linux-kernel - Re: [PATCH v10 0/7] Rust Abstractions for PWM subsystem with TH1520 PWM driver

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Message-ID: <cbxpqormchajfcnf7xxopd7j7igriqus4cuu5jfvxb3mbfb5tu@qz4rc67vjyif>
Date: Thu, 10 Jul 2025 22:57:12 +0200
From: Uwe Kleine-König <ukleinek@...nel.org>
To: Danilo Krummrich <dakr@...nel.org>
Cc: Michal Wilczynski <m.wilczynski@...sung.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>, Andreas Hindborg <a.hindborg@...nel.org>, 
	Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>, Guo Ren <guoren@...nel.org>, 
	Fu Wei <wefu@...hat.com>, Rob Herring <robh@...nel.org>, 
	Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley <conor+dt@...nel.org>, 
	Paul Walmsley <paul.walmsley@...ive.com>, Palmer Dabbelt <palmer@...belt.com>, 
	Albert Ou <aou@...s.berkeley.edu>, Alexandre Ghiti <alex@...ti.fr>, 
	Marek Szyprowski <m.szyprowski@...sung.com>, Benno Lossin <lossin@...nel.org>, 
	Michael Turquette <mturquette@...libre.com>, Drew Fustini <fustini@...nel.org>, linux-kernel@...r.kernel.org, 
	linux-pwm@...r.kernel.org, rust-for-linux@...r.kernel.org, linux-riscv@...ts.infradead.org, 
	devicetree@...r.kernel.org, Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
Subject: Re: [PATCH v10 0/7] Rust Abstractions for PWM subsystem with TH1520
 PWM driver

On Thu, Jul 10, 2025 at 06:06:26PM +0200, Danilo Krummrich wrote:
> On Thu Jul 10, 2025 at 5:25 PM CEST, Uwe Kleine-König wrote:
> > Hello Michal,
> >
> > On Thu, Jul 10, 2025 at 03:48:08PM +0200, Michal Wilczynski wrote:
> >> On 7/10/25 15:10, Uwe Kleine-König wrote:
> >> > On Thu, Jul 10, 2025 at 10:42:07AM +0200, Michal Wilczynski wrote:
> >> >> On 7/7/25 11:48, Michal Wilczynski wrote:
> >> >>> The series is structured as follows:
> >> >>>  - Expose static function pwmchip_release.
> >> > 
> >> > Is this really necessary? I didn't try to understand the requirements
> >> > yet, but I wonder about that. If you get the pwmchip from
> >> > __pwmchip_add() the right thing to do to release it is to call
> >> > pwmchip_remove(). Feels like a layer violation.
> >> 
> >> It's required to prevent a memory leak in a specific, critical failure
> >> scenario. The sequence of events is as follows:
> >> 
> >>     pwm::Chip::new() succeeds, allocating both the C struct pwm_chip and
> >>     the Rust drvdata.
> >> 
> >>     pwm::Registration::register() (which calls pwmchip_add()) fails for
> >>     some reason.
> >
> 
> (Just trying to help clear up the confusion.)

Very appreciated!

> > If you called pwmchip_alloc() but not yet pwmchip_add(), the right
> > function to call for cleanup is pwmchip_put().
> 
> That is exactly what is happening when ARef<Chip> is dropped. If the reference
> count drops to zero, pwmchip_release() is called, which frees the chip. However,
> this would leave the driver's private data allocation behind, which is owned by
> the Chip instance.

I don't understand that. The chip and the driver private data both are
located in the same allocation. How is this a problem of the driver
private data only then? The kfree() in pwmchip_release() is good enough
for both?!

> So, in Rust we not only have to free the chip itself on release, but also the
> driver's private data. The solution Michal went for is overwriting the PWM
> chip's dev->release() with a callback that drops the driver's private data and
> subsequently calls the "original" pwmchip_release().
> 
> This is a common pattern in Rust that we use in DRM as well. One thing that is
> different in DRM is, that a struct drm_device (equivalent of struct pwm_chip in
> this case), has it's own release callback for drivers that we can attach to.
> 
> PWM does not have such a callback AFAICS, hence the Rust abstraction uses the
> underlying device's release callback and then forwards to pwmchip_release().
> 
> Hope this helps. :)

Not yet ... :-)

Best regards
Uwe

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