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Date:   Mon, 4 Nov 2019 16:11:03 +0100
From:   Miquel Raynal <miquel.raynal@bootlin.com>
To:     Andy Shevchenko <andy.shevchenko@gmail.com>
Cc:     Linus Walleij <linus.walleij@linaro.org>,
        Bartosz Golaszewski <bgolaszewski@baylibre.com>,
        Thierry Reding <thierry.reding@gmail.com>,
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        <u.kleine-koenig@pengutronix.de>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@vger.kernel.org>,
        linux-pwm@vger.kernel.org,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Subject: Re: [PATCH] gpio: pca953x: Add Maxim MAX7313 PWM support
Message-ID: <20191104161103.64995b8a@xps13>
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Hi Linus,

Andy Shevchenko <andy.shevchenko@gmail.com> wrote on Tue, 15 Oct 2019
17:55:33 +0300:

> On Tue, Oct 15, 2019 at 5:30 PM Miquel Raynal <miquel.raynal@bootlin.com> wrote:
> >
> > Hi Andy,
> >
> > Thanks for the feedback.
> >
> > Andy Shevchenko <andy.shevchenko@gmail.com> wrote on Mon, 14 Oct 2019
> > 20:59:01 +0300:
> >  
> > > On Mon, Oct 14, 2019 at 4:09 PM Miquel Raynal <miquel.raynal@bootlin.com> wrote:  
> > > >
> > > > The MAX7313 chip is fully compatible with the PCA9535 on its basic
> > > > functions but can also manage the intensity on each of its ports with
> > > > PWM. Each output is independent and may be tuned with 16 values (4
> > > > bits per output). The period is always 32kHz, only the duty-cycle may
> > > > be changed. One can use any output as GPIO or PWM.  
> > >
> > > Can we rather not contaminate driver with this?
> > >
> > > Just register a separate PWM driver and export its functionality to
> > > GPIO, or other way around (in the latter case we actually have PCA8685
> > > which provides a GPIO fgunctionality).
> > >  
> >
> > I understand your concern but I am not sure to understand which
> > solution you have in mind. In the former case, could you explain a bit
> > more what you are thinking about? Would linking the PWM support with
> > the GPIO driver (code would be located in another .c file) work for
> > you? Or maybe you would prefer an MFD on top of the GPIO driver?
> >
> > As for the later case, I am not willing to re-implement GPIO support in
> > an alternate driver for an already supported chip, it is too much work
> > for the time I can spend on it.  
> 
> 
> drivers/pwm/pwm-max7313.c:
> 
> probe(platform_device)
> {
>   struct regmap = pdata;
>   ...
> }
> 
> --- 8< --- 8< ---
> drivers/gpio/gpio-pca953x.c:
> 
> probe()
> {
>   struct regmap rm;
> ...
>   if (dev_has_pwm)
>    pca953x_register_pwm_driver(rm);
> ...
> }
> 
> In the above regmap may be replaced with some (shared) container.
> 
> Or other way around. PWM registers GPIO (which actually I prefer since
> we have PCA9685 case where PWM provides GPIO functionality, though via
> different means)
> 

Can I have your input on this proposal?

On one hand I agree that the GPIO driver is already quite big due to
its genericity and the amount of controllers it supports, on the other
hand:
1/ Registering a PWM driver from the GPIO core seems strange. Maybe
registering a platform device could do the trick but I am not convinced
it is worth the trouble.
2/ Putting the PWM logic in the drivers/pwm/ directory is not very
convenient as the object file will have to be embedded within the GPIO
one; this line in drivers/gpio/Makefile would be horrible:
... += gpio-pca953x.o ../pwm/pwm-max7313.o (not even sure it works)
3/ In any cases, the regmap's ->readable_reg(), ->writable_reg()
callbacks shall be tweaked to turn the PWM registers accessible, so we
would still have PWM related code in the PCA953x GPIO driver.

In the end, I wonder if keeping everything in one file is not better?
Eventually I can create a separate file and fill it with just the PWM
helpers/hooks. Please advise on the better solution for you, I'll wait
your feedback before addressing Thierry Reding's other review and
resubmit.


Thanks,
Miquèl