linux-kernel - Re: [PATCH v2 0/7] Rust Abstractions for PWM subsystem with TH1520 PWM driver

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Message-ID: <b3252042-e7fd-4ac9-a4ad-0226d21fd477@samsung.com>
Date: Thu, 12 Jun 2025 15:27:09 +0200
From: Michal Wilczynski <m.wilczynski@...sung.com>
To: Uwe Kleine-König <ukleinek@...nel.org>, Drew Fustini
	<drew@...7.com>
Cc: 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>, Danilo Krummrich <dakr@...nel.org>, 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>,
	Stephen Boyd <sboyd@...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,
	linux-clk@...r.kernel.org
Subject: Re: [PATCH v2 0/7] Rust Abstractions for PWM subsystem with TH1520
 PWM driver



On 6/12/25 07:01, Uwe Kleine-König wrote:
> Hello Drew,
> 
> On Wed, Jun 11, 2025 at 04:52:22PM -0700, Drew Fustini wrote:
>> I also enabled the pwm fan driver. However, there is a probe failure:
>>
>> [    1.250921] pwm-fan pwm-fan: Failed to configure PWM: -524
>> [    1.256546] pwm-fan pwm-fan: probe with driver pwm-fan failed with error -524
> 
> 524 = ENOTSUPP, so it seems the request had duty_offset > 0. Does your
> fan use PWM_POLARITY_INVERTED? If so, try without that flag. If your fan
> really needs an inverted PWM this of course makes fan control buggy.
> With the next revision it should work fine (as a duty_offset > 0 should
> get rounded down to 0).

Since we're running the same DT, the polarity shouldn't be inverted. I
see you have CONFIG_PWM_DEBUG=y enabled, which is most likely the reason
the probe fails.

With that option, the following check is performed in __pwm_apply:

if (IS_ENABLED(CONFIG_PWM_DEBUG)) {
	struct pwm_waveform wf_rounded;

		err = __pwm_round_waveform_fromhw(chip, pwm, &wfhw, &wf_rounded);
		if (err)
			return err;

In this revision of the driver, I have not implemented the read-waveform
callbacks, so the Rust PWM abstractions correctly return -ENOTSUPP.

Uwe, this poses a problem, as reading from the duty and period registers
on the TH1520 SoC's PWM controller appears to be broken.


> 
> Best regards
> Uwe

Best regards,
-- 
Michal Wilczynski <m.wilczynski@...sung.com>