watchdog/hardlockup: HAVE_NMI_WATCHDOG must implement watchdog_hardlockup_probe()

Right now there is one arch (sparc64) that selects HAVE_NMI_WATCHDOG
without selecting HAVE_HARDLOCKUP_DETECTOR_ARCH.  Because of that one
architecture, we have some special case code in the watchdog core to
handle the fact that watchdog_hardlockup_probe() isn't implemented.

Let's implement watchdog_hardlockup_probe() for sparc64 and get rid of the
special case.

As a side effect of doing this, code inspection tells us that we could fix
a minor bug where the system won't properly realize that NMI watchdogs are
disabled.  Specifically, on powerpc if CONFIG_PPC_WATCHDOG is turned off
the arch might still select CONFIG_HAVE_HARDLOCKUP_DETECTOR_ARCH which
selects CONFIG_HAVE_NMI_WATCHDOG.  Since CONFIG_PPC_WATCHDOG was off then
nothing will override the "weak" watchdog_hardlockup_probe() and we'll
fallback to looking at CONFIG_HAVE_NMI_WATCHDOG.

Link: https://lkml.kernel.org/r/20230526184139.2.Ic6ebbf307ca0efe91f08ce2c1eb4a037ba6b0700@changeidSigned-off-by: Douglas Anderson <dianders@chromium.org>
Suggested-by: Petr Mladek <pmladek@suse.com>
Reviewed-by: Petr Mladek <pmladek@suse.com>
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>

arch/Kconfig

@@ -405,7 +405,8 @@ config HAVE_NMI_WATCHDOG
 	bool
 	help
 	  The arch provides a low level NMI watchdog. It provides
-	  asm/nmi.h, and defines its own arch_touch_nmi_watchdog().
+	  asm/nmi.h, and defines its own watchdog_hardlockup_probe() and
+	  arch_touch_nmi_watchdog().
 
 config HAVE_HARDLOCKUP_DETECTOR_ARCH
 	bool

arch/sparc/kernel/nmi.c

@@ -65,6 +65,11 @@ void arch_touch_nmi_watchdog(void)
 }
 EXPORT_SYMBOL(arch_touch_nmi_watchdog);
 
+int __init watchdog_hardlockup_probe(void)
+{
+	return 0;
+}
+
 static void die_nmi(const char *str, struct pt_regs *regs, int do_panic)
 {
 	int this_cpu = smp_processor_id();

kernel/watchdog.c

@@ -217,19 +217,6 @@ void __weak watchdog_hardlockup_disable(unsigned int cpu) { }
  */
 int __weak __init watchdog_hardlockup_probe(void)
 {
-	/*
-	 * If CONFIG_HAVE_NMI_WATCHDOG is defined then an architecture
-	 * is assumed to have the hard watchdog available and we return 0.
-	 */
-	if (IS_ENABLED(CONFIG_HAVE_NMI_WATCHDOG))
-		return 0;
-
-	/*
-	 * Hardlockup detectors other than those using CONFIG_HAVE_NMI_WATCHDOG
-	 * are required to implement a non-weak version of this probe function
-	 * to tell whether they are available. If they don't override then
-	 * we'll return -ENODEV.
-	 */
 	return -ENODEV;
 }