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

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Date:   Mon, 23 Nov 2020 22:36:13 +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 Sun, Nov 22, 2020 at 01:33:01PM +0000, Barnabás Pőcze wrote:
>Hi
>
>
>2020. november 22., vasárnap 11:15 keltezéssel, Coiby Xu írta:
>
>> [...]
>> >> +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.

>
>> >> +
>> >> +	if (status < 0) {
>> >> +		dev_warn(&client->dev,
>> >> +			 "Failed to get GPIO Interrupt line status for %s",
>> >> +			 client->name);
>> >
>> >I think it's possible that the kernel message buffer is flooded with these
>> >messages, which is not optimal in my opinion.
>> >
>> Thank you! Replaced with dev_dbg in v4.
>> [...]
>
>Have you looked at `dev_{warn,dbg,...}_ratelimited()`?
>
Thank you for pointing me to these functions!
>
>Regards,
>Barnabás Pőcze

--
Best regards,
Coiby