feat: Power-off fix for the Steam Machine; the kernel pin is optional

Kernels >= 7.2 keep the firmware's S4/S5 wake bit on GPIO pin 18, so the
Steam Machine starts again right after powering off. Steam Machine
support now builds a DKMS module (patches/steamify-fremont-poweroff.c)
that clears it before power-off, for every kernel. The kernel pin is no
longer ticked by default. Module sources and patches move to patches/,
embedded by the single-file build.

patches/steamify-fremont-poweroff.c

@@ -0,0 +1,80 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Steam Machine (Fremont): power off instead of booting up again.
+ *
+ * Since Linux 7.2 ("pinctrl-amd: Don't clear S4 wake bits at probe") the
+ * S4/S5 wake bit the firmware leaves set on GPIO pin 18 (_SB.PCI0.GPP6)
+ * stays set, and the machine starts again right after powering off.
+ * Valve's kernel clears that bit at probe on Fremont (not for upstream);
+ * this does the same right before power-off, for kernels without it.
+ */
+#include <linux/acpi.h>
+#include <linux/bits.h>
+#include <linux/dmi.h>
+#include <linux/io.h>
+#include <linux/module.h>
+#include <linux/platform_device.h>
+#include <linux/reboot.h>
+
+#define FREMONT_GPP6_PIN	18
+#define WAKE_CNTRL_OFF_S4	15
+
+static void __iomem *pin_reg;
+static struct sys_off_handler *handler;
+
+static int fremont_poweroff_prepare(struct sys_off_data *data)
+{
+	u32 v = readl(pin_reg);
+
+	if (v & BIT(WAKE_CNTRL_OFF_S4))
+		writel(v & ~BIT(WAKE_CNTRL_OFF_S4), pin_reg);
+	return NOTIFY_DONE;
+}
+
+static int __init fremont_poweroff_init(void)
+{
+	struct acpi_device *adev;
+	struct device *dev;
+	struct resource *res;
+	u32 v;
+
+	if (!dmi_match(DMI_BOARD_NAME, "Fremont"))
+		return -ENODEV;
+	adev = acpi_dev_get_first_match_dev("AMDI0030", NULL, -1);
+	if (!adev)
+		return -ENODEV;
+	dev = bus_find_device_by_acpi_dev(&platform_bus_type, adev);
+	acpi_dev_put(adev);
+	if (!dev)
+		return -ENODEV;
+	res = platform_get_resource(to_platform_device(dev), IORESOURCE_MEM, 0);
+	put_device(dev);
+	if (!res || resource_size(res) < (FREMONT_GPP6_PIN + 1) * 4)
+		return -ENODEV;
+	/* pinctrl-amd owns the region; only this one register is touched. */
+	pin_reg = ioremap(res->start + FREMONT_GPP6_PIN * 4, 4);
+	if (!pin_reg)
+		return -ENOMEM;
+	v = readl(pin_reg);
+	pr_info("GPIO %d register 0x%08x, S4/S5 wake %s\n", FREMONT_GPP6_PIN, v,
+		v & BIT(WAKE_CNTRL_OFF_S4) ? "set (cleared at power-off)" : "clear");
+	handler = register_sys_off_handler(SYS_OFF_MODE_POWER_OFF_PREPARE,
+					   SYS_OFF_PRIO_DEFAULT, fremont_poweroff_prepare, NULL);
+	if (IS_ERR(handler)) {
+		iounmap(pin_reg);
+		return PTR_ERR(handler);
+	}
+	return 0;
+}
+
+static void __exit fremont_poweroff_exit(void)
+{
+	unregister_sys_off_handler(handler);
+	iounmap(pin_reg);
+}
+
+module_init(fremont_poweroff_init);
+module_exit(fremont_poweroff_exit);
+MODULE_DESCRIPTION("Steam Machine: clear GPIO 18's S4 wake bit before power-off");
+MODULE_LICENSE("GPL");
+MODULE_ALIAS("dmi:*:rnFremont:*");