dm: core: Add livetree documentation

Add some documentation for the live device tree support in U-Boot. This
was missing from the initial series.

Signed-off-by: Simon Glass <sjg@chromium.org>
Suggested-by: Lukasz Majewski <lukma@denx.de>
Reviewed-by: Łukasz Majewski <lukma@denx.de>

doc/driver-model/livetree.txt

@@ -0,0 +1,272 @@
+Driver Model with Live Device Tree
+==================================
+
+
+Introduction
+------------
+
+Traditionally U-Boot has used a 'flat' device tree. This means that it
+reads directly from the device tree binary structure. It is called a flat
+device tree because nodes are listed one after the other, with the
+hierarchy detected by tags in the format.
+
+This document describes U-Boot's support for a 'live' device tree, meaning
+that the tree is loaded into a hierarchical data structure within U-Boot.
+
+
+Motivation
+----------
+
+The flat device tree has several advantages:
+
+- it is the format produced by the device tree compiler, so no translation
+is needed
+
+- it is fairly compact (e.g. there is no need for pointers)
+
+- it is accessed by the libfdt library, which is well tested and stable
+
+
+However the flat device tree does have some limitations. Adding new
+properties can involve copying large amounts of data around to make room.
+The overall tree has a fixed maximum size so sometimes the tree must be
+rebuilt in a new location to create more space. Even if not adding new
+properties or nodes, scanning the tree can be slow. For example, finding
+the parent of a node is a slow process. Reading from nodes involves a
+small amount parsing which takes a little time.
+
+Driver model scans the entire device tree sequentially on start-up which
+avoids the worst of the flat tree's limitations. But if the tree is to be
+modified at run-time, a live tree is much faster. Even if no modification
+is necessary, parsing the tree once and using a live tree from then on
+seems to save a little time.
+
+
+Implementation
+--------------
+
+In U-Boot a live device tree ('livetree') is currently supported only
+after relocation. Therefore we need a mechanism to specify a device
+tree node regardless of whether it is in the flat tree or livetree.
+
+The 'ofnode' type provides this. An ofnode can point to either a flat tree
+node (when the live tree node is not yet set up) or a livetree node. The
+caller of an ofnode function does not need to worry about these details.
+
+The main users of the information in a device tree are  drivers. These have
+a 'struct udevice *' which is attached to a device tree node. Therefore it
+makes sense to be able to read device tree  properties using the
+'struct udevice *', rather than having to obtain the ofnode first.
+
+The 'dev_read_...()' interface provides this. It allows properties to be
+easily read from the device tree using only a device pointer. Under the
+hood it uses ofnode so it works with both flat and live device trees.
+
+
+Enabling livetree
+-----------------
+
+CONFIG_OF_LIVE enables livetree. When this option is enabled, the flat
+tree will be used in SPL and before relocation in U-Boot proper. Just
+before relocation a livetree is built, and this is used for U-Boot proper
+after relocation.
+
+Most checks for livetree use CONFIG_IS_ENABLED(OF_LIVE). This means that
+for SPL, the CONFIG_SPL_OF_LIVE option is checked. At present this does
+not exist, since SPL does not support livetree.
+
+
+Porting drivers
+---------------
+
+Many existing drivers use the fdtdec interface to read device tree
+properties. This only works with a flat device tree. The drivers should be
+converted to use the dev_read_() interface.
+
+For example, the old code may be like this:
+
+    struct udevice *bus;
+    const void *blob = gd->fdt_blob;
+    int node = dev_of_offset(bus);
+
+    i2c_bus->regs = (struct i2c_ctlr *)devfdt_get_addr(dev);
+    plat->frequency = fdtdec_get_int(blob, node, "spi-max-frequency", 500000);
+
+The new code is:
+
+    struct udevice *bus;
+
+    i2c_bus->regs = (struct i2c_ctlr *)dev_read_addr(dev);
+    plat->frequency = dev_read_u32_default(bus, "spi-max-frequency", 500000);
+
+The dev_read_...() interface is more convenient and works with both the
+flat and live device trees. See include/dm/read.h for a list of functions.
+
+Where properties must be read from sub-nodes or other nodes, you must fall
+back to using ofnode. For example, for old code like this:
+
+    const void *blob = gd->fdt_blob;
+    int subnode;
+
+    fdt_for_each_subnode(subnode, blob, dev_of_offset(dev)) {
+        freq = fdtdec_get_int(blob, node, "spi-max-frequency", 500000);
+        ...
+    }
+
+you should use:
+
+    ofnode subnode;
+
+    ofnode_for_each_subnode(subnode, dev_ofnode(dev)) {
+        freq = ofnode_read_u32(node, "spi-max-frequency", 500000);
+        ...
+    }
+
+
+Useful ofnode functions
+-----------------------
+
+The internal data structures of the livetree are defined in include/dm/of.h :
+
+   struct device_node - holds information about a device tree node
