[PATCH 1/3] platform/surface: Add DTX driver

[Maximilian Luz <luzmaximilian@gmail.com>]

The Microsoft Surface Book series devices consist of a so-called
clipboard part (containing the CPU, touchscreen, and primary battery)
and a base part (containing keyboard, secondary battery, and optional
discrete GPU). These parts can be separated, i.e. the clipboard can be
detached and used as tablet.

This detachment process is initiated by pressing a button. On the
Surface Book 2 and 3 (targeted with this commit), the Surface Aggregator
Module (i.e. the embedded controller on those devices) attempts to send
a notification to any listening client driver and waits for further
instructions (i.e. whether the detachment process should continue or be
aborted). If it does not receive a response in a certain time-frame, the
detachment process (by default) continues and the clipboard can be
physically separated. In other words, (by default and) without a driver,
the detachment process takes about 10 seconds to complete.

This commit introduces a driver for this detachment system (called DTX).
This driver allows a user-space daemon to control and influence the
detachment behavior. Specifically, it forwards any detachment requests
to user-space, allows user-space to make such requests itself, and
allows handling of those requests. Requests can be handled by either
aborting, continuing/allowing, or delaying (i.e. resetting the timeout
via a heartbeat commend). The user-space API is implemented via the
/dev/surface/dtx miscdevice.

In addition, user-space can change the default behavior on timeout from
allowing detachment to disallowing it, which is useful if the (optional)
discrete GPU is in use.

Furthermore, this driver allows user-space to receive notifications
about the state of the base, specifically when it is physically removed
(as opposed to detachment requested), in what manner it is connected
(i.e. in reverse-/tent-/studio- or laptop-mode), and what type of base
is connected. Based on this information, the driver also provides a
simple tablet-mode switch (aliasing all modes without keyboard access,
i.e. tablet-mode and studio-mode to its reported tablet-mode).

An implementation of such a user-space daemon, allowing configuration of
detachment behavior via scripts (e.g. safely unmounting USB devices
connected to the base before continuing) can be found at [1].

[1]: https://github.com/linux-surface/surface-dtx-daemon

Signed-off-by: Maximilian Luz <luzmaximilian@gmail.com>
---
 .../userspace-api/ioctl/ioctl-number.rst      |    2 +
 MAINTAINERS                                   |    7 +
 drivers/platform/surface/Kconfig              |   16 +
 drivers/platform/surface/Makefile             |    1 +
 drivers/platform/surface/surface_dtx.c        | 1201 +++++++++++++++++
 include/uapi/linux/surface_aggregator/dtx.h   |  146 ++
 6 files changed, 1373 insertions(+)
 create mode 100644 drivers/platform/surface/surface_dtx.c
 create mode 100644 include/uapi/linux/surface_aggregator/dtx.h

diff --git a/Documentation/userspace-api/ioctl/ioctl-number.rst b/Documentation/userspace-api/ioctl/ioctl-number.rst
index 599bd4493944..1c28b8ef6677 100644
--- a/Documentation/userspace-api/ioctl/ioctl-number.rst
+++ b/Documentation/userspace-api/ioctl/ioctl-number.rst
@@ -327,6 +327,8 @@ Code  Seq#    Include File                                           Comments
 0xA4  00-1F  uapi/asm/sgx.h                                          <mailto:linux-sgx@vger.kernel.org>
 0xA5  01     linux/surface_aggregator/cdev.h                         Microsoft Surface Platform System Aggregator
                                                                      <mailto:luzmaximilian@gmail.com>
+0xA5  20-2F  linux/surface_aggregator/dtx.h                          Microsoft Surface DTX driver
+                                                                     <mailto:luzmaximilian@gmail.com>
 0xAA  00-3F  linux/uapi/linux/userfaultfd.h
 0xAB  00-1F  linux/nbd.h
 0xAC  00-1F  linux/raw.h
diff --git a/MAINTAINERS b/MAINTAINERS
index cf4cb8892623..adfc3a437db7 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -11861,6 +11861,13 @@ F:	drivers/scsi/smartpqi/smartpqi*.[ch]
 F:	include/linux/cciss*.h
 F:	include/uapi/linux/cciss*.h
 
+MICROSOFT SURFACE DTX DRIVER
+M:	Maximilian Luz <luzmaximilian@gmail.com>
+L:	platform-driver-x86@vger.kernel.org
+S:	Maintained
+F:	drivers/platform/surface/surface_dtx.c
+F:	include/uapi/linux/surface_aggregator/dtx.h
+
 MICROSOFT SURFACE GPE LID SUPPORT DRIVER
 M:	Maximilian Luz <luzmaximilian@gmail.com>
 L:	platform-driver-x86@vger.kernel.org
diff --git a/drivers/platform/surface/Kconfig b/drivers/platform/surface/Kconfig
index a045425026ae..98cf564fb17a 100644
--- a/drivers/platform/surface/Kconfig
+++ b/drivers/platform/surface/Kconfig
@@ -104,6 +104,22 @@ config SURFACE_AGGREGATOR_REGISTRY
 	  the respective client devices. Drivers for these devices still need to
 	  be selected via the other options.
 
+config SURFACE_DTX
+	tristate "Surface DTX (Detachment System) Driver"
+	depends on SURFACE_AGGREGATOR
+	depends on INPUT
+	help
+	  Driver for the Surface Book clipboard detachment system (DTX).
+
+	  On the Surface Book series devices, the display part containing the
+	  CPU (called the clipboard) can be detached from the base (containing a
+	  battery, the keyboard, and, optionally, a discrete GPU) by (if
+	  necessary) unlocking and opening the latch connecting both parts.
+
+	  This driver provides a user-space interface that can influence the
+	  behavior of this process, which includes the option to abort it in
+	  case the base is still in use or speed it up in case it is not.
+
 config SURFACE_GPE
 	tristate "Surface GPE/Lid Support Driver"
 	depends on DMI
diff --git a/drivers/platform/surface/Makefile b/drivers/platform/surface/Makefile
index 99372c427b73..32889482de55 100644
--- a/drivers/platform/surface/Makefile
+++ b/drivers/platform/surface/Makefile
@@ -11,6 +11,7 @@ obj-$(CONFIG_SURFACE_ACPI_NOTIFY)	+= surface_acpi_notify.o
 obj-$(CONFIG_SURFACE_AGGREGATOR)	+= aggregator/
 obj-$(CONFIG_SURFACE_AGGREGATOR_CDEV)	+= surface_aggregator_cdev.o
 obj-$(CONFIG_SURFACE_AGGREGATOR_REGISTRY) += surface_aggregator_registry.o
+obj-$(CONFIG_SURFACE_DTX)		+= surface_dtx.o
 obj-$(CONFIG_SURFACE_GPE)		+= surface_gpe.o
 obj-$(CONFIG_SURFACE_HOTPLUG)		+= surface_hotplug.o
 obj-$(CONFIG_SURFACE_PLATFORM_PROFILE)	+= surface_platform_profile.o
diff --git a/drivers/platform/surface/surface_dtx.c b/drivers/platform/surface/surface_dtx.c
new file mode 100644
index 000000000000..1301fab0ea14
--- /dev/null
+++ b/drivers/platform/surface/surface_dtx.c
@@ -0,0 +1,1201 @@
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * Surface Book (gen. 2 and later) detachment system (DTX) driver.
+ *
+ * Provides a user-space interface to properly handle clipboard/tablet
+ * (containing screen and processor) detachment from the base of the device
+ * (containing the keyboard and optionally a discrete GPU). Allows to
+ * acknowledge (to speed things up), abort (e.g. in case the dGPU is still in
+ * use), or request detachment via user-space.
