File-copy from v4.4.100
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Change-Id: I8a9ee2aea93cd29c52c847d0ce33091a73ae6afe
diff --git a/Documentation/sgi-ioc4.txt b/Documentation/sgi-ioc4.txt
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+The SGI IOC4 PCI device is a bit of a strange beast, so some notes on
+it are in order.
+
+First, even though the IOC4 performs multiple functions, such as an
+IDE controller, a serial controller, a PS/2 keyboard/mouse controller,
+and an external interrupt mechanism, it's not implemented as a
+multifunction device. The consequence of this from a software
+standpoint is that all these functions share a single IRQ, and
+they can't all register to own the same PCI device ID. To make
+matters a bit worse, some of the register blocks (and even registers
+themselves) present in IOC4 are mixed-purpose between these several
+functions, meaning that there's no clear "owning" device driver.
+
+The solution is to organize the IOC4 driver into several independent
+drivers, "ioc4", "sgiioc4", and "ioc4_serial". Note that there is no
+PS/2 controller driver as this functionality has never been wired up
+on a shipping IO card.
+
+ioc4
+====
+This is the core (or shim) driver for IOC4. It is responsible for
+initializing the basic functionality of the chip, and allocating
+the PCI resources that are shared between the IOC4 functions.
+
+This driver also provides registration functions that the other
+IOC4 drivers can call to make their presence known. Each driver
+needs to provide a probe and remove function, which are invoked
+by the core driver at appropriate times. The interface of these
+IOC4 function probe and remove operations isn't precisely the same
+as PCI device probe and remove operations, but is logically the
+same operation.
+
+sgiioc4
+=======
+This is the IDE driver for IOC4. Its name isn't very descriptive
+simply for historical reasons (it used to be the only IOC4 driver
+component). There's not much to say about it other than it hooks
+up to the ioc4 driver via the appropriate registration, probe, and
+remove functions.
+
+ioc4_serial
+===========
+This is the serial driver for IOC4. There's not much to say about it
+other than it hooks up to the ioc4 driver via the appropriate registration,
+probe, and remove functions.