linux-kernel - Re: [PATCH v15 0/7] Rust Abstractions for PWM subsystem with TH1520 PWM driver

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Message-ID: <ayuqnx7fjfe3zwicvdbdr3qsgb4w6s2gwjt7r3m5ikzrylmium@hsrxazwelnvp>
Date: Mon, 13 Oct 2025 18:48:18 +0200
From: Uwe Kleine-König <ukleinek@...nel.org>
To: Michal Wilczynski <m.wilczynski@...sung.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>, 
	Drew Fustini <fustini@...nel.org>, Daniel Almeida <daniel.almeida@...labora.com>, 
	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, 
	Elle Rhumsaa <elle@...thered-steel.dev>, Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
Subject: Re: [PATCH v15 0/7] Rust Abstractions for PWM subsystem with TH1520
 PWM driver

Hello,

my diff on top of your changes looks as follows:

diff --git a/drivers/pwm/Kconfig b/drivers/pwm/Kconfig
index dd6db01832ee..e7f770ecfe84 100644
--- a/drivers/pwm/Kconfig
+++ b/drivers/pwm/Kconfig
@@ -812,9 +812,8 @@ config PWM_XILINX
 	  will be called pwm-xilinx.
 
  config RUST_PWM_ABSTRACTIONS
-	bool "Rust PWM abstractions support"
+	bool
 	depends on RUST
-	depends on PWM=y
 	help
 	  This option enables the safe Rust abstraction layer for the PWM
 	  subsystem. It provides idiomatic wrappers and traits necessary for

i.e. make RUST_PWM_ABSTRACTIONS invisible, it is only supposed to be
selected and there is little (or even no?) use to enable it without a
selector.

diff --git a/drivers/pwm/pwm_th1520.rs b/drivers/pwm/pwm_th1520.rs
index c9fd1d8d17bc..a5666052b7ce 100644
--- a/drivers/pwm/pwm_th1520.rs
+++ b/drivers/pwm/pwm_th1520.rs
@@ -121,6 +121,7 @@ fn round_waveform_tohw(
         wf: &pwm::Waveform,
     ) -> Result<pwm::RoundedWaveform<Self::WfHw>> {
         let data = chip.drvdata();
+        let status = 0;
 
         if wf.period_length_ns == 0 {
             dev_dbg!(chip.device(), "Requested period is 0, disabling PWM.\n");
@@ -141,18 +142,13 @@ fn round_waveform_tohw(
         if period_cycles == 0 {
             dev_dbg!(
                 chip.device(),
-                "Requested period {} ns is too small for clock rate {} Hz, disabling PWM.\n",
+                "Requested period {} ns is too small for clock rate {} Hz, rounding up.\n",
                 wf.period_length_ns,
                 rate_hz
             );
 
-            return Ok(pwm::RoundedWaveform {
-                status: 0,
-                hardware_waveform: Th1520WfHw {
-                    enabled: false,
-                    ..Default::default()
-                },
-            });
+            period_cycles = 1;
+            status = 1;
         }
 
         let mut duty_cycles = ns_to_cycles(wf.duty_length_ns, rate_hz).min(u64::from(u32::MAX));

i.e. round up for too small period requests ...

@@ -189,7 +185,7 @@ fn round_waveform_tohw(
         );
 
         Ok(pwm::RoundedWaveform {
-            status: 0,
+            status: status,
             hardware_waveform: wfhw,
         })
     }

... and return 1 then

@@ -355,7 +351,7 @@ fn probe(
                 "Clock rate {} Hz is too high, not supported.\n",
                 rate_hz
             );
-            return Err(ERANGE);
+            return Err(EINVAL);
         }
 
         let chip = pwm::Chip::new(

at least pwm-stm32 uses EINVAL. Having said that, I wonder if this check
is sensible here at all. pwm-stm32 does it to ensure that
mul_u64_u64_div_u64() does the right thing, but this exact issue doesn't
exist here.

Otherwise looks fine I think.

Best regards
Uwe

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