+ *
+ * Copyright (C) 2019-2021 Maximilian Luz <luzmaximilian@gmail.com>
+ */
+
+#include <linux/fs.h>
+#include <linux/input.h>
+#include <linux/ioctl.h>
+#include <linux/kernel.h>
+#include <linux/kfifo.h>
+#include <linux/kref.h>
+#include <linux/miscdevice.h>
+#include <linux/module.h>
+#include <linux/mutex.h>
+#include <linux/platform_device.h>
+#include <linux/poll.h>
+#include <linux/rwsem.h>
+#include <linux/slab.h>
+#include <linux/workqueue.h>
+
+#include <linux/surface_aggregator/controller.h>
+#include <linux/surface_aggregator/dtx.h>
+
+
+/* -- SSAM interface. ------------------------------------------------------- */
+
+enum sam_event_cid_bas {
+	SAM_EVENT_CID_DTX_CONNECTION			= 0x0c,
+	SAM_EVENT_CID_DTX_REQUEST			= 0x0e,
+	SAM_EVENT_CID_DTX_CANCEL			= 0x0f,
+	SAM_EVENT_CID_DTX_LATCH_STATUS			= 0x11,
+};
+
+enum ssam_bas_base_state {
+	SSAM_BAS_BASE_STATE_DETACH_SUCCESS		= 0x00,
+	SSAM_BAS_BASE_STATE_ATTACHED			= 0x01,
+	SSAM_BAS_BASE_STATE_NOT_FEASIBLE		= 0x02,
+};
+
+enum ssam_bas_latch_status {
+	SSAM_BAS_LATCH_STATUS_CLOSED			= 0x00,
+	SSAM_BAS_LATCH_STATUS_OPENED			= 0x01,
+	SSAM_BAS_LATCH_STATUS_FAILED_TO_OPEN		= 0x02,
+	SSAM_BAS_LATCH_STATUS_FAILED_TO_REMAIN_OPEN	= 0x03,
+	SSAM_BAS_LATCH_STATUS_FAILED_TO_CLOSE		= 0x04,
+};
+
+enum ssam_bas_cancel_reason {
+	SSAM_BAS_CANCEL_REASON_NOT_FEASIBLE		= 0x00,  /* Low battery. */
+	SSAM_BAS_CANCEL_REASON_TIMEOUT			= 0x02,
+	SSAM_BAS_CANCEL_REASON_FAILED_TO_OPEN		= 0x03,
+	SSAM_BAS_CANCEL_REASON_FAILED_TO_REMAIN_OPEN	= 0x04,
+	SSAM_BAS_CANCEL_REASON_FAILED_TO_CLOSE		= 0x05,
+};
+
+struct ssam_bas_base_info {
+	u8 state;
+	u8 base_id;
+} __packed;
+
+static_assert(sizeof(struct ssam_bas_base_info) == 2);
+
+SSAM_DEFINE_SYNC_REQUEST_N(ssam_bas_latch_lock, {
+	.target_category = SSAM_SSH_TC_BAS,
+	.target_id       = 0x01,
+	.command_id      = 0x06,
+	.instance_id     = 0x00,
+});
+
+SSAM_DEFINE_SYNC_REQUEST_N(ssam_bas_latch_unlock, {
+	.target_category = SSAM_SSH_TC_BAS,
+	.target_id       = 0x01,
+	.command_id      = 0x07,
+	.instance_id     = 0x00,
+});
+
+SSAM_DEFINE_SYNC_REQUEST_N(ssam_bas_latch_request, {
+	.target_category = SSAM_SSH_TC_BAS,
+	.target_id       = 0x01,
+	.command_id      = 0x08,
+	.instance_id     = 0x00,
+});
+
+SSAM_DEFINE_SYNC_REQUEST_N(ssam_bas_latch_confirm, {
+	.target_category = SSAM_SSH_TC_BAS,
+	.target_id       = 0x01,
+	.command_id      = 0x09,
+	.instance_id     = 0x00,
+});
+
+SSAM_DEFINE_SYNC_REQUEST_N(ssam_bas_latch_heartbeat, {
+	.target_category = SSAM_SSH_TC_BAS,
+	.target_id       = 0x01,
+	.command_id      = 0x0a,
+	.instance_id     = 0x00,
+});
+
+SSAM_DEFINE_SYNC_REQUEST_N(ssam_bas_latch_cancel, {
+	.target_category = SSAM_SSH_TC_BAS,
+	.target_id       = 0x01,
+	.command_id      = 0x0b,
+	.instance_id     = 0x00,
+});
+
+SSAM_DEFINE_SYNC_REQUEST_R(ssam_bas_get_base, struct ssam_bas_base_info, {
+	.target_category = SSAM_SSH_TC_BAS,
+	.target_id       = 0x01,
+	.command_id      = 0x0c,
+	.instance_id     = 0x00,
+});
+
+SSAM_DEFINE_SYNC_REQUEST_R(ssam_bas_get_device_mode, u8, {
+	.target_category = SSAM_SSH_TC_BAS,
+	.target_id       = 0x01,
+	.command_id      = 0x0d,
+	.instance_id     = 0x00,
+});
+
+SSAM_DEFINE_SYNC_REQUEST_R(ssam_bas_get_latch_status, u8, {
+	.target_category = SSAM_SSH_TC_BAS,
+	.target_id       = 0x01,
+	.command_id      = 0x11,
+	.instance_id     = 0x00,
+});
+
+
+/* -- Main structures. ------------------------------------------------------ */
+
+enum sdtx_device_state {
+	SDTX_DEVICE_SHUTDOWN_BIT    = BIT(0),
+	SDTX_DEVICE_DIRTY_BASE_BIT  = BIT(1),
+	SDTX_DEVICE_DIRTY_MODE_BIT  = BIT(2),
+	SDTX_DEVICE_DIRTY_LATCH_BIT = BIT(3),
+};
+
+struct sdtx_device {
+	struct kref kref;
+	struct rw_semaphore lock;         /* Guards device and controller reference. */
+
+	struct device *dev;
+	struct ssam_controller *ctrl;
+	unsigned long flags;
+
+	struct miscdevice mdev;
+	wait_queue_head_t waitq;
+	struct mutex write_lock;          /* Guards order of events/notifications. */
+	struct rw_semaphore client_lock;  /* Guards client list.                   */
+	struct list_head client_list;
+
+	struct delayed_work state_work;
+	struct {
+		struct ssam_bas_base_info base;
+		u8 device_mode;
+		u8 latch_status;
+	} state;
+
+	struct delayed_work mode_work;
+	struct input_dev *mode_switch;
+
+	struct ssam_event_notifier notif;
+};
+
+enum sdtx_client_state {
+	SDTX_CLIENT_EVENTS_ENABLED_BIT = BIT(0),
+};
+
+struct sdtx_client {
+	struct sdtx_device *ddev;
+	struct list_head node;
+	unsigned long flags;
+
+	struct fasync_struct *fasync;
+
+	struct mutex read_lock;           /* Guards FIFO buffer read access. */
+	DECLARE_KFIFO(buffer, u8, 512);
+};
+
+static void __sdtx_device_release(struct kref *kref)
+{
+	struct sdtx_device *ddev = container_of(kref, struct sdtx_device, kref);
+
+	mutex_destroy(&ddev->write_lock);
+	kfree(ddev);
+}
+
+static struct sdtx_device *sdtx_device_get(struct sdtx_device *ddev)
+{
+	if (ddev)
+		kref_get(&ddev->kref);
+
+	return ddev;
+}
+
+static void sdtx_device_put(struct sdtx_device *ddev)
+{
+	if (ddev)
+		kref_put(&ddev->kref, __sdtx_device_release);
+}
+
+
+/* -- Firmware value translations. ------------------------------------------ */
+
+static u16 sdtx_translate_base_state(struct sdtx_device *ddev, u8 state)
+{
+	switch (state) {
+	case SSAM_BAS_BASE_STATE_ATTACHED:
+		return SDTX_BASE_ATTACHED;
+
+	case SSAM_BAS_BASE_STATE_DETACH_SUCCESS:
