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

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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