linux-kernel - Re: [PATCH 2/2] USB: serial: cp210x: add gpio-configuration debug printk

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Date:   Fri, 9 Apr 2021 18:32:48 +0200
From:   Johan Hovold <johan@...nel.org>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
Cc:     Bartosz Golaszewski <bgolaszewski@...libre.com>,
        Pho Tran <photranvan0712@...il.com>, Hung.Nguyen@...abs.com,
        Tung.Pham@...abs.com, USB <linux-usb@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] USB: serial: cp210x: add gpio-configuration debug
 printk

On Fri, Apr 09, 2021 at 07:22:26PM +0300, Andy Shevchenko wrote:
> On Fri, Apr 9, 2021 at 6:52 PM Johan Hovold <johan@...nel.org> wrote:
> >
> > Add a debug printk to dump the GPIO configuration stored in EEPROM
> > during probe.
> >
> > Signed-off-by: Johan Hovold <johan@...nel.org>
> > ---
> >  drivers/usb/serial/cp210x.c | 6 ++++++
> >  1 file changed, 6 insertions(+)
> >
> > diff --git a/drivers/usb/serial/cp210x.c b/drivers/usb/serial/cp210x.c
> > index ceb3a656a075..ee595d1bea0a 100644
> > --- a/drivers/usb/serial/cp210x.c
> > +++ b/drivers/usb/serial/cp210x.c
> > @@ -1543,10 +1543,16 @@ static int cp210x_gpio_init_valid_mask(struct gpio_chip *gc,
> >  {
> >         struct usb_serial *serial = gpiochip_get_data(gc);
> >         struct cp210x_serial_private *priv = usb_get_serial_data(serial);
> > +       struct device *dev = &serial->interface->dev;
> >         unsigned long altfunc_mask = priv->gpio_altfunc;
> >
> >         bitmap_complement(valid_mask, &altfunc_mask, ngpios);
> >
> > +       if (bitmap_empty(valid_mask, ngpios))
> > +               dev_dbg(dev, "no pin configured for GPIO\n");
> 
> Shouldn't we drop the GPIO device completely in such a case?

I considered it when we first added support for GPIOs to this driver but
decided not to. The reason being that we want to to tell user-space that
the device has gpio capability even if the GPIOs are currently muxed (in
EEPROM) for other functionality.

> Bart, wouldn't it be a good idea for GPIO library to do something like
> this on driver's behalf?

I'd say this is mostly useful for hotpluggable devices with EEPROM
configuration and is probably best handled by the drivers.

> > +       else
> > +               dev_dbg(dev, "GPIO.%*pbl configured for GPIO\n", ngpios,
> > +                               valid_mask);
> 
> A nit-pick:
> I would change GPIO -> pin in the second message in the first occurrence.

"GPIO.n" are the actual pin names from the datasheet (cf. ftdi_sio which
use "CBUSn" here). It's just a debug statement anyway.

Johan