platform/surface: Add DTX driver

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>
Link: https://lore.kernel.org/r/20210308184819.437438-2-luzmaximilian@gmail.com
Signed-off-by: Hans de Goede <hdegoede@redhat.com>

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

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

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

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