platform/surface: Add Surface Hot-Plug driver

Some Surface Book 2 and 3 models have a discrete GPU (dGPU) that is
hot-pluggable. On those devices, the dGPU is contained in the base,
which can be separated from the tablet part (containing CPU and
touchscreen) while the device is running.

It (in general) is presented as/behaves like a standard PCIe hot-plug
capable device, however, this device can also be put into D3cold. In
D3cold, the device itself is turned off and can thus not submit any
standard PCIe hot-plug events. To properly detect hot-(un)plugging while
the dGPU is in D3cold, out-of-band signaling is required. Without this,
the device state will only get updated during the next bus-check, eg.
via a manually issued lspci call.

This commit adds a driver to handle out-of-band PCIe hot-(un)plug events
on Microsoft Surface devices. On those devices, said events can be
detected via GPIO interrupts, which are then forwarded to the
corresponding ACPI DSM calls by this driver. The DSM then takes care of
issuing the appropriate bus-/device-check, causing the PCI core to
properly pick up the device change.

Signed-off-by: Maximilian Luz <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Hans de Goede <[e-mail]>

MAINTAINERS

···
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/pdx86/platform-drivers-x86.git
 F:	drivers/platform/surface/
 
+MICROSOFT SURFACE HOT-PLUG DRIVER
+M:	Maximilian Luz <[e-mail]>
+L:	[e-mail]
+S:	Maintained
+F:	drivers/platform/surface/surface_hotplug.c
+
 MICROSOFT SURFACE PRO 3 BUTTON DRIVER
 M:	Chen Yu <[e-mail]>
 L:	[e-mail]

drivers/platform/surface/Kconfig

···
 	  accordingly. It is required on those devices to allow wake-ups from
 	  suspend by opening the lid.
 
+config SURFACE_HOTPLUG
+	tristate "Surface Hot-Plug Driver"
+	depends on GPIOLIB
+	help
+	  Driver for out-of-band hot-plug event signaling on Microsoft Surface
+	  devices with hot-pluggable PCIe cards.
+
+	  This driver is used on Surface Book (2 and 3) devices with a
+	  hot-pluggable discrete GPU (dGPU). When not in use, the dGPU on those
+	  devices can enter D3cold, which prevents in-band (standard) PCIe
+	  hot-plug signaling. Thus, without this driver, detaching the base
+	  containing the dGPU will not correctly update the state of the
+	  corresponding PCIe device if it is in D3cold. This driver adds support
+	  for out-of-band hot-plug notifications, ensuring that the device state
+	  is properly updated even when the device in question is in D3cold.
+
+	  Select M or Y here, if you want to (fully) support hot-plugging of
+	  dGPU devices on the Surface Book 2 and/or 3 during D3cold.
+
 config SURFACE_PRO3_BUTTON
 	tristate "Power/home/volume buttons driver for Microsoft Surface Pro 3/4 tablet"
 	depends on INPUT

drivers/platform/surface/Makefile

···
 obj-$(CONFIG_SURFACE_AGGREGATOR)	+= aggregator/
 obj-$(CONFIG_SURFACE_AGGREGATOR_CDEV)	+= surface_aggregator_cdev.o
 obj-$(CONFIG_SURFACE_GPE)		+= surface_gpe.o
+obj-$(CONFIG_SURFACE_HOTPLUG)		+= surface_hotplug.o
 obj-$(CONFIG_SURFACE_PRO3_BUTTON)	+= surfacepro3_button.o

