[Bug 206035] gpiobus causes panic when attached to gpio with a single pin

[bugzilla-noreply at freebsd.org]

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=206035

            Bug ID: 206035
           Summary: gpiobus causes panic when attached to gpio with a
                    single pin
           Product: Base System
           Version: 11.0-CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: sgalabov at gmail.com

Created attachment 165265
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=165265&action=edit
Suggested fix for the problem (please see comments for details)

While attaching a gpiobus to a GPIO controller with a single pin, the system
panics due to a failed KASSERT in gpiobus_init_softc().

This is due to the fact that gpiobus obtains the maximum pin number from the
GPIO controller (via GPIO_PIN_MAX) and immediately does a KASSERT, expecting
that the obtained value is != 0.

In the case of a GPIO controller which controls a single pin, the returned
value will be 0 (0 is the maximum pin number), which is absolutely valid.

Moreover, right after the KASSERT the code in gpiobus_init_softc() actually
adjusts the number of pins to the right value.

The suggestion is to change the KASSERT like so:
-       KASSERT(sc->sc_npins != 0, ("GPIO device with no pins"));
+       KASSERT(sc->sc_npins >= 0, ("GPIO device with no pins"));

This, I believe, is a better approach than, say, simply postponing the KASSERT
until after gpiobus has adjusted the number of pins, since if a broken GPIO
controller driver returns a < 0 value we'll be in even more trouble.

I've attached a patch with the suggested fix.

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