of: Documentation regarding attaching OF Selftest testdata

This patch add a document that explains how the selftest test data is
dynamically attached into the live device tree irrespective of the
machine's architecture.
Signed-off-by: NGaurav Minocha <gaurav.minocha.os@gmail.com>
Signed-off-by: NGrant Likely <grant.likely@linaro.org>

Documentation/devicetree/of_selftest.txt

+Open Firmware Device Tree Selftest
+----------------------------------
+
+Author: Gaurav Minocha <gaurav.minocha.os@gmail.com>
+
+1. Introduction
+
+This document explains how the test data required for executing OF selftest
+is attached to the live tree dynamically, independent of the machine's
+architecture.
+
+It is recommended to read the following documents before moving ahead.
+
+[1] Documentation/devicetree/usage-model.txt
+[2] http://www.devicetree.org/Device_Tree_Usage
+
+OF Selftest has been designed to test the interface (include/linux/of.h)
+provided to device driver developers to fetch the device information..etc.
+from the unflattened device tree data structure. This interface is used by
+most of the device drivers in various use cases.
+
+
+2. Test-data
+
+The Device Tree Source file (drivers/of/testcase-data/testcases.dts) contains
+the test data required for executing the unit tests automated in
+drivers/of/selftests.c. Currently, following Device Tree Source Include files
+(.dtsi) are included in testcase.dts:
+
+drivers/of/testcase-data/tests-interrupts.dtsi
+drivers/of/testcase-data/tests-platform.dtsi
+drivers/of/testcase-data/tests-phandle.dtsi
+drivers/of/testcase-data/tests-match.dtsi
+
+When the kernel is build with OF_SELFTEST enabled, then the following make rule
+
+$(obj)/%.dtb: $(src)/%.dts FORCE
+	$(call if_changed_dep, dtc)
+
+is used to compile the DT source file (testcase.dts) into a binary blob
+(testcase.dtb), also referred as flattened DT.
+
+After that, using the following rule the binary blob above is wrapped as an
+assembly file (testcase.dtb.S).
+
+$(obj)/%.dtb.S: $(obj)/%.dtb
+	$(call cmd, dt_S_dtb)
+
+The assembly file is compiled into an object file (testcase.dtb.o), and is
+linked into the kernel image.
+
+
+2.1. Adding the test data
+
+Un-flattened device tree structure:
+
+Un-flattened device tree consists of connected device_node(s) in form of a tree
+structure described below.
+
+// following struct members are used to construct the tree
+struct device_node {
+    ...
+    struct  device_node *parent;
+    struct  device_node *child;
+    struct  device_node *sibling;
+    struct  device_node *allnext;   /* next in list of all nodes */
+    ...
+ };
+
+Figure 1, describes a generic structure of machine’s un-flattened device tree
+considering only child and sibling pointers. There exists another pointer,
+*parent, that is used to traverse the tree in the reverse direction. So, at
+a particular level the child node and all the sibling nodes will have a parent
+pointer pointing to a common node (e.g. child1, sibling2, sibling3, sibling4’s
+parent points to root node)
+
+root (‘/’)
+   |
+child1 -> sibling2 -> sibling3 -> sibling4 -> null
+   |         |           |           |
+   |         |           |          null
+   |         |           |
+   |         |        child31 -> sibling32 -> null
+   |         |           |          |
+   |         |          null       null
+   |         |
+   |      child21 -> sibling22 -> sibling23 -> null
+   |         |          |            |
+   |        null       null         null
+   |
+child11 -> sibling12 -> sibling13 -> sibling14 -> null
+   |           |           |            |
+   |           |           |           null
+   |           |           |
+  null        null       child131 -> null
+                           |
+                          null
+
+Figure 1: Generic structure of un-flattened device tree
+
+
+*allnext: it is used to link all the nodes of DT into a list. So, for the
+ above tree the list would be as follows:
+
+root->child1->child11->sibling12->sibling13->child131->sibling14->sibling2->
+child21->sibling22->sibling23->sibling3->child31->sibling32->sibling4->null
+
+Before executing OF selftest, it is required to attach the test data to
+machine's device tree (if present). So, when selftest_data_add() is called,
+at first it reads the flattened device tree data linked into the kernel image
+via the following kernel symbols:
+
+__dtb_testcases_begin - address marking the start of test data blob