drivers/platform/surface/surface_hotplug.c

···
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * Surface Book (2 and later) hot-plug driver.
+ *
+ * Surface Book devices (can) have a hot-pluggable discrete GPU (dGPU). This
+ * driver is responsible for out-of-band hot-plug event signaling on these
+ * devices. It is specifically required when the hot-plug device is in D3cold
+ * and can thus not generate PCIe hot-plug events itself.
+ *
+ * Event signaling is handled via ACPI, which will generate the appropriate
+ * device-check notifications to be picked up by the PCIe hot-plug driver.
+ *
+ * Copyright (C) 2019-2021 Maximilian Luz <[e-mail]>
+ */
+
+#include <linux/acpi.h>
+#include <linux/gpio.h>
+#include <linux/interrupt.h>
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/mutex.h>
+#include <linux/platform_device.h>
+
+static const struct acpi_gpio_params shps_base_presence_int   = { 0, 0, false };
+static const struct acpi_gpio_params shps_base_presence       = { 1, 0, false };
+static const struct acpi_gpio_params shps_device_power_int    = { 2, 0, false };
+static const struct acpi_gpio_params shps_device_power        = { 3, 0, false };
+static const struct acpi_gpio_params shps_device_presence_int = { 4, 0, false };
+static const struct acpi_gpio_params shps_device_presence     = { 5, 0, false };
+
+static const struct acpi_gpio_mapping shps_acpi_gpios[] = {
+	{ "base_presence-int-gpio",   &shps_base_presence_int,   1 },
+	{ "base_presence-gpio",       &shps_base_presence,       1 },
+	{ "device_power-int-gpio",    &shps_device_power_int,    1 },
+	{ "device_power-gpio",        &shps_device_power,        1 },
+	{ "device_presence-int-gpio", &shps_device_presence_int, 1 },
+	{ "device_presence-gpio",     &shps_device_presence,     1 },
+	{ },
+};
+
+/* 5515a847-ed55-4b27-8352-cd320e10360a */
+static const guid_t shps_dsm_guid =
+	GUID_INIT(0x5515a847, 0xed55, 0x4b27, 0x83, 0x52, 0xcd, 0x32, 0x0e, 0x10, 0x36, 0x0a);
+
+#define SHPS_DSM_REVISION		1
+
+enum shps_dsm_fn {
+	SHPS_DSM_FN_PCI_NUM_ENTRIES	= 0x01,
+	SHPS_DSM_FN_PCI_GET_ENTRIES	= 0x02,
+	SHPS_DSM_FN_IRQ_BASE_PRESENCE	= 0x03,
+	SHPS_DSM_FN_IRQ_DEVICE_POWER	= 0x04,
+	SHPS_DSM_FN_IRQ_DEVICE_PRESENCE	= 0x05,
+};
+
+enum shps_irq_type {
+	/* NOTE: Must be in order of enum shps_dsm_fn above. */
+	SHPS_IRQ_TYPE_BASE_PRESENCE	= 0,
+	SHPS_IRQ_TYPE_DEVICE_POWER	= 1,
+	SHPS_IRQ_TYPE_DEVICE_PRESENCE	= 2,
+	SHPS_NUM_IRQS,
+};
+
+static const char *const shps_gpio_names[] = {
+	[SHPS_IRQ_TYPE_BASE_PRESENCE]	= "base_presence",
+	[SHPS_IRQ_TYPE_DEVICE_POWER]	= "device_power",
+	[SHPS_IRQ_TYPE_DEVICE_PRESENCE]	= "device_presence",
+};
+
+struct shps_device {
+	struct mutex lock[SHPS_NUM_IRQS];  /* Protects update in shps_dsm_notify_irq() */
+	struct gpio_desc *gpio[SHPS_NUM_IRQS];
+	unsigned int irq[SHPS_NUM_IRQS];
+};
+
+#define SHPS_IRQ_NOT_PRESENT		((unsigned int)-1)
