Date: Thu, 24 Oct 2019 17:04:53 +0300 From: Andriy Gapon <avg@FreeBSD.org> To: FreeBSD Current <freebsd-current@FreeBSD.org> Subject: gpiobus: setting output value while in input mode Message-ID: <e576cd5c-0585-fabf-67f2-1afedb4fafb8@FreeBSD.org>
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For a lack of a more specific mailing list (or my not being aware of it), asking
here.
gpioiic, a very simple driver, has this code:
===========================================================================
static void
gpioiic_setsda(device_t dev, int val)
{
struct gpioiic_softc *sc = device_get_softc(dev);
if (val == 0) {
GPIOBUS_PIN_SET(sc->sc_busdev, sc->sc_dev, sc->sda_pin, 0);
GPIOBUS_PIN_SETFLAGS(sc->sc_busdev, sc->sc_dev, sc->sda_pin,
GPIO_PIN_OUTPUT);
} else {
GPIOBUS_PIN_SETFLAGS(sc->sc_busdev, sc->sc_dev, sc->sda_pin,
GPIO_PIN_INPUT);
}
}
===========================================================================
The interesting case is val == 0.
I know that there are GPIO controllers where input and output values are
represented by different bits, so it is possible to set a future output value
while the pin is in input mode.
At the same time, there are controllers where there is a single bit per pin and
while the pin is input mode the bit is read-only. So, it is impossible to
preset the pin.
How should controllers of the second type handle GPIOBUS_PIN_SET when the pin is
in input mode?
I see three options:
1) just silently ignore GPIOBUS_PIN_SET or do it in a more glorious way: go all
the way to the hardware without caring if that does anything;
2) return an error that would hint that the operation is not possible at this time;
3) try to emulate the first class of controllers; that is, stash the value to
apply it when the pin is switched to output mode.
I personally prefer 2, it's not hard to do (unlike 3) and there would be at
least some visibility into the problem.
--
Andriy Gapon
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