Re: [PATCH v3 1/3] pwm: make it possible to apply pwm changes in atomic context - Uwe Kleine-König

From: "Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>
To: Sean Young <sean@mess.org>
Cc: "Daniel Thompson" <daniel.thompson@linaro.org>,
	"Hans de Goede" <hdegoede@redhat.com>,
	linux-media@vger.kernel.org, linux-pwm@vger.kernel.org,
	"Ivaylo Dimitrov" <ivo.g.dimitrov.75@gmail.com>,
	"Thierry Reding" <thierry.reding@gmail.com>,
	"Jonathan Corbet" <corbet@lwn.net>,
	"Jani Nikula" <jani.nikula@linux.intel.com>,
	"Joonas Lahtinen" <joonas.lahtinen@linux.intel.com>,
	"Rodrigo Vivi" <rodrigo.vivi@intel.com>,
	"Tvrtko Ursulin" <tvrtko.ursulin@linux.intel.com>,
	"David Airlie" <airlied@gmail.com>,
	"Daniel Vetter" <daniel@ffwll.ch>,
	"Javier Martinez Canillas" <javierm@redhat.com>,
	"Jean Delvare" <jdelvare@suse.com>,
	"Guenter Roeck" <linux@roeck-us.net>,
	"Support Opensource" <support.opensource@diasemi.com>,
	"Dmitry Torokhov" <dmitry.torokhov@gmail.com>,
	"Pavel Machek" <pavel@ucw.cz>, "Lee Jones" <lee@kernel.org>,
	"Mauro Carvalho Chehab" <mchehab@kernel.org>,
	"Ilpo Järvinen" <ilpo.jarvinen@linux.intel.com>,
	"Mark Gross" <markgross@kernel.org>,
	"Liam Girdwood" <lgirdwood@gmail.com>,
	"Mark Brown" <broonie@kernel.org>,
	"Jingoo Han" <jingoohan1@gmail.com>,
	"Helge Deller" <deller@gmx.de>,
	linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
	intel-gfx@lists.freedesktop.org, dri-devel@lists.freedesktop.org,
	linux-hwmon@vger.kernel.org, linux-input@vger.kernel.org,
	linux-leds@vger.kernel.org, platform-driver-x86@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org,
	linux-fbdev@vger.kernel.org
Subject: Re: [PATCH v3 1/3] pwm: make it possible to apply pwm changes in atomic context
Date: Wed, 25 Oct 2023 12:47:43 +0200	[thread overview]
Message-ID: <20231025104743.56elaloj3jmojz2v@pengutronix.de> (raw)
In-Reply-To: <ZTjll7oTNVWqygbD@gofer.mess.org>

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Hello,

On Wed, Oct 25, 2023 at 10:53:27AM +0100, Sean Young wrote:
> On Mon, Oct 23, 2023 at 02:34:17PM +0100, Daniel Thompson wrote:
> > On Sun, Oct 22, 2023 at 11:46:22AM +0100, Sean Young wrote:
> > > On Sat, Oct 21, 2023 at 11:08:22AM +0200, Hans de Goede wrote:
> > > > On 10/19/23 12:51, Uwe Kleine-König wrote:
> > > > > On Wed, Oct 18, 2023 at 03:57:48PM +0200, Hans de Goede wrote:
> > > > >> On 10/17/23 11:17, Sean Young wrote:
> > > > > I think it's very subjective if you consider this
> > > > > churn or not.
> > > >
> > > > I consider it churn because I don't think adding a postfix
> > > > for what is the default/expected behavior is a good idea
> > > > (with GPIOs not sleeping is the expected behavior).
> > > >
> > > > I agree that this is very subjective and very much goes
> > > > into the territory of bikeshedding. So please consider
> > > > the above my 2 cents on this and lets leave it at that.
> > >
> > > You have a valid point. Let's focus on having descriptive function names.
> > 
> > For a couple of days I've been trying to resist the bikeshedding (esp.
> > given the changes to backlight are tiny) so I'll try to keep it as
> > brief as I can:
> > 
> > 1. I dislike the do_it() and do_it_cansleep() pairing. It is
> >    difficult to detect when a client driver calls do_it() by mistake.
> >    In fact a latent bug of this nature can only be detected by runtime
> >    testing with the small number of PWMs that do not support
> >    configuration from an atomic context.
> > 
> >    In contrast do_it() and do_it_atomic()[*] means that although
> >    incorrectly calling do_it() from an atomic context can be pretty
> >    catastrophic it is also trivially detected (with any PWM driver)
> >    simply by running with CONFIG_DEBUG_ATOMIC_SLEEP.

Wrongly calling the atomic variant (no matter how it's named) in a
context where sleeping is possible is only a minor issue. Being faster
than necessary is hardly a problem, so it only hurts by not being an
preemption point with PREEMPT_VOLUNTARY which might not even be relevant
because we're near to a system call anyhow.

For me the naming is only very loosely related to the possible bugs. I
think calling the wrong function happens mainly because the driver author
isn't aware in which context the call happens and not because of wrong
assumptions about the sleepiness of a certain function call.
If you consider this an argument however, do_it + do_it_cansleep is
better than do_it_atomic + do_it as wrongly assuming do_it would sleep
is less bad than wrongly assuming do_it wouldn't sleep. (The latter is
catched by CONFIG_DEBUG_ATOMIC_SLEEP, but only if it's enabled.)

Having said that while my subjective preference ordering is (with first
= best):

	do_it + do_it_cansleep
	do_it_atomic + do_it_cansleep
	do_it_atomic + do_it

wi(th a _might_sleep or _mightsleep suffix ranging below _cansleep)
I wouldn't get sleepless nights when I get overruled here
(uwe_cansleep :-).

> >    No objections (beyond churn) to fully spelt out pairings such as
> >    do_it_cansleep() and do_it_atomic()[*]!
> 
> I must say I do like the look of this. Uwe, how do you feel about:
> pwm_apply_cansleep() and pwm_apply_atomic()? I know we've talked about
> pwm_apply_atomic in the past, however I think this this the best 
> option I've seen so far.
> 
> > 2. If there is an API rename can we make sure the patch contains no
> >    other changes (e.g. don't introduce any new API in the same patch).
> >    Seperating renames makes the patches easier to review!
> >    It makes each one smaller and easier to review!
> 
> Yes, this should have been separated out. Will fix for next version.

+1

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | https://www.pengutronix.de/ |

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