linux-kernel - Re: [PATCH 1/3] pwm: pca9685: Switch to atomic API

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Date:   Thu, 19 Nov 2020 09:58:26 -0500
From:   Sven Van Asbroeck <thesven73@...il.com>
To:     Clemens Gruber <clemens.gruber@...ruber.com>
Cc:     linux-pwm@...r.kernel.org,
        Thierry Reding <thierry.reding@...il.com>,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>, Lee Jones <lee.jones@...aro.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        David Jander <david@...tonic.nl>
Subject: Re: [PATCH 1/3] pwm: pca9685: Switch to atomic API

On Thu, Nov 19, 2020 at 5:00 AM Clemens Gruber
<clemens.gruber@...ruber.com> wrote:
>
> > You appear to mix cached and uncached uses of prescale,
> > is there a need for this? If not, perhaps pick one and use
> > it consistently?
>
> Yes, sticking to the cached value is probably the way to go.
>

I would suggest going one step further, and turn on the cache in
regmap, i.e. .cache_type = REGCACHE_RBTREE, then:
- no need to cache pca->prescale explicitly, you can just read it with
  regmap_read() every time, and it won't result in bus activity.
  then you can eliminate pca->prescale, which simplifies the driver.
- pca9685_pwm_get_state() no longer results in bus reads, every regmap_read()
  is cached, this is extremely efficient.
- pca9685_pwm_apply() and pca9685_pwm_gpio_set() now only does bus writes if
  registers actually change, i.e. calling pwm_apply() multiple times in a row
  with the same parameters, writes the registers only once.

We can do this safely because this chip never actively writes to its
registers (as far as I know).

But maybe that's a suggestion for a follow-up patch...

> > Also, if the prescale register contains an invalid value
> > during probe(), e.g. 0x00 or 0x01, would it make sense
> > to explicitly overwrite it with a valid setting?
>
> As long as it is overwritten with a correct setting when the PWM is used
> for the first time, it should be OK?

I'm not sure. Consider the following scenario:
- prescale register is invalid at probe, say it contains 0x02
- user calls pwm_apply() but with an invalid period, which results
  in a calculated prescale value of 0x02
- pca9685_pwm_apply() skips prescale setup because prescale does not
  change, returns OK(0)
- user believes setup was ok, actually it's broken...

Also, some people use this chip exclusively as a gpiochip, in that
case the prescale register is never touched. So an invalid prescale
at probe is never corrected.

Speaking of the gpiochip side, would it make sense to call
pca9685_pwm_full_on()/_off() in pca9685_pwm_gpio_set() too?