linux-kernel - RE: [PATCH v2] gpio: mmio: handle "ngpios" properly in bgpio

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Date:   Fri, 24 Feb 2023 21:31:23 +0000
From:   Asmaa Mnebhi <asmaa@...dia.com>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
CC:     "linus.walleij@...aro.org" <linus.walleij@...aro.org>,
        "linux-gpio@...r.kernel.org" <linux-gpio@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v2] gpio: mmio: handle "ngpios" properly in bgpio_init()

> > bgpio_init() uses "sz" argument to populate ngpio, which is not
> > accurate. Instead, read the "ngpios" property from the DT and if it
> > doesn't exist, use the "sz" argument. With this change, drivers no
> > longer need to overwrite the ngpio variable after calling bgpio_init.
> 
> ...
> 
> > +       ret = device_property_present(dev, "ngpios");
> > +       if (!ret)
> 
> ret is not of the correct type for this call.
> 
> Why not simply
> 
>     if (!device_property_present(dev, "ngpios"))
> 
> > +               gc->ngpio = gc->bgpio_bits;
> 
> ...
> 
> The problem with this change is that you need to provide bgpio_bits.
> 
> So, if there is a property when bgpio_bits will be updated?
> 
> That's said what you need is something like this:
> 1) split "ngpios" handling code (lines ~718-744 in gpiolib.c)  into a helper
> function that is available inside drivers/gpio;
> 2) use it in bgpio_init() by overriding bgpio_bits.
> 
> ret = new_helper();
> if (ret)
>     gc->bgpio_bits = sz * 8;
> else
>     ... = gc->ngpio;

So this new_helper() which reads the ngpios property, would be called from bgpio_init() and 
From gpiochip_add_data_with_key() correct? Since there are some drivers that don’t use
bgpio_init().
Also, shouldn't "gc->bgpio_bits = = sz * 8;" be independent of new_helper() function?
"bgpio_bits" is described as "number of register bits used for a generic GPIO i.e. <register width> * 8" 
which seems different from "ngpio" described as "the number of GPIOs handled by this controller"