[question] Does gpiod_set_value have a "guarantee"?

[Vincent Fazio <vfazio@xxxxxxxxx>]

Is there any implicit or expected guarantee that after having returned from
gpiod_set_value that the GPIO will reflect the new value externally?

Some drivers that leverage GPIO to emulate buses, like i2c-gpio, may be relying
on this to be true in order to make a "stable" clock.

I was glancing at https://github.com/raspberrypi/linux/issues/5554 where
someone ran into an issue and it looks like, at least on the RPi 4 platform,
that there may need to be some "flush" mechanism to guarantee that a GPIO has
been written out.

If it's the responsibility of gpio_chip->set to do this, then I'm guessing the
pinctrl driver may need to be updated, but that does incur a performance hit
for every GPIO write.

If it's up to the bus emulator to do this, short of sampling the pin, should
there be some API or mechanism to assist with flushing writes out? Is there
already a mechanism to do this?

Thanks,
-Vincent