+__dtb_testcases_end   - address marking the end of test data blob
+
+Secondly, it calls of_fdt_unflatten_device_tree() to unflatten the flattened
+blob. And finally, if the machine’s device tree (i.e live tree) is present,
+then it attaches the unflattened test data tree to the live tree, else it
+attaches itself as a live device tree.
+
+attach_node_and_children() uses of_attach_node() to attach the nodes into the
+live tree as explained below. To explain the same, the test data tree described
+ in Figure 2 is attached to the live tree described in Figure 1.
+
+root (‘/’)
+    |
+ testcase-data
+    |
+ test-child0 -> test-sibling1 -> test-sibling2 -> test-sibling3 -> null
+    |               |                |                |
+ test-child01      null             null             null
+
+
+allnext list:
+
+root->testcase-data->test-child0->test-child01->test-sibling1->test-sibling2
+->test-sibling3->null
+
+Figure 2: Example test data tree to be attached to live tree.
+
+According to the scenario above, the live tree is already present so it isn’t
+required to attach the root(‘/’) node. All other nodes are attached by calling
+of_attach_node() on each node.
+
+In the function of_attach_node(), the new node is attached as the child of the
+given parent in live tree. But, if parent already has a child then the new node
+replaces the current child and turns it into its sibling. So, when the testcase
+data node is attached to the live tree above (Figure 1), the final structure is
+ as shown in Figure 3.
+
+root (‘/’)
+   |
+testcase-data -> child1 -> sibling2 -> sibling3 -> sibling4 -> null
+   |               |          |           |           |
+ (...)             |          |           |          null
+                   |          |         child31 -> sibling32 -> null
+                   |          |           |           |
+                   |          |          null        null
+                   |          |
+                   |        child21 -> sibling22 -> sibling23 -> null
+                   |          |           |            |
+                   |         null        null         null
+                   |
+                child11 -> sibling12 -> sibling13 -> sibling14 -> null
+                   |          |            |            |
+                  null       null          |           null
+                                           |
+                                        child131 -> null
+                                           |
+                                          null
+-----------------------------------------------------------------------
+
+root (‘/’)
+   |
+testcase-data -> child1 -> sibling2 -> sibling3 -> sibling4 -> null
+   |               |          |           |           |
+   |             (...)      (...)       (...)        null
+   |
+test-sibling3 -> test-sibling2 -> test-sibling1 -> test-child0 -> null
+   |                |                   |                |
+  null             null                null         test-child01
+
+
+Figure 3: Live device tree structure after attaching the testcase-data.
+
+
+Astute readers would have noticed that test-child0 node becomes the last
+sibling compared to the earlier structure (Figure 2). After attaching first
+test-child0 the test-sibling1 is attached that pushes the child node
+(i.e. test-child0) to become a sibling and makes itself a child node,
+ as mentioned above.
+
+If a duplicate node is found (i.e. if a node with same full_name property is
+already present in the live tree), then the node isn’t attached rather its
+properties are updated to the live tree’s node by calling the function
+update_node_properties().
+
+
+2.2. Removing the test data
+
+Once the test case execution is complete, selftest_data_remove is called in
+order to remove the device nodes attached initially (first the leaf nodes are
+detached and then moving up the parent nodes are removed, and eventually the
+whole tree). selftest_data_remove() calls detach_node_and_children() that uses
+of_detach_node() to detach the nodes from the live device tree.
+
+To detach a node, of_detach_node() first updates all_next linked list, by
+attaching the previous node’s allnext to current node’s allnext pointer. And
+then, it either updates the child pointer of given node’s parent to its
+sibling or attaches the previous sibling to the given node’s sibling, as
+appropriate. That is it :)