linux-kernel - Re: [PATCH v2 1/3] pwm: make it possible to apply pwm changes in atomic context

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Message-ID: <20231013180449.mcdmklbsz2rlymzz@pengutronix.de>
Date:   Fri, 13 Oct 2023 20:04:49 +0200
From:   Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
To:     Thierry Reding <thierry.reding@...il.com>
Cc:     Sean Young <sean@...s.org>, linux-media@...r.kernel.org,
        Ivaylo Dimitrov <ivo.g.dimitrov.75@...il.com>,
        linux-pwm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/3] pwm: make it possible to apply pwm changes in
 atomic context

Hello,

On Fri, Oct 13, 2023 at 05:34:34PM +0200, Thierry Reding wrote:
> On Fri, Oct 13, 2023 at 03:58:30PM +0100, Sean Young wrote:
> > On Fri, Oct 13, 2023 at 01:51:40PM +0200, Thierry Reding wrote:
> > > On Fri, Oct 13, 2023 at 11:46:14AM +0100, Sean Young wrote:
> > > [...]
> > > > diff --git a/include/linux/pwm.h b/include/linux/pwm.h
> > > > index d2f9f690a9c1..93f166ab03c1 100644
> > > > --- a/include/linux/pwm.h
> > > > +++ b/include/linux/pwm.h
> > > > @@ -267,6 +267,7 @@ struct pwm_capture {
> > > >   * @get_state: get the current PWM state. This function is only
> > > >   *	       called once per PWM device when the PWM chip is
> > > >   *	       registered.
> > > > + * @atomic: can the driver execute pwm_apply_state in atomic context
> > > >   * @owner: helps prevent removal of modules exporting active PWMs
> > > >   */
> > > >  struct pwm_ops {
> > > > @@ -278,6 +279,7 @@ struct pwm_ops {
> > > >  		     const struct pwm_state *state);
> > > >  	int (*get_state)(struct pwm_chip *chip, struct pwm_device *pwm,
> > > >  			 struct pwm_state *state);
> > > > +	bool atomic;
> > > >  	struct module *owner;
> > > >  };
> > > 
> > > As I mentioned earlier, this really belongs in struct pwm_chip rather
> > > than struct pwm_ops. I know that Uwe said this is unlikely to happen,
> > > and that may be true, but at the same time it's not like I'm asking
> > > much. Whether you put this in struct pwm_ops or struct pwm_chip is
> > > about the same amount of code, and putting it into pwm_chip is much
> > > more flexible, so it's really a no-brainer.
> > 
> > Happy to change this of course. I changed it and then changed it back after
> > Uwe's comment, I'll fix this in the next version.
> > 
> > One tiny advantage is that pwm_ops is static const while pwm_chip is
> > allocated per-pwm, so will need instructions for setting the value. Having
> > said that, the difference is tiny, it's a single bool.
> 
> Yeah, it's typically a single assignment, so from a code point of view
> it should be pretty much the same. I suppose from an instruction level
> point of view, yes, this might add a teeny-tiny bit of overhead.
> 
> On the other hand it lets us do interesting things like initialize
> chip->atomic = !regmap_might_sleep() for those drivers that use regmap
> and then not worry about it any longer.
> 
> Given that, I'm also wondering if we should try to keep the terminology
> a bit more consistent. "Atomic" is somewhat overloaded because ->apply()
> and ->get_state() are part of the "atomic" PWM API (in the sense that
> applying changes are done as a single, atomic operation, rather than in
> the sense of "non-sleeping" operation).
> 
> So pwm_apply_state_atomic() is then doubly atomic, which is a bit weird.
> On the other hand it's a bit tedious to convert all existing users to
> pwm_apply_state_might_sleep().
> 
> Perhaps as a compromise we can add pwm_apply_state_might_sleep() and
> make pwm_apply_state() a (deprecated) alias for that, so that existing
> drivers can be converted one by one.

To throw in my green for our bike shed: I'd pick

	pwm_apply_state_cansleep()

to match what gpio does (with gpiod_set_value_cansleep()). (Though I
have to admit that semantically Thierry's might_sleep is nicer as it
matches might_sleep().)

If we don't want to have an explicit indicator for the atomic/fast
variant (again similar to the gpio framework), maybe we can drop
"_state" which I think is somehow redundant and go for:

	pwm_apply (fast)
	pwm_apply_cansleep (sleeping)
	pwm_apply_state (compat alias for pwm_apply_cansleep())

(maybe replace cansleep with might_sleep). Similar for pwm_get_state()
we could use the opportunity and make

	pwm_get()

actually call ->get_state() and introduce

	pwm_get_lastapplied()

with the semantic of todays pwm_get_state(). Do we need a
pwm_get_cansleep/might_sleep()?

Best regards
Uwe

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

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