+		return SDTX_BASE_DETACHED;
+
+	case SSAM_BAS_BASE_STATE_NOT_FEASIBLE:
+		return SDTX_DETACH_NOT_FEASIBLE;
+
+	default:
+		dev_err(ddev->dev, "unknown base state: %#04x\n", state);
+		return SDTX_UNKNOWN(state);
+	}
+}
+
+static u16 sdtx_translate_latch_status(struct sdtx_device *ddev, u8 status)
+{
+	switch (status) {
+	case SSAM_BAS_LATCH_STATUS_CLOSED:
+		return SDTX_LATCH_CLOSED;
+
+	case SSAM_BAS_LATCH_STATUS_OPENED:
+		return SDTX_LATCH_OPENED;
+
+	case SSAM_BAS_LATCH_STATUS_FAILED_TO_OPEN:
+		return SDTX_ERR_FAILED_TO_OPEN;
+
+	case SSAM_BAS_LATCH_STATUS_FAILED_TO_REMAIN_OPEN:
+		return SDTX_ERR_FAILED_TO_REMAIN_OPEN;
+
+	case SSAM_BAS_LATCH_STATUS_FAILED_TO_CLOSE:
+		return SDTX_ERR_FAILED_TO_CLOSE;
+
+	default:
+		dev_err(ddev->dev, "unknown latch status: %#04x\n", status);
+		return SDTX_UNKNOWN(status);
+	}
+}
+
+static u16 sdtx_translate_cancel_reason(struct sdtx_device *ddev, u8 reason)
+{
+	switch (reason) {
+	case SSAM_BAS_CANCEL_REASON_NOT_FEASIBLE:
+		return SDTX_DETACH_NOT_FEASIBLE;
+
+	case SSAM_BAS_CANCEL_REASON_TIMEOUT:
+		return SDTX_DETACH_TIMEDOUT;
+
+	case SSAM_BAS_CANCEL_REASON_FAILED_TO_OPEN:
+		return SDTX_ERR_FAILED_TO_OPEN;
+
+	case SSAM_BAS_CANCEL_REASON_FAILED_TO_REMAIN_OPEN:
+		return SDTX_ERR_FAILED_TO_REMAIN_OPEN;
+
+	case SSAM_BAS_CANCEL_REASON_FAILED_TO_CLOSE:
+		return SDTX_ERR_FAILED_TO_CLOSE;
+
+	default:
+		dev_err(ddev->dev, "unknown cancel reason: %#04x\n", reason);
+		return SDTX_UNKNOWN(reason);
+	}
+}
+
+
+/* -- IOCTLs. --------------------------------------------------------------- */
+
+static int sdtx_ioctl_get_base_info(struct sdtx_device *ddev,
+				    struct sdtx_base_info __user *buf)
+{
+	struct ssam_bas_base_info raw;
+	struct sdtx_base_info info;
+	int status;
+
+	lockdep_assert_held_read(&ddev->lock);
+
+	status = ssam_retry(ssam_bas_get_base, ddev->ctrl, &raw);
+	if (status < 0)
+		return status;
+
+	info.state = sdtx_translate_base_state(ddev, raw.state);
+	info.base_id = SDTX_BASE_TYPE_SSH(raw.base_id);
+
+	if (copy_to_user(buf, &info, sizeof(info)))
+		return -EFAULT;
+
+	return 0;
+}
+
+static int sdtx_ioctl_get_device_mode(struct sdtx_device *ddev, u16 __user *buf)
+{
+	u8 mode;
+	int status;
+
+	lockdep_assert_held_read(&ddev->lock);
+
+	status = ssam_retry(ssam_bas_get_device_mode, ddev->ctrl, &mode);
+	if (status < 0)
+		return status;
+
+	return put_user(mode, buf);
+}
+
+static int sdtx_ioctl_get_latch_status(struct sdtx_device *ddev, u16 __user *buf)
+{
+	u8 latch;
+	int status;
+
+	lockdep_assert_held_read(&ddev->lock);
+
+	status = ssam_retry(ssam_bas_get_latch_status, ddev->ctrl, &latch);
+	if (status < 0)
+		return status;
+
+	return put_user(sdtx_translate_latch_status(ddev, latch), buf);
+}
+
+static long __surface_dtx_ioctl(struct sdtx_client *client, unsigned int cmd, unsigned long arg)
+{
+	struct sdtx_device *ddev = client->ddev;
+
+	lockdep_assert_held_read(&ddev->lock);
+
+	switch (cmd) {
+	case SDTX_IOCTL_EVENTS_ENABLE:
+		set_bit(SDTX_CLIENT_EVENTS_ENABLED_BIT, &client->flags);
+		return 0;
+
+	case SDTX_IOCTL_EVENTS_DISABLE:
+		clear_bit(SDTX_CLIENT_EVENTS_ENABLED_BIT, &client->flags);
+		return 0;
+
+	case SDTX_IOCTL_LATCH_LOCK:
+		return ssam_retry(ssam_bas_latch_lock, ddev->ctrl);
+
+	case SDTX_IOCTL_LATCH_UNLOCK:
+		return ssam_retry(ssam_bas_latch_unlock, ddev->ctrl);
+
+	case SDTX_IOCTL_LATCH_REQUEST:
+		return ssam_retry(ssam_bas_latch_request, ddev->ctrl);
+
+	case SDTX_IOCTL_LATCH_CONFIRM:
+		return ssam_retry(ssam_bas_latch_confirm, ddev->ctrl);
+
+	case SDTX_IOCTL_LATCH_HEARTBEAT:
+		return ssam_retry(ssam_bas_latch_heartbeat, ddev->ctrl);
+
+	case SDTX_IOCTL_LATCH_CANCEL:
+		return ssam_retry(ssam_bas_latch_cancel, ddev->ctrl);
+
+	case SDTX_IOCTL_GET_BASE_INFO:
+		return sdtx_ioctl_get_base_info(ddev, (struct sdtx_base_info __user *)arg);
+
+	case SDTX_IOCTL_GET_DEVICE_MODE:
+		return sdtx_ioctl_get_device_mode(ddev, (u16 __user *)arg);
+
+	case SDTX_IOCTL_GET_LATCH_STATUS:
+		return sdtx_ioctl_get_latch_status(ddev, (u16 __user *)arg);
+
+	default:
+		return -EINVAL;
+	}
+}
+
+static long surface_dtx_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
+{
+	struct sdtx_client *client = file->private_data;
+	long status;
+
+	if (down_read_killable(&client->ddev->lock))
+		return -ERESTARTSYS;
+
+	if (test_bit(SDTX_DEVICE_SHUTDOWN_BIT, &client->ddev->flags)) {
+		up_read(&client->ddev->lock);
+		return -ENODEV;
+	}
+
+	status = __surface_dtx_ioctl(client, cmd, arg);
+
+	up_read(&client->ddev->lock);
+	return status;
+}
+
+
+/* -- File operations. ------------------------------------------------------ */
+
+static int surface_dtx_open(struct inode *inode, struct file *file)
+{
+	struct sdtx_device *ddev = container_of(file->private_data, struct sdtx_device, mdev);
+	struct sdtx_client *client;
+
+	/* Initialize client. */
+	client = kzalloc(sizeof(*client), GFP_KERNEL);
+	if (!client)
+		return -ENOMEM;
+
+	client->ddev = sdtx_device_get(ddev);
+
+	INIT_LIST_HEAD(&client->node);
+
+	mutex_init(&client->read_lock);
+	INIT_KFIFO(client->buffer);
+
+	file->private_data = client;
+
+	/* Attach client. */
+	down_write(&ddev->client_lock);
+
+	/*
+	 * Do not add a new client if the device has been shut down. Note that
+	 * it's enough to hold the client_lock here as, during shutdown, we
+	 * only acquire that lock and remove clients after marking the device
+	 * as shut down.
