linux-kernel - Re: [PATCH v4 1/8] pwm: Declare waveform stubs for when PWM is not reachable

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Message-ID: <xklptdwk7skvil6axqeef5xs7jqeotbo5g6r62tvdfpluzozfe@sdrmdzmxoz7x>
Date: Mon, 13 Oct 2025 10:58:38 +0200
From: Uwe Kleine-König <ukleinek@...nel.org>
To: David Lechner <dlechner@...libre.com>
Cc: Marcelo Schmitt <marcelo.schmitt@...log.com>, 
	linux-iio@...r.kernel.org, devicetree@...r.kernel.org, linux-doc@...r.kernel.org, 
	linux-pwm@...r.kernel.org, linux-spi@...r.kernel.org, linux-kernel@...r.kernel.org, 
	jic23@...nel.org, michael.hennerich@...log.com, nuno.sa@...log.com, 
	eblanc@...libre.com, andy@...nel.org, robh@...nel.org, krzk+dt@...nel.org, 
	conor+dt@...nel.org, corbet@....net, marcelo.schmitt1@...il.com, 
	kernel test robot <lkp@...el.com>
Subject: Re: [PATCH v4 1/8] pwm: Declare waveform stubs for when PWM is not
 reachable

On Fri, Oct 10, 2025 at 11:34:49AM -0500, David Lechner wrote:
> On 10/9/25 11:58 AM, Uwe Kleine-König wrote:
> > Hello,
> > 
> > On Wed, Oct 08, 2025 at 10:49:44AM -0300, Marcelo Schmitt wrote:
> >> Previously, the PWM waveform consumer API would not be declared if
> >> CONFIG_PWM was not reachable. That caused kernel builds to fail if a
> >> consumer driver was enabled but PWM disabled. Add stubs for PWM waveform
> >> functions so client drivers that use, but don't depend on PWM, can build if
> >> PWM is disabled.
> >>
> >> Fixes: 6c5126c6406d ("pwm: Provide new consumer API functions for waveforms")
> >> Reported-by: kernel test robot <lkp@...el.com>
> >> Closes: https://lore.kernel.org/oe-kbuild-all/202509272028.0zLNiR5w-lkp@intel.com/
> >> Signed-off-by: Marcelo Schmitt <marcelo.schmitt@...log.com>
> >> ---
> >> I've included this patch in this series because it should save us from being
> >> notified by 0-day about the build failure this patch fixes. From contributor's
> >> perspective, it's easier to have this patch together with the rest of ad4030
> >> series. Though, no objection if kernel maintainers decide to pick it [1] through
> >> the PWM tree.
> >>
> >> [1]: https://lore.kernel.org/linux-pwm/1ac0fc529e02744aacfcb9140ed597ff60886f39.1759873890.git.marcelo.schmitt@analog.com/
> > 
> > TL;DR: nack
> > 
> > I replied to the original submission about why this patch is wrong. See
> > there for the details.
> 
> If we want to avoid this patch, then it sounds like we should use:
> 
> #if IS_REACHABLE(CONFIG_PWM)
> 
> in the ADC driver around any PWM waveform code.

Or use IS_REACHABLE(CONFIG_SPI_OFFLOAD_TRIGGER_PWM) which might be
nearer to the actual dependency. (Hmm, but looking in the driver, it
handles the pwm device and setting directly, I would have expected that
code specific to SPI_OFFLOAD_TRIGGER_PWM does that and the driver
doesn't even see the pwm.)

Or make the driver depend on CONFIG_PWM, which is cheap.

Or if the PWM support is really optional, convince me to add the stubs.

Best regards
Uwe

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