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Message-ID: <f0158d9e98734d325fbde643ca982332c40980cd.camel@posteo.de>
Date: Thu, 20 Nov 2025 13:00:09 +0000
From: Markus Probst <markus.probst@...teo.de>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Lee Jones <lee@...nel.org>, Pavel Machek <pavel@...nel.org>, Greg
Kroah-Hartman <gregkh@...uxfoundation.org>, Dave Ertman
<david.m.ertman@...el.com>, Ira Weiny <ira.weiny@...el.com>, Leon
Romanovsky <leon@...nel.org>, 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 <lossin@...nel.org>, Andreas
Hindborg <a.hindborg@...nel.org>, Trevor Gross <tmgross@...ch.edu>, Danilo
Krummrich <dakr@...nel.org>, "Rafael J. Wysocki" <rafael@...nel.org>,
Bjorn Helgaas <bhelgaas@...gle.com>, Krzysztof Wilczyński
<kwilczynski@...nel.org>, rust-for-linux@...r.kernel.org,
linux-leds@...r.kernel.org, linux-kernel@...r.kernel.org,
linux-pci@...r.kernel.org
Subject: Re: [PATCH v9 1/3] rust: leds: add basic led classdev abstractions
On Thu, 2025-11-20 at 11:34 +0000, Alice Ryhl wrote:
> On Wed, Nov 19, 2025 at 02:11:21PM +0000, Markus Probst wrote:
> > Implement the core abstractions needed for led class devices, including:
> >
> > * `led::LedOps` - the trait for handling leds, including
> > `brightness_set`, `brightness_get` and `blink_set`
> >
> > * `led::InitData` - data set for the led class device
> >
> > * `led::Device` - a safe wrapper around `led_classdev`
> >
> > Signed-off-by: Markus Probst <markus.probst@...teo.de>
> > ---
> > MAINTAINERS | 7 +
> > rust/kernel/led.rs | 472 +++++++++++++++++++++++++++++++++++++++++++++++++++++
> > rust/kernel/lib.rs | 1 +
> > 3 files changed, 480 insertions(+)
> >
> > diff --git a/MAINTAINERS b/MAINTAINERS
> > index b71ea515240a..80cb030911b7 100644
> > --- a/MAINTAINERS
> > +++ b/MAINTAINERS
> > @@ -14112,6 +14112,13 @@ F: drivers/leds/
> > F: include/dt-bindings/leds/
> > F: include/linux/leds.h
> >
> > +LED SUBSYSTEM [RUST]
> > +M: Markus Probst <markus.probst@...teo.de>
> > +L: linux-leds@...r.kernel.org
> > +L: rust-for-linux@...r.kernel.org
> > +S: Maintained
> > +F: rust/kernel/led.rs
> > +
> > LEGO MINDSTORMS EV3
> > R: David Lechner <david@...hnology.com>
> > S: Maintained
> > diff --git a/rust/kernel/led.rs b/rust/kernel/led.rs
> > new file mode 100644
> > index 000000000000..fca55f02be8d
> > --- /dev/null
> > +++ b/rust/kernel/led.rs
> > @@ -0,0 +1,472 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +
> > +//! Abstractions for the leds driver model.
> > +//!
> > +//! C header: [`include/linux/leds.h`](srctree/include/linux/leds.h)
> > +
> > +use core::{
> > + marker::PhantomData,
> > + mem::transmute,
> > + pin::Pin,
> > + ptr::NonNull, //
> > +};
> > +
> > +use pin_init::{
> > + pin_data,
> > + pinned_drop,
> > + PinInit, //
> > +};
> > +
> > +use crate::{
> > + build_error,
> > + container_of,
> > + device::{
> > + self,
> > + property::FwNode,
> > + AsBusDevice,
> > + Bound, //
> > + },
> > + devres::Devres,
> > + error::{
> > + code::EINVAL,
> > + from_result,
> > + to_result,
> > + Error,
> > + Result,
> > + VTABLE_DEFAULT_ERROR, //
> > + },
> > + macros::vtable,
> > + str::CStr,
> > + try_pin_init,
> > + types::{
> > + ARef,
> > + Opaque, //
> > + }, //
> > +};
>
> Please import kernel::prelude::* and remove all the imports that are
> available from the prelude.
>
> > +impl<'a> InitData<'a> {
> > + /// Sets the firmware node
> > + pub fn fwnode(self, fwnode: Option<ARef<FwNode>>) -> Self {
>
> I'm thinking that perhaps this should just be a `&'a FwNode` instead?
> That way, you can increment the refcount in Device::new() if
> registration is successful.
This was the way I have done it in v8. I issue with this approch is, if
the fwnode is optional, you have to do this ugly code:
let mut init_data = InitData::new()
.default_label(c_str!(":label"))
.devicename(c_str!("devicename"));
let child_fwnode = fwnode.child_by_name(c_str!("led"));
if let Some(child_fwnode_ref) = &child_fwnode {
init_data = init_data.fwnode(child_fwnode_ref);
}
Instead of
let mut init_data = InitData::new()
.fwnode(fwnode.child_by_name(c_str!("led")))
.default_label(c_str!(":label"))
.devicename(c_str!("devicename"));
Furthermore, most of the functions in the rust abstractions return a
`ARef<FwNode>` anyway. The only exception I found is
`device::Device::fwnode()`, but an led driver usually has more than one
led to its unlikely that the root fwnode will be directly passed to an
led classdev. As a result with the `&'a FwNode` approach, the led
abstraction would increment the reference count once, and the driver
will then decrement it once (dropping of the ARef<FwNode>).
Sounds like a tiny overhead to me.
>
> > + Self { fwnode, ..self }
> > + }
> > +
> > + /// Sets a default label
>
> There are many missing periods in doc-comments.
>
> > +/// Trait defining the operations for a LED driver.
> > +///
> > +/// # Examples
> > +///
> > +///```
> > +/// # use kernel::{
> > +/// # c_str, device, devres::Devres,
> > +/// # error::Result, led,
> > +/// # macros::vtable, platform, prelude::*,
> > +/// # };
> > +/// # use core::pin::Pin;
> > +///
> > +/// struct MyLedOps;
>
> When using # in examples, please do not have an empty line before
> beginning the example. It shows up as a weird extra empty line in the
> rendered docs.
>
> You could consider just making the imports displayed here also.
>
> Also the ``` both above and below the example usually has a space:
>
> /// ```
>
> rather than
>
> ///```
>
> > + // SAFETY:
> > + // - `parent.as_raw()` is guaranteed to be a pointer to a valid `device`
> > + // or a null pointer.
> > + // - `ptr` is guaranteed to be a pointer to an initialized `led_classdev`.
> > + to_result(unsafe {
> > + bindings::led_classdev_register_ext(
> > + parent.as_ref().as_raw(),
> > + ptr,
> > + &mut init_data_raw,
> > + )
> > + })?;
> > +
> > + core::mem::forget(init_data.fwnode); // keep the reference count incremented
>
> This led abstraction implicitly takes a refcount on the fwnode and then
> drops it when the device is unbound.
I did look through the led classdev code and noticed that the fwnode
refcount isn't incremented. From what I can tell, the driver is
responsible to ensure the fwnode refcount never hits 0 while the led
classdev is registered. Thats why I incremented the refcount of the
fwnode to ensure the safety requirement is met.
Thanks
- Markus Probst
>
> Lee, can you confirm that taking a refcount on the fwnode is the right
> way to use the LED subsytem?
>
> Alice
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