+	 */
+	if (test_bit(SDTX_DEVICE_SHUTDOWN_BIT, &ddev->flags)) {
+		up_write(&ddev->client_lock);
+		sdtx_device_put(client->ddev);
+		kfree(client);
+		return -ENODEV;
+	}
+
+	list_add_tail(&client->node, &ddev->client_list);
+	up_write(&ddev->client_lock);
+
+	stream_open(inode, file);
+	return 0;
+}
+
+static int surface_dtx_release(struct inode *inode, struct file *file)
+{
+	struct sdtx_client *client = file->private_data;
+
+	/* Detach client. */
+	down_write(&client->ddev->client_lock);
+	list_del(&client->node);
+	up_write(&client->ddev->client_lock);
+
+	/* Free client. */
+	sdtx_device_put(client->ddev);
+	mutex_destroy(&client->read_lock);
+	kfree(client);
+
+	return 0;
+}
+
+static ssize_t surface_dtx_read(struct file *file, char __user *buf, size_t count, loff_t *offs)
+{
+	struct sdtx_client *client = file->private_data;
+	struct sdtx_device *ddev = client->ddev;
+	unsigned int copied;
+	int status = 0;
+
+	if (down_read_killable(&ddev->lock))
+		return -ERESTARTSYS;
+
+	/* Make sure we're not shut down. */
+	if (test_bit(SDTX_DEVICE_SHUTDOWN_BIT, &ddev->flags)) {
+		up_read(&ddev->lock);
+		return -ENODEV;
+	}
+
+	do {
+		/* Check availability, wait if necessary. */
+		if (kfifo_is_empty(&client->buffer)) {
+			up_read(&ddev->lock);
+
+			if (file->f_flags & O_NONBLOCK)
+				return -EAGAIN;
+
+			status = wait_event_interruptible(ddev->waitq,
+							  !kfifo_is_empty(&client->buffer) ||
+							  test_bit(SDTX_DEVICE_SHUTDOWN_BIT,
+								   &ddev->flags));
+			if (status < 0)
+				return status;
+
+			if (down_read_killable(&client->ddev->lock))
+				return -ERESTARTSYS;
+
+			/* Need to check that we're not shut down again. */
+			if (test_bit(SDTX_DEVICE_SHUTDOWN_BIT, &ddev->flags)) {
+				up_read(&ddev->lock);
+				return -ENODEV;
+			}
+		}
+
+		/* Try to read from FIFO. */
+		if (mutex_lock_interruptible(&client->read_lock)) {
+			up_read(&ddev->lock);
+			return -ERESTARTSYS;
+		}
+
+		status = kfifo_to_user(&client->buffer, buf, count, &copied);
+		mutex_unlock(&client->read_lock);
+
+		if (status < 0) {
+			up_read(&ddev->lock);
+			return status;
+		}
+
+		/* We might not have gotten anything, check this here. */
+		if (copied == 0 && (file->f_flags & O_NONBLOCK)) {
+			up_read(&ddev->lock);
+			return -EAGAIN;
+		}
+	} while (copied == 0);
+
+	up_read(&ddev->lock);
+	return copied;
+}
+
+static __poll_t surface_dtx_poll(struct file *file, struct poll_table_struct *pt)
+{
+	struct sdtx_client *client = file->private_data;
+	__poll_t events = 0;
+
+	if (down_read_killable(&client->ddev->lock))
+		return -ERESTARTSYS;
+
+	if (test_bit(SDTX_DEVICE_SHUTDOWN_BIT, &client->ddev->flags)) {
+		up_read(&client->ddev->lock);
+		return EPOLLHUP | EPOLLERR;
+	}
+
+	poll_wait(file, &client->ddev->waitq, pt);
+
+	if (!kfifo_is_empty(&client->buffer))
+		events |= EPOLLIN | EPOLLRDNORM;
+
+	up_read(&client->ddev->lock);
+	return events;
+}
+
+static int surface_dtx_fasync(int fd, struct file *file, int on)
+{
+	struct sdtx_client *client = file->private_data;
+
+	return fasync_helper(fd, file, on, &client->fasync);
+}
+
+static const struct file_operations surface_dtx_fops = {
+	.owner          = THIS_MODULE,
+	.open           = surface_dtx_open,
+	.release        = surface_dtx_release,
+	.read           = surface_dtx_read,
+	.poll           = surface_dtx_poll,
+	.fasync         = surface_dtx_fasync,
+	.unlocked_ioctl = surface_dtx_ioctl,
+	.compat_ioctl   = surface_dtx_ioctl,
+	.llseek         = no_llseek,
+};
+
+
+/* -- Event handling/forwarding. -------------------------------------------- */
+
+/*
+ * The device operation mode is not immediately updated on the EC when the
+ * base has been connected, i.e. querying the device mode inside the
+ * connection event callback yields an outdated value. Thus, we can only
+ * determine the new tablet-mode switch and device mode values after some
+ * time.
+ *
+ * These delays have been chosen by experimenting. We first delay on connect
+ * events, then check and validate the device mode against the base state and
+ * if invalid delay again by the "recheck" delay.
