+Device Sequence Numbers
+-----------------------
+
+U-Boot numbers devices from 0 in many situations, such as in the command
+line for I2C and SPI buses, and the device names for serial ports (serial0,
+serial1, ...). Driver model supports this numbering and permits devices
+to be locating by their 'sequence'. This numbering uniquely identifies a
+device in its uclass, so no two devices within a particular uclass can have
+the same sequence number.
+
+Sequence numbers start from 0 but gaps are permitted. For example, a board
+may have I2C buses 1, 4, 5 but no 0, 2 or 3. The choice of how devices are
+numbered is up to a particular board, and may be set by the SoC in some
+cases. While it might be tempting to automatically renumber the devices
+where there are gaps in the sequence, this can lead to confusion and is
+not the way that U-Boot works.
+
+Each device can request a sequence number. If none is required then the
+device will be automatically allocated the next available sequence number.
+
+To specify the sequence number in the device tree an alias is typically
+used. Make sure that the uclass has the DM_UC_FLAG_SEQ_ALIAS flag set.
+
+aliases {
+ serial2 = "/serial@22230000";
+};
+
+This indicates that in the uclass called "serial", the named node
+("/serial@22230000") will be given sequence number 2. Any command or driver
+which requests serial device 2 will obtain this device.
+
+More commonly you can use node references, which expand to the full path:
+
+aliases {
+ serial2 = &serial_2;
+};
+...
+serial_2: serial@22230000 {
+...
+};
+
+The alias resolves to the same string in this case, but this version is
+easier to read.
+
+Device sequence numbers are resolved when a device is probed. Before then
+the sequence number is only a request which may or may not be honoured,
+depending on what other devices have been probed. However the numbering is
+entirely under the control of the board author so a conflict is generally
+an error.
+
+
+Bus Drivers
+-----------
+
+A common use of driver model is to implement a bus, a device which provides
+access to other devices. Example of buses include SPI and I2C. Typically
+the bus provides some sort of transport or translation that makes it
+possible to talk to the devices on the bus.
+
+Driver model provides some useful features to help with implementing buses.
+Firstly, a bus can request that its children store some 'parent data' which
+can be used to keep track of child state. Secondly, the bus can define
+methods which are called when a child is probed or removed. This is similar
+to the methods the uclass driver provides. Thirdly, per-child platform data
+can be provided to specify things like the child's address on the bus. This
+persists across child probe()/remove() cycles.
+
+For consistency and ease of implementation, the bus uclass can specify the
+per-child platform data, so that it can be the same for all children of buses
+in that uclass. There are also uclass methods which can be called when
+children are bound and probed.
+
+Here an explanation of how a bus fits with a uclass may be useful. Consider
+a USB bus with several devices attached to it, each from a different (made
+up) uclass:
+
+ xhci_usb (UCLASS_USB)
+ eth (UCLASS_ETHERNET)
+ camera (UCLASS_CAMERA)
+ flash (UCLASS_FLASH_STORAGE)
+
+Each of the devices is connected to a different address on the USB bus.
+The bus device wants to store this address and some other information such
+as the bus speed for each device.
+
+To achieve this, the bus device can use dev->parent_platdata in each of its
+three children. This can be auto-allocated if the bus driver (or bus uclass)
+has a non-zero value for per_child_platdata_auto_alloc_size. If not, then
+the bus device or uclass can allocate the space itself before the child
+device is probed.
+
+Also the bus driver can define the child_pre_probe() and child_post_remove()
+methods to allow it to do some processing before the child is activated or
+after it is deactivated.
+
+Similarly the bus uclass can define the child_post_bind() method to obtain
+the per-child platform data from the device tree and set it up for the child.
+The bus uclass can also provide a child_pre_probe() method. Very often it is
+the bus uclass that controls these features, since it avoids each driver
+having to do the same processing. Of course the driver can still tweak and
+override these activities.
+
+Note that the information that controls this behaviour is in the bus's
+driver, not the child's. In fact it is possible that child has no knowledge
+that it is connected to a bus. The same child device may even be used on two
+different bus types. As an example. the 'flash' device shown above may also
+be connected on a SATA bus or standalone with no bus:
+
+ xhci_usb (UCLASS_USB)
+ flash (UCLASS_FLASH_STORAGE) - parent data/methods defined by USB bus
+
+ sata (UCLASS_SATA)
+ flash (UCLASS_FLASH_STORAGE) - parent data/methods defined by SATA bus
+
+ flash (UCLASS_FLASH_STORAGE) - no parent data/methods (not on a bus)
+
+Above you can see that the driver for xhci_usb/sata controls the child's
+bus methods. In the third example the device is not on a bus, and therefore
+will not have these methods at all. Consider the case where the flash
+device defines child methods. These would be used for *its* children, and
+would be quite separate from the methods defined by the driver for the bus
+that the flash device is connetced to. The act of attaching a device to a
+parent device which is a bus, causes the device to start behaving like a
+bus device, regardless of its own views on the matter.
+
+The uclass for the device can also contain data private to that uclass.
+But note that each device on the bus may be a memeber of a different
+uclass, and this data has nothing to do with the child data for each child
+on the bus. It is the bus' uclass that controls the child with respect to
+the bus.
+
+