+
+static enum shps_dsm_fn shps_dsm_fn_for_irq(enum shps_irq_type type)
+{
+	return SHPS_DSM_FN_IRQ_BASE_PRESENCE + type;
+}
+
+static void shps_dsm_notify_irq(struct platform_device *pdev, enum shps_irq_type type)
+{
+	struct shps_device *sdev = platform_get_drvdata(pdev);
+	acpi_handle handle = ACPI_HANDLE(&pdev->dev);
+	union acpi_object *result;
+	union acpi_object param;
+	int value;
+
+	mutex_lock(&sdev->lock[type]);
+
+	value = gpiod_get_value_cansleep(sdev->gpio[type]);
+	if (value < 0) {
+		mutex_unlock(&sdev->lock[type]);
+		dev_err(&pdev->dev, "failed to get gpio: %d (irq=%d)\n", type, value);
+		return;
+	}
+
+	dev_dbg(&pdev->dev, "IRQ notification via DSM (irq=%d, value=%d)\n", type, value);
+
+	param.type = ACPI_TYPE_INTEGER;
+	param.integer.value = value;
+
+	result = acpi_evaluate_dsm(handle, &shps_dsm_guid, SHPS_DSM_REVISION,
+				   shps_dsm_fn_for_irq(type), &param);
+
+	if (!result) {
+		dev_err(&pdev->dev, "IRQ notification via DSM failed (irq=%d, gpio=%d)\n",
+			type, value);
+
+	} else if (result->type != ACPI_TYPE_BUFFER) {
+		dev_err(&pdev->dev,
+			"IRQ notification via DSM failed: unexpected result type (irq=%d, gpio=%d)\n",
+			type, value);
+
+	} else if (result->buffer.length != 1 || result->buffer.pointer[0] != 0) {
+		dev_err(&pdev->dev,
+			"IRQ notification via DSM failed: unexpected result value (irq=%d, gpio=%d)\n",
+			type, value);
+	}
+
+	mutex_unlock(&sdev->lock[type]);
+
+	if (result)
+		ACPI_FREE(result);
+}
+
+static irqreturn_t shps_handle_irq(int irq, void *data)
+{
+	struct platform_device *pdev = data;
+	struct shps_device *sdev = platform_get_drvdata(pdev);
+	int type;
+
+	/* Figure out which IRQ we're handling. */
+	for (type = 0; type < SHPS_NUM_IRQS; type++)
+		if (irq == sdev->irq[type])
+			break;
+
+	/* We should have found our interrupt, if not: this is a bug. */
+	if (WARN(type >= SHPS_NUM_IRQS, "invalid IRQ number: %d\n", irq))
+		return IRQ_HANDLED;
+
+	/* Forward interrupt to ACPI via DSM. */
+	shps_dsm_notify_irq(pdev, type);
+	return IRQ_HANDLED;
+}
+
+static int shps_setup_irq(struct platform_device *pdev, enum shps_irq_type type)
+{
+	unsigned long flags = IRQF_ONESHOT | IRQF_TRIGGER_FALLING | IRQF_TRIGGER_RISING;
+	struct shps_device *sdev = platform_get_drvdata(pdev);
+	struct gpio_desc *gpiod;
+	acpi_handle handle = ACPI_HANDLE(&pdev->dev);
+	const char *irq_name;
+	const int dsm = shps_dsm_fn_for_irq(type);
+	int status, irq;
+
+	/*
+	 * Only set up interrupts that we actually need: The Surface Book 3
+	 * does not have a DSM for base presence, so don't set up an interrupt
+	 * for that.
+	 */
+	if (!acpi_check_dsm(handle, &shps_dsm_guid, SHPS_DSM_REVISION, BIT(dsm))) {
+		dev_dbg(&pdev->dev, "IRQ notification via DSM not present (irq=%d)\n", type);
+		return 0;
+	}
+
+	gpiod = devm_gpiod_get(&pdev->dev, shps_gpio_names[type], GPIOD_ASIS);