+ */
+#define SDTX_DEVICE_MODE_DELAY_CONNECT	msecs_to_jiffies(100)
+#define SDTX_DEVICE_MODE_DELAY_RECHECK	msecs_to_jiffies(100)
+
+struct sdtx_status_event {
+	struct sdtx_event e;
+	__u16 v;
+} __packed;
+
+struct sdtx_base_info_event {
+	struct sdtx_event e;
+	struct sdtx_base_info v;
+} __packed;
+
+union sdtx_generic_event {
+	struct sdtx_event common;
+	struct sdtx_status_event status;
+	struct sdtx_base_info_event base;
+};
+
+static void sdtx_update_device_mode(struct sdtx_device *ddev, unsigned long delay);
+
+/* Must be executed with ddev->write_lock held. */
+static void sdtx_push_event(struct sdtx_device *ddev, struct sdtx_event *evt)
+{
+	const size_t len = sizeof(struct sdtx_event) + evt->length;
+	struct sdtx_client *client;
+
+	lockdep_assert_held(&ddev->write_lock);
+
+	down_read(&ddev->client_lock);
+	list_for_each_entry(client, &ddev->client_list, node) {
+		if (!test_bit(SDTX_CLIENT_EVENTS_ENABLED_BIT, &client->flags))
+			continue;
+
+		if (likely(kfifo_avail(&client->buffer) >= len))
+			kfifo_in(&client->buffer, (const u8 *)evt, len);
+		else
+			dev_warn(ddev->dev, "event buffer overrun\n");
+
+		kill_fasync(&client->fasync, SIGIO, POLL_IN);
+	}
+	up_read(&ddev->client_lock);
+
+	wake_up_interruptible(&ddev->waitq);
+}
+
+static u32 sdtx_notifier(struct ssam_event_notifier *nf, const struct ssam_event *in)
+{
+	struct sdtx_device *ddev = container_of(nf, struct sdtx_device, notif);
+	union sdtx_generic_event event;
+	size_t len;
+
+	/* Validate event payload length. */
+	switch (in->command_id) {
+	case SAM_EVENT_CID_DTX_CONNECTION:
+		len = 2 * sizeof(u8);
+		break;
+
+	case SAM_EVENT_CID_DTX_REQUEST:
+		len = 0;
+		break;
+
+	case SAM_EVENT_CID_DTX_CANCEL:
+		len = sizeof(u8);
+		break;
+
+	case SAM_EVENT_CID_DTX_LATCH_STATUS:
+		len = sizeof(u8);
+		break;
+
+	default:
+		return 0;
+	};
+
+	if (in->length != len) {
+		dev_err(ddev->dev,
+			"unexpected payload size for event %#04x: got %u, expected %zu\n",
+			in->command_id, in->length, len);
+		return 0;
+	}
+
+	mutex_lock(&ddev->write_lock);
+
+	/* Translate event. */
+	switch (in->command_id) {
+	case SAM_EVENT_CID_DTX_CONNECTION:
+		clear_bit(SDTX_DEVICE_DIRTY_BASE_BIT, &ddev->flags);
+
+		/* If state has not changed: do not send new event. */
+		if (ddev->state.base.state == in->data[0] &&
+		    ddev->state.base.base_id == in->data[1])
+			goto out;
+
+		ddev->state.base.state = in->data[0];
+		ddev->state.base.base_id = in->data[1];
+
+		event.base.e.length = sizeof(struct sdtx_base_info);
+		event.base.e.code = SDTX_EVENT_BASE_CONNECTION;
+		event.base.v.state = sdtx_translate_base_state(ddev, in->data[0]);
+		event.base.v.base_id = SDTX_BASE_TYPE_SSH(in->data[1]);
+		break;
+
+	case SAM_EVENT_CID_DTX_REQUEST:
+		event.common.code = SDTX_EVENT_REQUEST;
+		event.common.length = 0;
+		break;
+
+	case SAM_EVENT_CID_DTX_CANCEL:
+		event.status.e.length = sizeof(u16);
+		event.status.e.code = SDTX_EVENT_CANCEL;
+		event.status.v = sdtx_translate_cancel_reason(ddev, in->data[0]);
+		break;
+
+	case SAM_EVENT_CID_DTX_LATCH_STATUS:
+		clear_bit(SDTX_DEVICE_DIRTY_LATCH_BIT, &ddev->flags);
+
+		/* If state has not changed: do not send new event. */
+		if (ddev->state.latch_status == in->data[0])
+			goto out;
+
+		ddev->state.latch_status = in->data[0];
+
+		event.status.e.length = sizeof(u16);
+		event.status.e.code = SDTX_EVENT_LATCH_STATUS;
+		event.status.v = sdtx_translate_latch_status(ddev, in->data[0]);
+		break;
+	}
+
+	sdtx_push_event(ddev, &event.common);
+
+	/* Update device mode on base connection change. */
+	if (in->command_id == SAM_EVENT_CID_DTX_CONNECTION) {
+		unsigned long delay;
+
+		delay = in->data[0] ? SDTX_DEVICE_MODE_DELAY_CONNECT : 0;
+		sdtx_update_device_mode(ddev, delay);
+	}
+
+out:
+	mutex_unlock(&ddev->write_lock);
+	return SSAM_NOTIF_HANDLED;
+}
+
+
+/* -- State update functions. ----------------------------------------------- */
+
+static bool sdtx_device_mode_invalid(u8 mode, u8 base_state)
+{
+	return ((base_state == SSAM_BAS_BASE_STATE_ATTACHED) &&
+		(mode == SDTX_DEVICE_MODE_TABLET)) ||
+	       ((base_state == SSAM_BAS_BASE_STATE_DETACH_SUCCESS) &&
+		(mode != SDTX_DEVICE_MODE_TABLET));
+}
+
+static void sdtx_device_mode_workfn(struct work_struct *work)
+{
+	struct sdtx_device *ddev = container_of(work, struct sdtx_device, mode_work.work);
+	struct sdtx_status_event event;
+	struct ssam_bas_base_info base;
+	int status, tablet;
+	u8 mode;
+
+	/* Get operation mode. */
+	status = ssam_retry(ssam_bas_get_device_mode, ddev->ctrl, &mode);
+	if (status) {
+		dev_err(ddev->dev, "failed to get device mode: %d\n", status);
+		return;
+	}
+
+	/* Get base info. */
+	status = ssam_retry(ssam_bas_get_base, ddev->ctrl, &base);
+	if (status) {
+		dev_err(ddev->dev, "failed to get base info: %d\n", status);
+		return;
+	}
+
+	/*
+	 * In some cases (specifically when attaching the base), the device
+	 * mode isn't updated right away. Thus we check if the device mode
+	 * makes sense for the given base state and try again later if it
+	 * doesn't.
+	 */
+	if (sdtx_device_mode_invalid(mode, base.state)) {
+		dev_dbg(ddev->dev, "device mode is invalid, trying again\n");
+		sdtx_update_device_mode(ddev, SDTX_DEVICE_MODE_DELAY_RECHECK);
+		return;
+	}
+
+	mutex_lock(&ddev->write_lock);
+	clear_bit(SDTX_DEVICE_DIRTY_MODE_BIT, &ddev->flags);
+
+	/* Avoid sending duplicate device-mode events. */
+	if (ddev->state.device_mode == mode) {
+		mutex_unlock(&ddev->write_lock);
+		return;
+	}
+
+	ddev->state.device_mode = mode;
+
+	event.e.length = sizeof(u16);
+	event.e.code = SDTX_EVENT_DEVICE_MODE;
+	event.v = mode;
+
+	sdtx_push_event(ddev, &event.e);
+
+	/* Send SW_TABLET_MODE event. */
+	tablet = mode != SDTX_DEVICE_MODE_LAPTOP;
+	input_report_switch(ddev->mode_switch, SW_TABLET_MODE, tablet);
+	input_sync(ddev->mode_switch);
+
+	mutex_unlock(&ddev->write_lock);
+}
+
+static void sdtx_update_device_mode(struct sdtx_device *ddev, unsigned long delay)
+{
+	schedule_delayed_work(&ddev->mode_work, delay);
+}
+
+/* Must be executed with ddev->write_lock held. */
+static void __sdtx_device_state_update_base(struct sdtx_device *ddev,
+					    struct ssam_bas_base_info info)
+{
+	struct sdtx_base_info_event event;
+
+	lockdep_assert_held(&ddev->write_lock);
+
+	/* Prevent duplicate events. */
+	if (ddev->state.base.state == info.state &&
+	    ddev->state.base.base_id == info.base_id)
+		return;
+
+	ddev->state.base = info;
+
+	event.e.length = sizeof(struct sdtx_base_info);
+	event.e.code = SDTX_EVENT_BASE_CONNECTION;
+	event.v.state = sdtx_translate_base_state(ddev, info.state);
+	event.v.base_id = SDTX_BASE_TYPE_SSH(info.base_id);
+
+	sdtx_push_event(ddev, &event.e);
+}
+
+/* Must be executed with ddev->write_lock held. */
+static void __sdtx_device_state_update_mode(struct sdtx_device *ddev, u8 mode)
+{
+	struct sdtx_status_event event;
+	int tablet;
+
+	/*
+	 * Note: This function must be called after updating the base state
+	 * via __sdtx_device_state_update_base(), as we rely on the updated
+	 * base state value in the validity check below.
