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

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Message-ID: <685e73cf.df0a0220.214b10.9998@mx.google.com>
Date: Fri, 27 Jun 2025 12:34:49 +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 01:25:48PM +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. 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.

kref I checked but not useful for the task.

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


-- 
	Ansuel