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Date:   Sun, 22 Nov 2020 13:33:01 +0000
To:     Coiby Xu <coiby.xu@gmail.com>
From:   =?utf-8?Q?Barnab=C3=A1s_P=C5=91cze?= <pobrn@protonmail.com>
Cc:     "linux-input@vger.kernel.org" <linux-input@vger.kernel.org>,
        Helmut Stult <helmut.stult@schinfo.de>,
        "stable@vger.kernel.org" <stable@vger.kernel.org>,
        Jiri Kosina <jikos@kernel.org>,
        Benjamin Tissoires <benjamin.tissoires@redhat.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Reply-To: =?utf-8?Q?Barnab=C3=A1s_P=C5=91cze?= <pobrn@protonmail.com>
Subject: Re: [PATCH v3] HID: i2c-hid: add polling mode based on connected GPIO chip's pin status
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References: <20201021134931.462560-1-coiby.xu@gmail.com> <qo0Y8DqV6mbQsSFabOaqRoxYhKdYCZPjqYuF811CTdPXRFFXpx7sNXYcW9OGI5PMyclgsTjI7Xj3Du3v4hYQVBWGJl3t0t8XSbTKE9uOJ2E=@protonmail.com> <20201122101525.j265hvj6lqgbtfi2@Rk>
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Hi


2020. november 22., vas=C3=A1rnap 11:15 keltez=C3=A9ssel, Coiby Xu =C3=
=ADrta:

> [...]
> >> +static int get_gpio_pin_state(struct irq_desc *irq_desc)
> >> +{
> >> +=09struct gpio_chip *gc =3D irq_data_get_irq_chip_data(&irq_desc->irq=
_data);
> >> +
> >> +=09return gc->get(gc, irq_desc->irq_data.hwirq);
> >> +}
> [...]
> >> +=09ssize_t=09status =3D get_gpio_pin_state(irq_desc);
> >
> >`get_gpio_pin_state()` returns an `int`, so I am not sure why `ssize_t` =
is used here.
> >
>
> I used `ssize_t` because I found gpiolib-sysfs.c uses `ssize_t`
>
>      // drivers/gpio/gpiolib-sysfs.c
>      static ssize_t value_show(struct device *dev,
>      =09=09struct device_attribute *attr, char *buf)
>      {
>      =09struct gpiod_data *data =3D dev_get_drvdata(dev);
>      =09struct gpio_desc *desc =3D data->desc;
>      =09ssize_t=09=09=09status;
>
>      =09mutex_lock(&data->mutex);
>
>      =09status =3D gpiod_get_value_cansleep(desc);
>          ...
>      =09return status;
>      }
>
> According to the book Advanced Programming in the UNIX Environment by
> W. Richard Stevens,
>      With the 1990 POSIX.1 standard, the primitive system data type
>      ssize_t was introduced to provide the signed return value...
>
> So ssize_t is fairly common, for example, the read and write syscall
> return a value of type ssize_t. But I haven't found out why ssize_t is
> better int.
> >

Sorry if I wasn't clear, what prompted me to ask that question is the follo=
wing:
`gc->get()` returns `int`, `get_gpio_pin_state()` returns `int`, yet you st=
ill
save the return value of `get_gpio_pin_state()` into a variable with type
`ssize_t` for no apparent reason. In the example you cited, `ssize_t` is us=
ed
because the show() callback of a sysfs attribute must return `ssize_t`, but=
 here,
`interrupt_line_active()` returns `bool`, so I don't see any advantage over=
 a
plain `int`. Anyways, I believe either one is fine, I just found it odd.


> >> +
> >> +=09if (status < 0) {
> >> +=09=09dev_warn(&client->dev,
> >> +=09=09=09 "Failed to get GPIO Interrupt line status for %s",
> >> +=09=09=09 client->name);
> >
> >I think it's possible that the kernel message buffer is flooded with the=
se
> >messages, which is not optimal in my opinion.
> >
> Thank you! Replaced with dev_dbg in v4.
> [...]

Have you looked at `dev_{warn,dbg,...}_ratelimited()`?


Regards,
Barnab=C3=A1s P=C5=91cze