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

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Message-ID: <aF544lt-9YJq8r0y@smile.fi.intel.com>
Date: Fri, 27 Jun 2025 13:56:34 +0300
From: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To: Christian Marangi <ansuelsmth@...il.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 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.

> 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.

> 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

-- 
With Best Regards,
Andy Shevchenko