watchdog/hardlockup: make the config checks more straightforward

watchdog/hardlockup: make the config checks more straightforward

There are four possible variants of hardlockup detectors:

  + buddy: available when SMP is set.

  + perf: available when HAVE_HARDLOCKUP_DETECTOR_PERF is set.

  + arch-specific: available when HAVE_HARDLOCKUP_DETECTOR_ARCH is set.

  + sparc64 special variant: available when HAVE_NMI_WATCHDOG is set
	and HAVE_HARDLOCKUP_DETECTOR_ARCH is not set.

The check for the sparc64 variant is more complicated because
HAVE_NMI_WATCHDOG is used to #ifdef code used by both arch-specific
and sparc64 specific variant. Therefore it is automatically
selected with HAVE_HARDLOCKUP_DETECTOR_ARCH.

This complexity is partly hidden in HAVE_HARDLOCKUP_DETECTOR_NON_ARCH.
It reduces the size of some checks but it makes them harder to follow.

Finally, the other temporary variable HARDLOCKUP_DETECTOR_NON_ARCH
is used to re-compute HARDLOCKUP_DETECTOR_PERF/BUDDY when the global
HARDLOCKUP_DETECTOR switch is enabled/disabled.

Make the logic more straightforward by the following changes:

  + Better explain the role of HAVE_HARDLOCKUP_DETECTOR_ARCH and
    HAVE_NMI_WATCHDOG in comments.

  + Add HAVE_HARDLOCKUP_DETECTOR_BUDDY so that there is separate
    HAVE_* for all four hardlockup detector variants.

    Use it in the other conditions instead of SMP. It makes it
    clear that it is about the buddy detector.

  + Open code HAVE_HARDLOCKUP_DETECTOR_NON_ARCH in HARDLOCKUP_DETECTOR
    and HARDLOCKUP_DETECTOR_PREFER_BUDDY. It helps to understand
    the conditions between the four hardlockup detector variants.

  + Define the exact conditions when HARDLOCKUP_DETECTOR_PERF/BUDDY
    can be enabled. It explains the dependency on the other
    hardlockup detector variants.

    Also it allows to remove HARDLOCKUP_DETECTOR_NON_ARCH by using "imply".
    It triggers re-evaluating HARDLOCKUP_DETECTOR_PERF/BUDDY when
    the global HARDLOCKUP_DETECTOR switch is changed.

  + Add dependency on HARDLOCKUP_DETECTOR so that the affected variables
    disappear when the hardlockup detectors are disabled.

    Another nice side effect is that HARDLOCKUP_DETECTOR_PREFER_BUDDY
    value is not preserved when the global switch is disabled.
    The user has to make the decision again when it gets re-enabled.

Link: https://lkml.kernel.org/r/20230616150618.6073-3-pmladek@suse.com
Signed-off-by: Petr Mladek <pmladek@suse.com>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>