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

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Date:   Wed, 25 Nov 2020 18:57:20 +0800
From:   Coiby Xu <coiby.xu@...il.com>
To:     Barnabás Pőcze <pobrn@...tonmail.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

On Mon, Nov 23, 2020 at 04:32:40PM +0000, Barnabás Pőcze wrote:
>> [...]
>> >> >> +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`.
>

Could you explain why `ssize_t` is a better choice? AFAIU, ssize_t
is used because we can return negative value to indicate an error. If
we use ssize_t here, it's a reminder that reading a GPIO pin's status
could fail. And ssize_t reminds us it's a operation similar to read
or write. So ssize_t is better than int here. And maybe it's the same
reason why "it was decided that way".

>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

--
Best regards,
Coiby