Re: GPIO static allocation warning with v6.2-rcX

[Robert Schwebel <r.schwebel@pengutronix.de>]

On Thu, Jan 26, 2023 at 11:49:27AM +0100, Sascha Hauer wrote:
> What's missing is a way to let a GPIO stay in the current state when I
> release a GPIO chip. Unlike the new features you listed above this is a
> feature that the sysfs API offers and that's important for us.

An example where it is used is labgrid: our test automation controller
(LXA-TAC) doesn't run any software for controlling power of the device-
under-test; to switch on a DuT, labgrid does

  ssh tac echo 1 > /sys/some/path/to/gpio

While this could also be done with a daemon offering a dbus api, this
would be significantly more complex. In a critical environment, one
needs to make sure that the daemon process never fails, otherwhise the
power of the DuT would maybe be in a random state. Then of course one
can add a watchdog, but with the current sysfs interface it's really
simple. Of course that would also work if the new interface would offer
a "keep this line as it is" feature, but adding a dbus daemon just for
keeping the state of a pin sounds overcomplex when the kernel could also
provide that functionality.

Another example that came up on friday when we talked about this is a
motor for an airplane: It doesn't have only one "safe state" it could
fall back to if something fails (i.e. daemon disappears). The safe state
on power-on (with uninitialized external hardware) might be different
from the one on the ground (motor-off) or while being in the air
(motor-on). Of course one would probably not build an airplane without
further safety mechanics, but we have several less-desasterous-but-
still-very-expensive-in-the-case-of-failure use cases in the field, like
multi hundret kilowatt motors in agricultural or heavy construction
machine equipment being switched on/off by a GPIO that cause significant
loss of material / work on failure.

I hope those examples help a bit to understand the issues. As Sascha
said: when the new interface provides the same features sysfs offers
today, without adding tons of new complexity, increasing the pressure on
people to move there is perfectly fine. 

rsc
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