+   struct property    - holds information about a property within a node
+
+Nodes have pointers to their first property, their parent, their first child
+and their sibling. This allows nodes to be linked together in a hierarchical
+tree.
+
+Properties have pointers to the next property. This allows all properties of
+a node to be linked together in a chain.
+
+It should not be necessary to use these data structures in normal code. In
+particular, you should refrain from using functions which access the livetree
+directly, such as of_read_u32(). Use ofnode functions instead, to allow your
+code to work with a flat tree also.
+
+Some conversion functions are used internally. Generally these are not needed
+for driver code. Note that they will not work if called in the wrong context.
+For example it is invalid to call ofnode_to_no() when a flat tree is being
+used. Similarly it is not possible to call ofnode_to_offset() on a livetree
+node.
+
+   ofnode_to_np() - converts ofnode to struct device_node *
+   ofnode_to_offset() - converts ofnode to offset
+
+   no_to_ofnode() - converts node pointer to ofnode
+   offset_to_ofnode() - converts offset to ofnode
+
+
+Other useful functions:
+
+   of_live_active() returns true if livetree is in use, false if flat tree
+   ofnode_valid() return true if a given node is valid
+   ofnode_is_np() returns true if a given node is a livetree node
+   ofnode_equal() compares two ofnodes
+   ofnode_null() returns a null ofnode (for which ofnode_valid() returns false)
+
+
+Phandles
+--------
+
+There is full phandle support for live tree. All functions make use of
+struct ofnode_phandle_args, which has an ofnode within it. This supports both
+livetree and flat tree transparently. See for example
+ofnode_parse_phandle_with_args().
+
+
+Reading addresses
+-----------------
+
+You should use dev_read_addr() and friends to read addresses from device-tree
+nodes.
+
+
+fdtdec
+------
+
+The existing fdtdec interface will eventually be retired. Please try to avoid
+using it in new code.
+
+
+Modifying the livetree
+----------------------
+
+This is not currently supported. Once implemented it should provide a much
+more efficient implementation for modification of the device tree than using
+the flat tree.
+
+
+Internal implementation
+-----------------------
+
+The dev_read_...() functions have two implementations. When
+CONFIG_DM_DEV_READ_INLINE is enabled, these functions simply call the ofnode
+functions directly. This is useful when livetree is not enabled. The ofnode
+functions call ofnode_is_np(node) which will always return false if livetree
+is disabled, just falling back to flat tree code.
+
+This optimisation means that without livetree enabled, the dev_read_...() and
+ofnode interfaces do not noticeably add to code size.
+
+The CONFIG_DM_DEV_READ_INLINE option defaults to enabled when livetree is
+disabled.
+
+Most livetree code comes directly from Linux and is modified as little as
+possible. This is deliberate since this code is fairly stable and does what
+we want. Some features (such as get/put) are not supported. Internal macros
+take care of removing these features silently.
+
+Within the of_access.c file there are pointers to the alias node, the chosen
+node and the stdout-path alias.
+
+
+Errors
+------
+
+With a flat device tree, libfdt errors are returned (e.g. -FDT_ERR_NOTFOUND).
+For livetree normal 'errno' errors are returned (e.g. -ENOTFOUND). At present
+the ofnode and dev_read_...() functions return either one or other type of
+error. This is clearly not desirable. Once tests are added for all the
+functions this can be tidied up.
+
+
+Adding new access functions
+---------------------------
+
+Adding a new function for device-tree access involves the following steps:
+
+   - Add two dev_read() functions:
+	- inline version in the read.h header file, which calls an ofnode
+		function
+	- standard version in the read.c file (or perhaps another file), which
+		also calls an ofnode function
+
+	The implementations of these functions can be the same. The purpose
+	of the inline version is purely to reduce code size impact.
+
+   - Add an ofnode function. This should call ofnode_is_np() to work out
+	whether a livetree or flat tree is used. For the livetree it should
+	call an of_...() function. For the flat tree it should call an
+	fdt_...() function. The livetree version will be optimised out at
+	compile time if livetree is not enabled.
+
+   - Add an of_...() function for the livetree implementation. If a similar
+	function is available in Linux, the implementation should be taken
+	from there and modified as little as possible (generally not at all).
+
+
+Future work
+-----------
+
+Live tree support was introduced in U-Boot 2017.07. There is still quite a bit
+of work to do to flesh this out:
+
+- tests for all access functions
+- support for livetree modification
+- addition of more access functions as needed
+- support for livetree in SPL and before relocation (if desired)
+
+
+--
+Simon Glass <sjg@chromium.org>
+5-Aug-17