linux-kernel - Re: [PATCH v18] pwm: airoha: Add support for EN7581 SoC

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Message-ID: <686264ac.df0a0220.feace.3044@mx.google.com>
Date: Mon, 30 Jun 2025 12:19:22 +0200
From: Christian Marangi <ansuelsmth@...il.com>
To: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc: Uwe Kleine-König <ukleinek@...nel.org>,
	linux-kernel@...r.kernel.org, linux-pwm@...r.kernel.org,
	Benjamin Larsson <benjamin.larsson@...exis.eu>,
	AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>,
	Lorenzo Bianconi <lorenzo@...nel.org>
Subject: Re: [PATCH v18] pwm: airoha: Add support for EN7581 SoC

On Fri, Jun 27, 2025 at 01:56:34PM +0300, Andy Shevchenko wrote:
> On Fri, Jun 27, 2025 at 12:34:49PM +0200, Christian Marangi wrote:
> > On Fri, Jun 27, 2025 at 01:25:48PM +0300, Andy Shevchenko wrote:
> > > On Fri, Jun 27, 2025 at 11:32:46AM +0200, Christian Marangi wrote:
> > > > On Fri, Jun 27, 2025 at 11:58:04AM +0300, Andy Shevchenko wrote:
> > > > > On Fri, Jun 27, 2025 at 12:47:53AM +0200, Christian Marangi wrote:
> 
> ...
> 
> > > > > > +	/* Global mutex to protect bucket used refcount_t */
> > > > > > +	struct mutex mutex;
> > > > > 
> > > > > This makes a little sense. Either you use refcount_t (which is atomic) or
> > > > > use mutex + regular variable.
> > > > 
> > > > Using a regular variable I lose all the benefits of refcount_t with
> > > > underflow and other checks.
> > > 
> > > Then drop the mutex, atomic operations do not need an additional
> > > synchronisation. Btw, have you looked at kref APIs? Maybe that
> > > would make the intention clearer?
> > 
> > It's needed for
> > 
> > +       mutex_lock(&pc->mutex);
> > +       if (refcount_read(&pc->buckets[bucket].used) == 0) {
> > +               config_bucket = true;
> > +               refcount_set(&pc->buckets[bucket].used, 1);
> > +       } else {
> > +               refcount_inc(&pc->buckets[bucket].used);
> > +       }
> > +       mutex_unlock(&pc->mutex);
> > 
> > the refcount_read + refcount_set.
> 
> Which is simply wrong. Nobody should use atomics in such a way.
> Imagine if somebody wants to copy something like this in their
> code (in case of no mutex is there), they most likely won't notice
> this subtle bug.
>

Yes I understand that someone might think the additional mutex can be
""optional""

> > As you explained there might be case where refcount_read is zero but nother
> > PWM channel is setting the value so one refcount gets lost.
> 
> Right, because you should use refcount_inc_and_test() and initialise it
> to -MAX instead of 0. Or something like this.
> 

Mhhh I think API for _inc_and_test doesn't currently exist and I don't
feel too confident implementing them currently.

> > kref I checked but not useful for the task.
> 
> Okay.
> 
> > The logic here is
> > 
> > - refcount init as 0 (bucket unused)
> > - refcount set to 1 on first bucket use (bucket get configured)
> > - refcount increased if already used
> > - refcount decreased when PWM channel released
> > - bucket gets flagged as unused when refcount goes to 0 again
>

Do you think I should bite the bullet and just drop using refcount and
implement a simple int variable protected by a mutex?

-- 
	Ansuel