+	if (IS_ERR(gpiod))
+		return PTR_ERR(gpiod);
+
+	irq = gpiod_to_irq(gpiod);
+	if (irq < 0)
+		return irq;
+
+	irq_name = devm_kasprintf(&pdev->dev, GFP_KERNEL, "shps-irq-%d", type);
+	if (!irq_name)
+		return -ENOMEM;
+
+	status = devm_request_threaded_irq(&pdev->dev, irq, NULL, shps_handle_irq,
+					   flags, irq_name, pdev);
+	if (status)
+		return status;
+
+	dev_dbg(&pdev->dev, "set up irq %d as type %d\n", irq, type);
+
+	sdev->gpio[type] = gpiod;
+	sdev->irq[type] = irq;
+
+	return 0;
+}
+
+static int surface_hotplug_remove(struct platform_device *pdev)
+{
+	struct shps_device *sdev = platform_get_drvdata(pdev);
+	int i;
+
+	/* Ensure that IRQs have been fully handled and won't trigger any more. */
+	for (i = 0; i < SHPS_NUM_IRQS; i++) {
+		if (sdev->irq[i] != SHPS_IRQ_NOT_PRESENT)
+			disable_irq(sdev->irq[i]);
+
+		mutex_destroy(&sdev->lock[i]);
+	}
+
+	return 0;
+}
+
+static int surface_hotplug_probe(struct platform_device *pdev)
+{
+	struct shps_device *sdev;
+	int status, i;
+
+	/*
+	 * The MSHW0153 device is also present on the Surface Laptop 3,
+	 * however that doesn't have a hot-pluggable PCIe device. It also
+	 * doesn't have any GPIO interrupts/pins under the MSHW0153, so filter
+	 * it out here.
+	 */
+	if (gpiod_count(&pdev->dev, NULL) < 0)
+		return -ENODEV;
+
+	status = devm_acpi_dev_add_driver_gpios(&pdev->dev, shps_acpi_gpios);
+	if (status)
+		return status;
+
+	sdev = devm_kzalloc(&pdev->dev, sizeof(*sdev), GFP_KERNEL);
+	if (!sdev)
+		return -ENOMEM;
+
+	platform_set_drvdata(pdev, sdev);
+
+	/*
+	 * Initialize IRQs so that we can safely call surface_hotplug_remove()
+	 * on errors.
+	 */
+	for (i = 0; i < SHPS_NUM_IRQS; i++)
+		sdev->irq[i] = SHPS_IRQ_NOT_PRESENT;
+
+	/* Set up IRQs. */
+	for (i = 0; i < SHPS_NUM_IRQS; i++) {
+		mutex_init(&sdev->lock[i]);
+
+		status = shps_setup_irq(pdev, i);
+		if (status) {
+			dev_err(&pdev->dev, "failed to set up IRQ %d: %d\n", i, status);
+			goto err;
+		}
+	}
+
+	/* Ensure everything is up-to-date. */
+	for (i = 0; i < SHPS_NUM_IRQS; i++)
+		if (sdev->irq[i] != SHPS_IRQ_NOT_PRESENT)
+			shps_dsm_notify_irq(pdev, i);
+
+	return 0;
+
+err:
+	surface_hotplug_remove(pdev);
+	return status;
+}
+
+static const struct acpi_device_id surface_hotplug_acpi_match[] = {
+	{ "MSHW0153", 0 },
+	{ },
+};
+MODULE_DEVICE_TABLE(acpi, surface_hotplug_acpi_match);
+
+static struct platform_driver surface_hotplug_driver = {
+	.probe = surface_hotplug_probe,
+	.remove = surface_hotplug_remove,
+	.driver = {
+		.name = "surface_hotplug",
+		.acpi_match_table = surface_hotplug_acpi_match,
+		.probe_type = PROBE_PREFER_ASYNCHRONOUS,
+	},
+};
+module_platform_driver(surface_hotplug_driver);
+
+MODULE_AUTHOR("Maximilian Luz <[e-mail]>");
+MODULE_DESCRIPTION("Surface Hot-Plug Signaling Driver for Surface Book Devices");
+MODULE_LICENSE("GPL");