+	 */
+
+	lockdep_assert_held(&ddev->write_lock);
+
+	if (sdtx_device_mode_invalid(mode, ddev->state.base.state)) {
+		dev_dbg(ddev->dev, "device mode is invalid, trying again\n");
+		sdtx_update_device_mode(ddev, SDTX_DEVICE_MODE_DELAY_RECHECK);
+		return;
+	}
+
+	/* Prevent duplicate events. */
+	if (ddev->state.device_mode == mode)
+		return;
+
+	ddev->state.device_mode = mode;
+
+	/* Send event. */
+	event.e.length = sizeof(u16);
+	event.e.code = SDTX_EVENT_DEVICE_MODE;
+	event.v = mode;
+
+	sdtx_push_event(ddev, &event.e);
+
+	/* Send SW_TABLET_MODE event. */
+	tablet = mode != SDTX_DEVICE_MODE_LAPTOP;
+	input_report_switch(ddev->mode_switch, SW_TABLET_MODE, tablet);
+	input_sync(ddev->mode_switch);
+}
+
+/* Must be executed with ddev->write_lock held. */
+static void __sdtx_device_state_update_latch(struct sdtx_device *ddev, u8 status)
+{
+	struct sdtx_status_event event;
+
+	lockdep_assert_held(&ddev->write_lock);
+
+	/* Prevent duplicate events. */
+	if (ddev->state.latch_status == status)
+		return;
+
+	ddev->state.latch_status = status;
+
+	event.e.length = sizeof(struct sdtx_base_info);
+	event.e.code = SDTX_EVENT_BASE_CONNECTION;
+	event.v = sdtx_translate_latch_status(ddev, status);
+
+	sdtx_push_event(ddev, &event.e);
+}
+
+static void sdtx_device_state_workfn(struct work_struct *work)
+{
+	struct sdtx_device *ddev = container_of(work, struct sdtx_device, state_work.work);
+	struct ssam_bas_base_info base;
+	u8 mode, latch;
+	int status;
+
+	/* Mark everything as dirty. */
+	set_bit(SDTX_DEVICE_DIRTY_BASE_BIT, &ddev->flags);
+	set_bit(SDTX_DEVICE_DIRTY_MODE_BIT, &ddev->flags);
+	set_bit(SDTX_DEVICE_DIRTY_LATCH_BIT, &ddev->flags);
+
+	/*
+	 * Ensure that the state gets marked as dirty before continuing to
+	 * query it. Necessary to ensure that clear_bit() calls in
+	 * sdtx_notifier() and sdtx_device_mode_workfn() actually clear these
+	 * bits if an event is received while updating the state here.
+	 */
+	smp_mb__after_atomic();
+
+	status = ssam_retry(ssam_bas_get_base, ddev->ctrl, &base);
+	if (status) {
+		dev_err(ddev->dev, "failed to get base state: %d\n", status);
+		return;
+	}
+
+	status = ssam_retry(ssam_bas_get_device_mode, ddev->ctrl, &mode);
+	if (status) {
+		dev_err(ddev->dev, "failed to get device mode: %d\n", status);
+		return;
+	}
+
+	status = ssam_retry(ssam_bas_get_latch_status, ddev->ctrl, &latch);
+	if (status) {
+		dev_err(ddev->dev, "failed to get latch status: %d\n", status);
+		return;
+	}
+
+	mutex_lock(&ddev->write_lock);
+
+	/*
+	 * If the respective dirty-bit has been cleared, an event has been
+	 * received, updating this state. The queried state may thus be out of
+	 * date. At this point, we can safely assume that the state provided
+	 * by the event is either up to date, or we're about to receive
+	 * another event updating it.
+	 */
+
+	if (test_and_clear_bit(SDTX_DEVICE_DIRTY_BASE_BIT, &ddev->flags))
+		__sdtx_device_state_update_base(ddev, base);
+
+	if (test_and_clear_bit(SDTX_DEVICE_DIRTY_MODE_BIT, &ddev->flags))
+		__sdtx_device_state_update_mode(ddev, mode);
+
+	if (test_and_clear_bit(SDTX_DEVICE_DIRTY_LATCH_BIT, &ddev->flags))
+		__sdtx_device_state_update_latch(ddev, latch);
+
+	mutex_unlock(&ddev->write_lock);
+}
+
+static void sdtx_update_device_state(struct sdtx_device *ddev, unsigned long delay)
+{
+	schedule_delayed_work(&ddev->state_work, delay);
+}
+
+
+/* -- Common device initialization. ----------------------------------------- */
+
+static int sdtx_device_init(struct sdtx_device *ddev, struct device *dev,
+			    struct ssam_controller *ctrl)
+{
+	int status, tablet_mode;
+
+	/* Basic initialization. */
+	kref_init(&ddev->kref);
+	init_rwsem(&ddev->lock);
+	ddev->dev = dev;
+	ddev->ctrl = ctrl;
+
+	ddev->mdev.minor = MISC_DYNAMIC_MINOR;
+	ddev->mdev.name = "surface_dtx";
+	ddev->mdev.nodename = "surface/dtx";
+	ddev->mdev.fops = &surface_dtx_fops;
+
+	ddev->notif.base.priority = 1;
+	ddev->notif.base.fn = sdtx_notifier;
+	ddev->notif.event.reg = SSAM_EVENT_REGISTRY_SAM;
+	ddev->notif.event.id.target_category = SSAM_SSH_TC_BAS;
+	ddev->notif.event.id.instance = 0;
+	ddev->notif.event.mask = SSAM_EVENT_MASK_NONE;
+	ddev->notif.event.flags = SSAM_EVENT_SEQUENCED;
+
+	init_waitqueue_head(&ddev->waitq);
+	mutex_init(&ddev->write_lock);
+	init_rwsem(&ddev->client_lock);
+	INIT_LIST_HEAD(&ddev->client_list);
+
+	INIT_DELAYED_WORK(&ddev->mode_work, sdtx_device_mode_workfn);
+	INIT_DELAYED_WORK(&ddev->state_work, sdtx_device_state_workfn);
+
+	/*
+	 * Get current device state. We want to guarantee that events are only
+	 * sent when state actually changes. Thus we cannot use special
+	 * "uninitialized" values, as that would cause problems when manually
+	 * querying the state in surface_dtx_pm_complete(). I.e. we would not
+	 * be able to detect state changes there if no change event has been
+	 * received between driver initialization and first device suspension.
+	 *
+	 * Note that we also need to do this before registering the event
+	 * notifier, as that may access the state values.
