[POWERPC] Document describing mpc52xx device tree binding conventions

This document describes the device tree expectations for mpc52xx based
boards.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Acked-by: Sylvain Munaut <tnt@246tNt.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>

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+MPC52xx Device Tree Bindings
+----------------------------
+
+(c) 2006 Secret Lab Technologies Ltd
+Grant Likely <grant.likely at secretlab.ca>
+
+I - Introduction
+================
+Boards supported by the arch/powerpc architecture require device tree be
+passed by the boot loader to the kernel at boot time.  The device tree
+describes what devices are present on the board and how they are
+connected.  The device tree can either be passed as a binary blob (as
+described in Documentation/powerpc/booting-without-of.txt), or passed
+by Open Firmare (IEEE 1275) compatible firmware using an OF compatible
+client interface API.
+
+This document specifies the requirements on the device-tree for mpc52xx
+based boards.  These requirements are above and beyond the details
+specified in either the OpenFirmware spec or booting-without-of.txt
+
+All new mpc52xx-based boards are expected to match this document.  In
+cases where this document is not sufficient to support a new board port,
+this document should be updated as part of adding the new board support.
+
+II - Philosophy
+===============
+The core of this document is naming convention.  The whole point of
+defining this convention is to reduce or eliminate the number of
+special cases required to support a 52xx board.  If all 52xx boards
+follow the same convention, then generic 52xx support code will work
+rather than coding special cases for each new board.
+
+This section tries to capture the thought process behind why the naming
+convention is what it is.
+
+1. Node names
+-------------
+There is strong convention/requirements already established for children
+of the root node.  'cpus' describes the processor cores, 'memory'
+describes memory, and 'chosen' provides boot configuration.  Other nodes
+are added to describe devices attached to the processor local bus.
+Following convention already established with other system-on-chip
+processors, MPC52xx boards must have an 'soc5200' node as a child of the
+root node.
+
+The soc5200 node holds child nodes for all on chip devices.  Child nodes
+are typically named after the configured function.  ie. the FEC node is
+named 'ethernet', and a PSC in uart mode is named 'serial'.
+
+2. device_type property
+-----------------------
+similar to the node name convention above; the device_type reflects the
+configured function of a device.  ie. 'serial' for a uart and 'spi' for
+an spi controller.  However, while node names *should* reflect the
+configured function, device_type *must* match the configured function
+exactly.
+
+3. compatible property
+----------------------
+Since device_type isn't enough to match devices to drivers, there also
+needs to be a naming convention for the compatible property.  Compatible
+is an list of device descriptions sorted from specific to generic.  For
+the mpc52xx, the required format for each compatible value is
+<chip>-<device>[-<mode>].  At the minimum, the list shall contain two
+items; the first specifying the exact chip, and the second specifying
+mpc52xx for the chip.
+
+ie. ethernet on mpc5200b: compatible = "mpc5200b-ethernet\0mpc52xx-ethernet"
+
+The idea here is that most drivers will match to the most generic field
+in the compatible list (mpc52xx-*), but can also test the more specific
+field for enabling bug fixes or extra features.
+
+Modal devices, like PSCs, also append the configured function to the
+end of the compatible field.  ie. A PSC in i2s mode would specify
+"mpc52xx-psc-i2s", not "mpc52xx-i2s".  This convention is chosen to
+avoid naming conflicts with non-psc devices providing the same
+function.  For example, "mpc52xx-spi" and "mpc52xx-psc-spi" describe
+the mpc5200 simple spi device and a PSC spi mode respectively.
+
+If the soc device is more generic and present on other SOCs, the
+compatible property can specify the more generic device type also.
+
+ie. mscan: compatible = "mpc5200-mscan\0mpc52xx-mscan\0fsl,mscan";
+
+At the time of writing, exact chip may be either 'mpc5200' or
+'mpc5200b'.
+
+Device drivers should always try to match as generically as possible.
+
+III - Structure
+===============
+The device tree for an mpc52xx board follows the structure defined in
+booting-without-of.txt with the following additional notes:
+
+0) the root node
+----------------
+Typical root description node; see booting-without-of
+
+1) The cpus node
+----------------
+The cpus node follows the basic layout described in booting-without-of.
