linux-kernel - Re: [PATCH v3] HID: i2c-hid: add polling mode based on connected GPIO chip's pin status

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Message-ID: <1FeR4cJ-m2i5GGyb68drDocoWP-yJ47BeKKEi2IkYbkppLFRCQPTQT4D6xqVCQcmUIjIsoe9HXhwycxxt5XxtsESO6w4uVMzISa987s_T-U=@protonmail.com>
Date:   Mon, 23 Nov 2020 16:32:40 +0000
From:   Barnabás Pőcze <pobrn@...tonmail.com>
To:     Coiby Xu <coiby.xu@...il.com>
Cc:     "linux-input@...r.kernel.org" <linux-input@...r.kernel.org>,
        Helmut Stult <helmut.stult@...info.de>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        Jiri Kosina <jikos@...nel.org>,
        Benjamin Tissoires <benjamin.tissoires@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3] HID: i2c-hid: add polling mode based on connected GPIO chip's pin status

> [...]
> >> >> +static int get_gpio_pin_state(struct irq_desc *irq_desc)
> >> >> +{
> >> >> +	struct gpio_chip *gc = irq_data_get_irq_chip_data(&irq_desc->irq_data);
> >> >> +
> >> >> +	return gc->get(gc, irq_desc->irq_data.hwirq);
> >> >> +}
> >> [...]
> >> >> +	ssize_t	status = 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,
> >>      		struct device_attribute *attr, char *buf)
> >>      {
> >>      	struct gpiod_data *data = dev_get_drvdata(dev);
> >>      	struct gpio_desc *desc = data->desc;
> >>      	ssize_t			status;
> >>
> >>      	mutex_lock(&data->mutex);
> >>
> >>      	status = gpiod_get_value_cansleep(desc);
> >>          ...
> >>      	return 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 following:
> >`gc->get()` returns `int`, `get_gpio_pin_state()` returns `int`, yet you still
> >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 used
> >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.
> >
> I don't understand why "the show() callback of a sysfs attribute
> must return `ssize_t`" instead of int. Do you think the rationale
> behind it is the same for this case? If yes, using "ssize_t" for
> status could be justified.
> [...]

Because it was decided that way, `ssize_t` is a better choice for that purpose
than plain `int`. You can see it in include/linux/device.h, that both the
show() and store() methods must return `ssize_t`.

What I'm arguing here, is that there is no reason to use `ssize_t` in this case.
Because `get_gpio_pin_state()` returns `int`. So when you do
```
ssize_t status = get_gpio_pin_state(...);
```
then the return value of `get_gpio_pin_state()` (which is an `int`), will be
converted to an `ssize_t`, and saved into `status`. I'm arguing that that is
unnecessary and a plain `int` would work perfectly well in this case. Anyways,
both work fine, I just found the unnecessary use of `ssize_t` here odd.


Regards,
Barnabás Pőcze