+	 */
+	status = ssam_retry(ssam_bas_get_base, ddev->ctrl, &ddev->state.base);
+	if (status)
+		return status;
+
+	status = ssam_retry(ssam_bas_get_device_mode, ddev->ctrl, &ddev->state.device_mode);
+	if (status)
+		return status;
+
+	status = ssam_retry(ssam_bas_get_latch_status, ddev->ctrl, &ddev->state.latch_status);
+	if (status)
+		return status;
+
+	/* Set up tablet mode switch. */
+	ddev->mode_switch = input_allocate_device();
+	if (!ddev->mode_switch)
+		return -ENOMEM;
+
+	ddev->mode_switch->name = "Microsoft Surface DTX Device Mode Switch";
+	ddev->mode_switch->phys = "ssam/01:11:01:00:00/input0";
+	ddev->mode_switch->id.bustype = BUS_HOST;
+	ddev->mode_switch->dev.parent = ddev->dev;
+
+	tablet_mode = (ddev->state.device_mode != SDTX_DEVICE_MODE_LAPTOP);
+	input_set_capability(ddev->mode_switch, EV_SW, SW_TABLET_MODE);
+	input_report_switch(ddev->mode_switch, SW_TABLET_MODE, tablet_mode);
+
+	status = input_register_device(ddev->mode_switch);
+	if (status) {
+		input_free_device(ddev->mode_switch);
+		return status;
+	}
+
+	/* Set up event notifier. */
+	status = ssam_notifier_register(ddev->ctrl, &ddev->notif);
+	if (status)
+		goto err_notif;
+
+	/* Register miscdevice. */
+	status = misc_register(&ddev->mdev);
+	if (status)
+		goto err_mdev;
+
+	/*
+	 * Update device state in case it has changed between getting the
+	 * initial mode and registering the event notifier.
+	 */
+	sdtx_update_device_state(ddev, 0);
+	return 0;
+
+err_notif:
+	ssam_notifier_unregister(ddev->ctrl, &ddev->notif);
+	cancel_delayed_work_sync(&ddev->mode_work);
+err_mdev:
+	input_unregister_device(ddev->mode_switch);
+	return status;
+}
+
+static struct sdtx_device *sdtx_device_create(struct device *dev, struct ssam_controller *ctrl)
+{
+	struct sdtx_device *ddev;
+	int status;
+
+	ddev = kzalloc(sizeof(*ddev), GFP_KERNEL);
+	if (!ddev)
+		return ERR_PTR(-ENOMEM);
+
+	status = sdtx_device_init(ddev, dev, ctrl);
+	if (status) {
+		sdtx_device_put(ddev);
+		return ERR_PTR(status);
+	}
+
+	return ddev;
+}
+
+static void sdtx_device_destroy(struct sdtx_device *ddev)
+{
+	struct sdtx_client *client;
+
+	/*
+	 * Mark device as shut-down. Prevent new clients from being added and
+	 * new operations from being executed.
+	 */
+	set_bit(SDTX_DEVICE_SHUTDOWN_BIT, &ddev->flags);
+
+	/* Disable notifiers, prevent new events from arriving. */
+	ssam_notifier_unregister(ddev->ctrl, &ddev->notif);
+
+	/* Stop mode_work, prevent access to mode_switch. */
+	cancel_delayed_work_sync(&ddev->mode_work);
+
+	/* Stop state_work. */
+	cancel_delayed_work_sync(&ddev->state_work);
+
+	/* With mode_work canceled, we can unregister the mode_switch. */
+	input_unregister_device(ddev->mode_switch);
+
+	/* Wake up async clients. */
+	down_write(&ddev->client_lock);
+	list_for_each_entry(client, &ddev->client_list, node) {
+		kill_fasync(&client->fasync, SIGIO, POLL_HUP);
+	}
+	up_write(&ddev->client_lock);
+
+	/* Wake up blocking clients. */
+	wake_up_interruptible(&ddev->waitq);
+
+	/*
+	 * Wait for clients to finish their current operation. After this, the
+	 * controller and device references are guaranteed to be no longer in
+	 * use.
+	 */
+	down_write(&ddev->lock);
+	ddev->dev = NULL;
+	ddev->ctrl = NULL;
+	up_write(&ddev->lock);
+
+	/* Finally remove the misc-device. */
+	misc_deregister(&ddev->mdev);
+
+	/*
+	 * We're now guaranteed that sdtx_device_open() won't be called any
+	 * more, so we can now drop out reference.
+	 */
+	sdtx_device_put(ddev);
+}
+
+
+/* -- PM ops. --------------------------------------------------------------- */
+
+#ifdef CONFIG_PM_SLEEP
+
+static void surface_dtx_pm_complete(struct device *dev)
+{
+	struct sdtx_device *ddev = dev_get_drvdata(dev);
+
+	/*
+	 * Normally, the EC will store events while suspended (i.e. in
+	 * display-off state) and release them when resumed (i.e. transitioned
+	 * to display-on state). During hibernation, however, the EC will be
+	 * shut down and does not store events. Furthermore, events might be
+	 * dropped during prolonged suspension (it is currently unknown how
+	 * big this event buffer is and how it behaves on overruns).
+	 *
+	 * To prevent any problems, we update the device state here. We do
+	 * this delayed to ensure that any events sent by the EC directly
+	 * after resuming will be handled first. The delay below has been
+	 * chosen (experimentally), so that there should be ample time for
+	 * these events to be handled, before we check and, if necessary,
+	 * update the state.
+	 */
+	sdtx_update_device_state(ddev, msecs_to_jiffies(1000));
+}
+
+static const struct dev_pm_ops surface_dtx_pm_ops = {
+	.complete = surface_dtx_pm_complete,
+};
+
+#else /* CONFIG_PM_SLEEP */
+
+static const struct dev_pm_ops surface_dtx_pm_ops = {};
+
+#endif /* CONFIG_PM_SLEEP */
+
+
+/* -- Platform driver. ------------------------------------------------------ */
+
+static int surface_dtx_platform_probe(struct platform_device *pdev)
+{
+	struct ssam_controller *ctrl;
+	struct sdtx_device *ddev;
+
+	/* Link to EC. */
+	ctrl = ssam_client_bind(&pdev->dev);
+	if (IS_ERR(ctrl))
+		return PTR_ERR(ctrl) == -ENODEV ? -EPROBE_DEFER : PTR_ERR(ctrl);
+
+	ddev = sdtx_device_create(&pdev->dev, ctrl);
+	if (IS_ERR(ddev))
+		return PTR_ERR(ddev);
+
+	platform_set_drvdata(pdev, ddev);
+	return 0;
+}
+
+static int surface_dtx_platform_remove(struct platform_device *pdev)
+{
+	sdtx_device_destroy(platform_get_drvdata(pdev));
+	return 0;
+}
+
+static const struct acpi_device_id surface_dtx_acpi_match[] = {
+	{ "MSHW0133", 0 },
+	{ },
+};
+MODULE_DEVICE_TABLE(acpi, surface_dtx_acpi_match);
+
+static struct platform_driver surface_dtx_platform_driver = {
+	.probe = surface_dtx_platform_probe,
+	.remove = surface_dtx_platform_remove,
+	.driver = {
+		.name = "surface_dtx_pltf",
+		.acpi_match_table = surface_dtx_acpi_match,
+		.pm = &surface_dtx_pm_ops,
+		.probe_type = PROBE_PREFER_ASYNCHRONOUS,
+	},
+};
+module_platform_driver(surface_dtx_platform_driver);
+
+MODULE_AUTHOR("Maximilian Luz <luzmaximilian@gmail.com>");
+MODULE_DESCRIPTION("Detachment-system driver for Surface System Aggregator Module");
+MODULE_LICENSE("GPL");
diff --git a/include/uapi/linux/surface_aggregator/dtx.h b/include/uapi/linux/surface_aggregator/dtx.h
new file mode 100644
index 000000000000..0833aab0d819
--- /dev/null
+++ b/include/uapi/linux/surface_aggregator/dtx.h
@@ -0,0 +1,146 @@
+/* SPDX-License-Identifier: GPL-2.0+ WITH Linux-syscall-note */
+/*
+ * Surface DTX (clipboard detachment system driver) user-space interface.
+ *
+ * Definitions, structs, and IOCTLs for the /dev/surface/dtx misc device. This
+ * device allows user-space to control the clipboard detachment process on
+ * Surface Book series devices.