+The bus-frequency property holds the XLB bus frequency
+The clock-frequency property holds the core frequency
+
+2) The memory node
+------------------
+Typical memory description node; see booting-without-of.
+
+3) The soc5200 node
+-------------------
+This node describes the on chip SOC peripherals.  Every mpc52xx based
+board will have this node, and as such there is a common naming
+convention for SOC devices.
+
+Required properties:
+name                    type            description
+----                    ----            -----------
+device_type             string          must be "soc"
+ranges                  int             should be <0 baseaddr baseaddr+10000>
+reg                     int             must be <baseaddr 10000>
+
+Recommended properties:
+name                    type            description
+----                    ----            -----------
+compatible              string          should be "<chip>-soc\0mpc52xx-soc"
+                                        ie. "mpc5200b-soc\0mpc52xx-soc"
+#interrupt-cells        int             must be <3>.  If it is not defined
+                                        here then it must be defined in every
+                                        soc device node.
+bus-frequency           int             IPB bus frequency in HZ.  Clock rate
+                                        used by most of the soc devices.
+                                        Defining it here avoids needing it
+                                        added to every device node.
+
+4) soc5200 child nodes
+----------------------
+Any on chip SOC devices available to Linux must appear as soc5200 child nodes.
+
+Note: in the tables below, '*' matches all <chip> values.  ie.
+*-pic would translate to "mpc5200-pic\0mpc52xx-pic"
+
+Required soc5200 child nodes:
+name            device_type             compatible      Description
+----            -----------             ----------      -----------
+cdm@<addr>      cdm                     *-cmd           Clock Distribution
+pic@<addr>      interrupt-controller    *-pic           need an interrupt
+                                                        controller to boot
+bestcomm@<addr> dma-controller          *-bestcomm      52xx pic also requires
+                                                        the bestcomm device
+
+Recommended soc5200 child nodes; populate as needed for your board
+name            device_type     compatible      Description
+----            -----------     ----------      -----------
+gpt@<addr>      gpt             *-gpt           General purpose timers
+rtc@<addr>      rtc             *-rtc           Real time clock
+mscan@<addr>    mscan           *-mscan         CAN bus controller
+pci@<addr>      pci             *-pci           PCI bridge
+serial@<addr>   serial          *-psc-uart      PSC in serial mode
+i2s@<addr>      i2s             *-psc-i2s       PSC in i2s mode
+ac97@<addr>     ac97            *-psc-ac97      PSC in ac97 mode
+spi@<addr>      spi             *-psc-spi       PSC in spi mode
+irda@<addr>     irda            *-psc-irda      PSC in IrDA mode
+spi@<addr>      spi             *-spi           MPC52xx spi device
+ethernet@<addr> network         *-fec           MPC52xx ethernet device
+ata@<addr>      ata             *-ata           IDE ATA interface
+i2c@<addr>      i2c             *-i2c           I2C controller
+usb@<addr>      usb-ohci-be     *-ohci,ohci-be  USB controller
+xlb@<addr>      xlb             *-xlb           XLB arbritrator
+
+IV - Extra Notes
+================
+
+1. Interrupt mapping
+--------------------
+The mpc52xx pic driver splits hardware IRQ numbers into two levels.  The
+split reflects the layout of the PIC hardware itself, which groups
+interrupts into one of three groups; CRIT, MAIN or PERP.  Also, the
+Bestcomm dma engine has it's own set of interrupt sources which are
+cascaded off of peripheral interrupt 0, which the driver interprets as a
+fourth group, SDMA.
+
+The interrupts property for device nodes using the mpc52xx pic consists
+of three cells; <L1 L2 level>
+
+    L1 := [CRIT=0, MAIN=1, PERP=2, SDMA=3]
+    L2 := interrupt number; directly mapped from the value in the
+          "ICTL PerStat, MainStat, CritStat Encoded Register"
+    level := [LEVEL_HIGH=0, EDGE_RISING=1, EDGE_FALLING=2, LEVEL_LOW=3]