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

[Coiby Xu]

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