ACPI: Add new tiny-power-button driver to directly signal init

Virtual machines often use an ACPI power button event to tell the
machine to shut down gracefully.

Provide an extremely lightweight "tiny power button" driver to handle
this event by signaling init directly, rather than running a separate
daemon (such as acpid or systemd-logind) that adds to startup time and
VM image complexity.

The kernel configuration defines the default signal to send init, and
userspace can change this signal via a module parameter.

Suggested-by: "Rafael J. Wysocki" <rafael@kernel.org>
Signed-off-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

1 files changed, 46 insertions, 0 deletions

diff --git a/drivers/acpi/tiny-power-button.c b/drivers/acpi/tiny-power-button.c
new file mode 100644
index 000000000000..6273d73c0b59
--- /dev/null
+++ b/drivers/acpi/tiny-power-button.c
@@ -0,0 +1,46 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+#include <linux/module.h>
+#include <linux/sched/signal.h>
+#include <linux/acpi.h>
+#include <acpi/button.h>
+
+ACPI_MODULE_NAME("tiny-power-button");
+MODULE_AUTHOR("Josh Triplett");
+MODULE_DESCRIPTION("ACPI Tiny Power Button Driver");
+MODULE_LICENSE("GPL");
+
+static int power_signal __read_mostly = CONFIG_ACPI_TINY_POWER_BUTTON_SIGNAL;
+module_param(power_signal, int, 0644);
+MODULE_PARM_DESC(power_signal, "Power button sends this signal to init");
+
+static const struct acpi_device_id tiny_power_button_device_ids[] = {
+	{ ACPI_BUTTON_HID_POWER, 0 },
+	{ ACPI_BUTTON_HID_POWERF, 0 },
+	{ "", 0 },
+};
+MODULE_DEVICE_TABLE(acpi, tiny_power_button_device_ids);
+
+static int acpi_noop_add_remove(struct acpi_device *device)
+{
+	return 0;
+}
+
+static void acpi_tiny_power_button_notify(struct acpi_device *device, u32 event)
+{
+	kill_cad_pid(power_signal, 1);
+}
+
+static struct acpi_driver acpi_tiny_power_button_driver = {
+	.name = "tiny-power-button",
+	.class = "tiny-power-button",
+	.ids = tiny_power_button_device_ids,
+	.ops = {
+		.add = acpi_noop_add_remove,
+		.remove = acpi_noop_add_remove,
+		.notify = acpi_tiny_power_button_notify,
+	},
+};
+
+module_driver(acpi_tiny_power_button_driver,
+		acpi_bus_register_driver,
+		acpi_bus_unregister_driver);