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:

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[...]

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+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);
+}

[...]

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+	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.

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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.

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+
+	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