Re: [PATCH v3] HID: i2c-hid: add polling mode based on connected GPIO chip's pin status
From: Coiby Xu
Date: Wed Nov 25 2020 - 08:19:08 EST
On Wed, Nov 25, 2020 at 12:39:02PM +0000, Barnabás Pőcze wrote:
2020. november 25., szerda 11:57 keltezéssel, Coiby Xu írta:
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.
ssize_t: "Signed integer type used for a count of bytes or an error indication."[1]
And POSIX mandates that the return type of read() and write() be `ssize_t`,
so it makes sense to keep a similar interface in the kernel since show() and store()
are called as a direct result of the user using the read() and write() system
calls, respectively.
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".
[...]
I believe it's more appropriate to use ssize_t when it's about a "count of elements",
but the GPIO pin state is a single boolean value (or an error indication), which
is returned as an `int`. Since it's returned as an `int` - I'm arguing that -
there is no reason to use `ssize_t` here. Anyways, both `ssize_t` and `int` work fine
in this case.
So value_show in gpiolib-sysfs.c is a kind of being forced to use
ssize_t. I'll use int instead to avoid confusion in v4. Thank you for
the explanation!
[1]: https://pubs.opengroup.org/onlinepubs/9699919799/functions/V2_chap02.html#tag_15_12
Regards,
Barnabás Pőcze
--
Best regards,
Coiby