> Also, this patch does three separate things.
>
>   (1) move property_present() to separate file
>   (2) implement FwNode abstraction
>   (3) implement Device::fwnode()
>
> I'd rather keep property_present() in device.rs and also move fwnode() into
> device.rs, even though in C it's in property.c. impl Device blocks should be in
> device.rs.

I was asked to move it by Rob Herring on Zulip[1]. Dirk Behme also
agreed with it[2]. I have no strong opinion on this, hopefully you can
come to an agreement. Whatever it is, I'll happily implement it.

>> +++ b/rust/kernel/property.rs
>> @@ -0,0 +1,75 @@
>> +// SPDX-License-Identifier: GPL-2.0
>> +
>> +//! Unified device property interface.
>> +//!
>> +//! C header: [`include/linux/property.h`](srctree/include/linux/property.h)
>> +
>> +use core::ptr;
>> +
>> +use crate::{bindings, device::Device, str::CStr, types::Opaque};
>> +
>> +impl Device {
>> +    /// Obtain the fwnode corresponding to the device.
>> +    fn fwnode(&self) -> &FwNode {
>> +        // SAFETY: `self` is valid.
>> +        let fwnode_handle = unsafe { bindings::__dev_fwnode(self.as_raw()) };
>> +        if fwnode_handle.is_null() {
>> +            panic!("fwnode_handle cannot be null");
>
> Please don't use panic!() unless you hit an error condition that is impossible
> to handle.
>
> If __dev_fwnode() can ever return NULL, make this function return
> Option<&FwNode> instead.

Rob Herring mentioned this in the previous thread already:

> Users/drivers testing fwnode_handle/of_node for NULL is pretty common.
> Though often that's a legacy code path, so maybe not allowing NULL is
> fine for now.

I asked a follow-up question about how to proceed but got no reply[3].

Best regards,
Remo

Link: https://rust-for-linux.zulipchat.com/#narrow/channel/288089-General/topic/DS90UB954.20driver.20done.2C.20ready.20to.20upstream.3F/near/495807346 [1]
Link: https://rust-for-linux.zulipchat.com/#narrow/channel/288089-General/topic/DS90UB954.20driver.20done.2C.20ready.20to.20upstream.3F/near/495945200 [2]
Link: https://lore.kernel.org/rust-for-linux/D96HP1UV49SY.1U1BFA14P8XHE@buenzli.dev/T/#mc7967352b7bd1be812806939150511c1a5018bb1 [3]