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

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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



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