USB: misc: Add onboard_usb_hub driver

The main issue this driver addresses is that a USB hub needs to be
powered before it can be discovered. For discrete onboard hubs (an
example for such a hub is the Realtek RTS5411) this is often solved
by supplying the hub with an 'always-on' regulator, which is kind
of a hack. Some onboard hubs may require further initialization
steps, like changing the state of a GPIO or enabling a clock, which
requires even more hacks. This driver creates a platform device
representing the hub which performs the necessary initialization.
Currently it only supports switching on a single regulator, support
for multiple regulators or other actions can be added as needed.
Different initialization sequences can be supported based on the
compatible string.

Besides performing the initialization the driver can be configured
to power the hub off during system suspend. This can help to extend
battery life on battery powered devices which have no requirements
to keep the hub powered during suspend. The driver can also be
configured to leave the hub powered when a wakeup capable USB device
is connected when suspending, and power it off otherwise.

Technically the driver consists of two drivers, the platform driver
described above and a very thin USB driver that subclasses the
generic driver. The purpose of this driver is to provide the platform
driver with the USB devices corresponding to the hub(s) (a hub
controller may provide multiple 'logical' hubs, e.g. one to support
USB 2.0 and another for USB 3.x).

Note: the current series only supports hubs connected directly to
a root hub (through xhci-plat), support for other configurations
could be added if needed.

Co-developed-by: Ravi Chandra Sadineni <ravisadineni@chromium.org>
Signed-off-by: Ravi Chandra Sadineni <ravisadineni@chromium.org>
Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Acked-by: Alan Stern <stern@rowland.harvard.edu>

Documentation/ABI/testing/sysfs-bus-platform-onboard-usb-hub

@@ -0,0 +1,8 @@
+What:		/sys/bus/platform/devices/<dev>/always_powered_in_suspend
+Date:		March 2021
+KernelVersion:	5.13
+Contact:	Matthias Kaehlcke <matthias@kaehlcke.net>
+		linux-usb@vger.kernel.org
+Description:
+		(RW) Controls whether the USB hub remains always powered
+		during system suspend or not.

MAINTAINERS

@@ -13734,6 +13734,13 @@ S:	Maintained
 T:	git git://linuxtv.org/media_tree.git
 F:	drivers/media/i2c/ov9734.c
 
+ONBOARD USB HUB DRIVER
+M:	Matthias Kaehlcke <mka@chromium.org>
+L:	linux-usb@vger.kernel.org
+S:	Maintained
+F:	Documentation/devicetree/bindings/usb/onboard_usb_hub.yaml
+F:	drivers/usb/misc/onboard_usb_hub.c
+
 ONENAND FLASH DRIVER
 M:	Kyungmin Park <kyungmin.park@samsung.com>
 L:	linux-mtd@lists.infradead.org

drivers/usb/misc/Kconfig

@@ -284,3 +284,20 @@ config BRCM_USB_PINMAP
 	  This option enables support for remapping some USB external
 	  signals, which are typically on dedicated pins on the chip,
 	  to any gpio.
+
+config USB_ONBOARD_HUB
+	tristate "Onboard USB hub support"
+	depends on OF || COMPILE_TEST
+	help
+	  Say Y here if you want to support discrete onboard USB hubs that
+	  don't require an additional control bus for initialization, but
+	  need some nontrivial form of initialization, such as enabling a
+	  power regulator. An example for such a hub is the Realtek
+	  RTS5411.
+
+	  The driver can be configured to turn off the power of the hub
+	  during system suspend. This may reduce power consumption while
+	  the system is suspended.
+
+	  To compile this driver as a module, choose M here: the
+	  module will be called onboard_usb_hub.

drivers/usb/misc/Makefile

@@ -32,3 +32,4 @@ obj-$(CONFIG_USB_CHAOSKEY)		+= chaoskey.o
 obj-$(CONFIG_USB_SISUSBVGA)		+= sisusbvga/
 obj-$(CONFIG_USB_LINK_LAYER_TEST)	+= lvstest.o
 obj-$(CONFIG_BRCM_USB_PINMAP)		+= brcmstb-usb-pinmap.o
+obj-$(CONFIG_USB_ONBOARD_HUB)		+= onboard_usb_hub.o