On Wed, Nov 25, 2020 at 12:39:02PM +0000, Barnabás Pőcze wrote:
- 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!
Regards, Barnabás Pőcze
-- Best regards, Coiby