WIP: hid: transport: spi: add apple SPI transport

Keyboard and touchpad of Apple Sillicon SoCs (M1, M1 Pro/Max) laptops
are connected over SPI. The input devices it use HID.`

This is the same protocol as drivers/input/keyboard/applespi.c. At the
time it appears nobody noticed that it was using HID over SPI.
Adding support for ACPI based Intel MacBooks currently using applespi
should be possible.

Microsoft has a specification for HID over SPI [1] incompatible with the
transport protocol used by Apple.

[1] https://docs.microsoft.com/en-us/windows-hardware/drivers/hid/hid-over-spi

Signed-off-by: Janne Grunau <j@jannau.net>

drivers/hid/Kconfig

@@ -1268,4 +1268,6 @@ source "drivers/hid/amd-sfh-hid/Kconfig"
 
 source "drivers/hid/surface-hid/Kconfig"
 
+source "drivers/hid/spi-hid/Kconfig"
+
 endmenu

drivers/hid/Makefile

@@ -151,3 +151,5 @@ obj-$(INTEL_ISH_FIRMWARE_DOWNLOADER)	+= intel-ish-hid/
 obj-$(CONFIG_AMD_SFH_HID)       += amd-sfh-hid/
 
 obj-$(CONFIG_SURFACE_HID_CORE)  += surface-hid/
+
+obj-$(CONFIG_SPI_HID_APPLE)	+= spi-hid/

drivers/hid/spi-hid/Kconfig

@@ -0,0 +1,18 @@
+# SPDX-License-Identifier: GPL-2.0-only
+menu "SPI HID support"
+	depends on SPI
+
+config SPI_HID_APPLE
+	tristate "HID over SPI transport layer for Apple Silicon SoCs"
+	default n
+	depends on SPI && INPUT && OF
+	help
+	  Say Y here if you use Apple Silicon based laptop. The keyboard and
+	  touchpad are HID based devices connected via SPI.\
+
+	  If unsure, say N.
+
+	  This support is also available as a module.  If so, the module
+	  will be called spi-hid-apple.
+
+endmenu

drivers/hid/spi-hid/Makefile

@@ -0,0 +1,6 @@
+# SPDX-License-Identifier: GPL-2.0-only
+#
+# Makefile for SPI HID tarnsport drivers
+#
+
+obj-$(CONFIG_SPI_HID_APPLE)		+= spi-hid-apple.o