[RFC PATCH v3 15/21] watchdog/hardlockup/hpet: Only enable the HPET watchdog via a boot parameter

[Ricardo Neri <ricardo.neri-calderon@linux.intel.com>]

Keep the HPET-based hardlockup detector disabled unless explicitly enabled
via a command-line argument. If such parameter is not given, the
initialization of the hpet-based hardlockup detector fails and the NMI
watchdog will fallback to use the perf-based implementation.

Given that __setup("nmi_watchdog=") is already used to control the behavior
of the NMI watchdog (via hardlockup_panic_setup()), it cannot be used to
control of the hpet-based implementation. Instead, use a new
early_param("nmi_watchdog").

Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ashok Raj <ashok.raj@intel.com>
Cc: Andi Kleen <andi.kleen@intel.com>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Clemens Ladisch <clemens@ladisch.de>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Philippe Ombredanne <pombredanne@nexb.com>
Cc: Kate Stewart <kstewart@linuxfoundation.org>
Cc: "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>
Cc: Mimi Zohar <zohar@linux.ibm.com>
Cc: Jan Kiszka <jan.kiszka@siemens.com>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: Nayna Jain <nayna@linux.ibm.com>
Cc: Stephane Eranian <eranian@google.com>
Cc: Suravee Suthikulpanit <Suravee.Suthikulpanit@amd.com>
Cc: "Ravi V. Shankar" <ravi.v.shankar@intel.com>
Cc: x86@kernel.org
Signed-off-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>

--
checkpatch gives the following warning:

CHECK: __setup appears un-documented -- check Documentation/admin-guide/kernel-parameters.rst
+__setup("nmi_watchdog=", hardlockup_detector_hpet_setup);

This is a false-positive as the option nmi_watchdog is already
documented. The option is re-evaluated in this file as well.
---
 .../admin-guide/kernel-parameters.txt         |  8 ++++++-
 arch/x86/kernel/watchdog_hld_hpet.c           | 22 +++++++++++++++++++
 2 files changed, 29 insertions(+), 1 deletion(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index fd03e2b629bb..3c42205b469c 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -2801,7 +2801,7 @@
 			Format: [state][,regs][,debounce][,die]
 
 	nmi_watchdog=	[KNL,BUGS=X86] Debugging features for SMP kernels
-			Format: [panic,][nopanic,][num]
+			Format: [panic,][nopanic,][num,][hpet]
 			Valid num: 0 or 1
 			0 - turn hardlockup detector in nmi_watchdog off
 			1 - turn hardlockup detector in nmi_watchdog on
@@ -2811,6 +2811,12 @@
 			please see 'nowatchdog'.
 			This is useful when you use a panic=... timeout and
 			need the box quickly up again.
+			When hpet is specified, the NMI watchdog will be driven
+			by an HPET timer, if available in the system. Otherwise,
+			it falls back to the default implementation (perf or
+			architecture-specific). Specifying hpet has no effect
+			if the NMI watchdog is not enabled (either at build time
+			or via the command line).
 
 			These settings can be accessed at runtime via
 			the nmi_watchdog and hardlockup_panic sysctls.
diff --git a/arch/x86/kernel/watchdog_hld_hpet.c b/arch/x86/kernel/watchdog_hld_hpet.c
index 6f1f540cfee9..90680a8cf9fc 100644
--- a/arch/x86/kernel/watchdog_hld_hpet.c
+++ b/arch/x86/kernel/watchdog_hld_hpet.c
@@ -350,6 +350,28 @@ void hardlockup_detector_hpet_stop(void)
 	disable_timer(hld_data);
 }
 
+/**
+ * hardlockup_detector_hpet_setup() - Parse command-line parameters
+ * @str:	A string containing the kernel command line
+ *
+ * Parse the nmi_watchdog parameter from the kernel command line. If
+ * selected by the user, use this implementation to detect hardlockups.
+ */
+static int __init hardlockup_detector_hpet_setup(char *str)
+{
+	if (!str)
+		return -EINVAL;
+
+	if (parse_option_str(str, "hpet"))
+		hardlockup_use_hpet = true;
+
+	if (!nmi_watchdog_user_enabled && hardlockup_use_hpet)
+		pr_warn("Selecting HPET NMI watchdog has no effect with NMI watchdog disabled\n");
+
+	return 0;
+}
+early_param("nmi_watchdog", hardlockup_detector_hpet_setup);
+
 /**
  * hardlockup_detector_hpet_init() - Initialize the hardlockup detector
  *
-- 
2.17.1