Add architectural support for the cpuidle-haltpoll driver by defining
arch_haltpoll_*(). Also define ARCH_CPUIDLE_HALTPOLL to allow
cpuidle-haltpoll to be selected.

Haltpoll uses poll_idle() to do the actual polling. This in turn
uses smp_cond_load*() to wait until there's a specific store to
a cacheline. 
In the edge case -- no stores to the cacheline and no interrupt --
the event-stream provides the terminating condition ensuring we
don't wait forever. But because the event-stream runs at a fixed
frequency (configured at 10kHz) haltpoll might spend more time in
the polling stage than specified by cpuidle_poll_time().

This would only happen in the last iteration, since overshooting the
poll_limit means the governor will move out of the polling stage.

Tested-by: Haris Okanovic <redacted>
Tested-by: Misono Tomohiro <redacted>
Signed-off-by: Ankur Arora <redacted>
---
 arch/arm64/Kconfig                        |  6 ++++++
 arch/arm64/include/asm/cpuidle_haltpoll.h | 24 +++++++++++++++++++++++
 2 files changed, 30 insertions(+)
 create mode 100644 arch/arm64/include/asm/cpuidle_haltpoll.h

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index ef9c22c3cff2..5fc99eba22b2 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig

@@ -2415,6 +2415,12 @@ config ARCH_HIBERNATION_HEADER
 config ARCH_SUSPEND_POSSIBLE
 	def_bool y
 
+config ARCH_CPUIDLE_HALTPOLL
+	bool "Enable selection of the cpuidle-haltpoll driver"
+	help
+	  cpuidle-haltpoll allows for adaptive polling based on
+	  current load before entering the idle state.
+
 endmenu # "Power management options"
 
 menu "CPU Power Management"

diff --git a/arch/arm64/include/asm/cpuidle_haltpoll.h b/arch/arm64/include/asm/cpuidle_haltpoll.h
new file mode 100644
index 000000000000..91f0be707629
--- /dev/null
+++ b/arch/arm64/include/asm/cpuidle_haltpoll.h

@@ -0,0 +1,24 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+
+#ifndef _ARCH_HALTPOLL_H
+#define _ARCH_HALTPOLL_H
+
+static inline void arch_haltpoll_enable(unsigned int cpu) { }
+static inline void arch_haltpoll_disable(unsigned int cpu) { }
+
+static inline bool arch_haltpoll_want(bool force)
+{
+	/*
+	 * Enabling haltpoll requires two things:
+	 *
+	 * - Event stream support to provide a terminating condition to the
+	 *   WFE in the poll loop.
+	 *
+	 * - KVM support for arch_haltpoll_enable(), arch_haltpoll_disable().
+	 *
+	 * Given that the second is missing, only allow force loading for
+	 * haltpoll.
+	 */
+	return force;
+}
+#endif

-- 
2.43.5