+ *
+ * Copyright (C) 2020-2021 Maximilian Luz <luzmaximilian@gmail.com>
+ */
+
+#ifndef _UAPI_LINUX_SURFACE_AGGREGATOR_DTX_H
+#define _UAPI_LINUX_SURFACE_AGGREGATOR_DTX_H
+
+#include <linux/ioctl.h>
+#include <linux/types.h>
+
+/* Status/error categories */
+#define SDTX_CATEGORY_STATUS		0x0000
+#define SDTX_CATEGORY_RUNTIME_ERROR	0x1000
+#define SDTX_CATEGORY_HARDWARE_ERROR	0x2000
+#define SDTX_CATEGORY_UNKNOWN		0xf000
+
+#define SDTX_CATEGORY_MASK		0xf000
+#define SDTX_CATEGORY(value)		((value) & SDTX_CATEGORY_MASK)
+
+#define SDTX_STATUS(code)		((code) | SDTX_CATEGORY_STATUS)
+#define SDTX_ERR_RT(code)		((code) | SDTX_CATEGORY_RUNTIME_ERROR)
+#define SDTX_ERR_HW(code)		((code) | SDTX_CATEGORY_HARDWARE_ERROR)
+#define SDTX_UNKNOWN(code)		((code) | SDTX_CATEGORY_UNKNOWN)
+
+#define SDTX_SUCCESS(value)		(SDTX_CATEGORY(value) == SDTX_CATEGORY_STATUS)
+
+/* Latch status values */
+#define SDTX_LATCH_CLOSED		SDTX_STATUS(0x00)
+#define SDTX_LATCH_OPENED		SDTX_STATUS(0x01)
+
+/* Base state values */
+#define SDTX_BASE_DETACHED		SDTX_STATUS(0x00)
+#define SDTX_BASE_ATTACHED		SDTX_STATUS(0x01)
+
+/* Runtime errors (non-critical) */
+#define SDTX_DETACH_NOT_FEASIBLE	SDTX_ERR_RT(0x01)
+#define SDTX_DETACH_TIMEDOUT		SDTX_ERR_RT(0x02)
+
+/* Hardware errors (critical) */
+#define SDTX_ERR_FAILED_TO_OPEN		SDTX_ERR_HW(0x01)
+#define SDTX_ERR_FAILED_TO_REMAIN_OPEN	SDTX_ERR_HW(0x02)
+#define SDTX_ERR_FAILED_TO_CLOSE	SDTX_ERR_HW(0x03)
+
+/* Base types */
+#define SDTX_DEVICE_TYPE_HID		0x0100
+#define SDTX_DEVICE_TYPE_SSH		0x0200
+
+#define SDTX_DEVICE_TYPE_MASK		0x0f00
+#define SDTX_DEVICE_TYPE(value)		((value) & SDTX_DEVICE_TYPE_MASK)
+
+#define SDTX_BASE_TYPE_HID(id)		((id) | SDTX_DEVICE_TYPE_HID)
+#define SDTX_BASE_TYPE_SSH(id)		((id) | SDTX_DEVICE_TYPE_SSH)
+
+/**
+ * enum sdtx_device_mode - Mode describing how (and if) the clipboard is
+ * attached to the base of the device.
+ * @SDTX_DEVICE_MODE_TABLET: The clipboard is detached from the base and the
+ *                           device operates as tablet.
+ * @SDTX_DEVICE_MODE_LAPTOP: The clipboard is attached normally to the base
+ *                           and the device operates as laptop.
+ * @SDTX_DEVICE_MODE_STUDIO: The clipboard is attached to the base in reverse.
+ *                           The device operates as tablet with keyboard and
+ *                           touchpad deactivated, however, the base battery
+ *                           and, if present in the specific device model, dGPU
+ *                           are available to the system.
+ */
+enum sdtx_device_mode {
+	SDTX_DEVICE_MODE_TABLET		= 0x00,
+	SDTX_DEVICE_MODE_LAPTOP		= 0x01,
+	SDTX_DEVICE_MODE_STUDIO		= 0x02,
+};
+
+/**
+ * struct sdtx_event - Event provided by reading from the DTX device file.
+ * @length: Length of the event payload, in bytes.
+ * @code:   Event code, detailing what type of event this is.
+ * @data:   Payload of the event, containing @length bytes.
+ *
+ * See &enum sdtx_event_code for currently valid event codes.
+ */
+struct sdtx_event {
+	__u16 length;
+	__u16 code;
+	__u8 data[];
+} __attribute__((__packed__));
+
+/**
+ * enum sdtx_event_code - Code describing the type of an event.
+ * @SDTX_EVENT_REQUEST:         Detachment request event type.
+ * @SDTX_EVENT_CANCEL:          Cancel detachment process event type.
+ * @SDTX_EVENT_BASE_CONNECTION: Base/clipboard connection change event type.
+ * @SDTX_EVENT_LATCH_STATUS:    Latch status change event type.
+ * @SDTX_EVENT_DEVICE_MODE:     Device mode change event type.
+ *
+ * Used in &struct sdtx_event to describe the type of the event. Further event
+ * codes are reserved for future use. Any event parser should be able to
+ * gracefully handle unknown events, i.e. by simply skipping them.
+ *
+ * Consult the DTX user-space interface documentation for details regarding
+ * the individual event types.
+ */
+enum sdtx_event_code {
+	SDTX_EVENT_REQUEST		= 1,
+	SDTX_EVENT_CANCEL		= 2,
+	SDTX_EVENT_BASE_CONNECTION	= 3,
+	SDTX_EVENT_LATCH_STATUS		= 4,
+	SDTX_EVENT_DEVICE_MODE		= 5,
+};
+
+/**
+ * struct sdtx_base_info - Describes if and what type of base is connected.
+ * @state:   The state of the connection. Valid values are %SDTX_BASE_DETACHED,
+ *           %SDTX_BASE_ATTACHED, and %SDTX_DETACH_NOT_FEASIBLE (in case a base
+ *           is attached but low clipboard battery prevents detachment). Other
+ *           values are currently reserved.
+ * @base_id: The type of base connected. Zero if no base is connected.
+ */
+struct sdtx_base_info {
+	__u16 state;
+	__u16 base_id;
+} __attribute__((__packed__));
+
+/* IOCTLs */
+#define SDTX_IOCTL_EVENTS_ENABLE	_IO(0xa5, 0x21)
+#define SDTX_IOCTL_EVENTS_DISABLE	_IO(0xa5, 0x22)
+
+#define SDTX_IOCTL_LATCH_LOCK		_IO(0xa5, 0x23)
+#define SDTX_IOCTL_LATCH_UNLOCK		_IO(0xa5, 0x24)
+
+#define SDTX_IOCTL_LATCH_REQUEST	_IO(0xa5, 0x25)
+#define SDTX_IOCTL_LATCH_CONFIRM	_IO(0xa5, 0x26)
+#define SDTX_IOCTL_LATCH_HEARTBEAT	_IO(0xa5, 0x27)
+#define SDTX_IOCTL_LATCH_CANCEL		_IO(0xa5, 0x28)
+
+#define SDTX_IOCTL_GET_BASE_INFO	_IOR(0xa5, 0x29, struct sdtx_base_info)
+#define SDTX_IOCTL_GET_DEVICE_MODE	_IOR(0xa5, 0x2a, __u16)
+#define SDTX_IOCTL_GET_LATCH_STATUS	_IOR(0xa5, 0x2b, __u16)
+
+#endif /* _UAPI_LINUX_SURFACE_AGGREGATOR_DTX_H */
-- 
2.30.1