1 Open Firmware Device Tree Unittest
2 ----------------------------------
4 Author: Gaurav Minocha <gaurav.minocha.os@gmail.com>
8 This document explains how the test data required for executing OF unittest
9 is attached to the live tree dynamically, independent of the machine's
12 It is recommended to read the following documents before moving ahead.
14 [1] Documentation/devicetree/usage-model.txt
15 [2] http://www.devicetree.org/Device_Tree_Usage
17 OF Selftest has been designed to test the interface (include/linux/of.h)
18 provided to device driver developers to fetch the device information..etc.
19 from the unflattened device tree data structure. This interface is used by
20 most of the device drivers in various use cases.
25 The Device Tree Source file (drivers/of/unittest-data/testcases.dts) contains
26 the test data required for executing the unit tests automated in
27 drivers/of/unittest.c. Currently, following Device Tree Source Include files
28 (.dtsi) are included in testcases.dts:
30 drivers/of/unittest-data/tests-interrupts.dtsi
31 drivers/of/unittest-data/tests-platform.dtsi
32 drivers/of/unittest-data/tests-phandle.dtsi
33 drivers/of/unittest-data/tests-match.dtsi
35 When the kernel is build with OF_SELFTEST enabled, then the following make rule
37 $(obj)/%.dtb: $(src)/%.dts FORCE
38 $(call if_changed_dep, dtc)
40 is used to compile the DT source file (testcases.dts) into a binary blob
41 (testcases.dtb), also referred as flattened DT.
43 After that, using the following rule the binary blob above is wrapped as an
44 assembly file (testcases.dtb.S).
46 $(obj)/%.dtb.S: $(obj)/%.dtb
49 The assembly file is compiled into an object file (testcases.dtb.o), and is
50 linked into the kernel image.
53 2.1. Adding the test data
55 Un-flattened device tree structure:
57 Un-flattened device tree consists of connected device_node(s) in form of a tree
58 structure described below.
60 // following struct members are used to construct the tree
63 struct device_node *parent;
64 struct device_node *child;
65 struct device_node *sibling;
69 Figure 1, describes a generic structure of machine's un-flattened device tree
70 considering only child and sibling pointers. There exists another pointer,
71 *parent, that is used to traverse the tree in the reverse direction. So, at
72 a particular level the child node and all the sibling nodes will have a parent
73 pointer pointing to a common node (e.g. child1, sibling2, sibling3, sibling4's
74 parent points to root node)
78 child1 -> sibling2 -> sibling3 -> sibling4 -> null
82 | | child31 -> sibling32 -> null
86 | child21 -> sibling22 -> sibling23 -> null
90 child11 -> sibling12 -> sibling13 -> sibling14 -> null
94 null null child131 -> null
98 Figure 1: Generic structure of un-flattened device tree
101 Before executing OF unittest, it is required to attach the test data to
102 machine's device tree (if present). So, when selftest_data_add() is called,
103 at first it reads the flattened device tree data linked into the kernel image
104 via the following kernel symbols:
106 __dtb_testcases_begin - address marking the start of test data blob
107 __dtb_testcases_end - address marking the end of test data blob
109 Secondly, it calls of_fdt_unflatten_tree() to unflatten the flattened
110 blob. And finally, if the machine's device tree (i.e live tree) is present,
111 then it attaches the unflattened test data tree to the live tree, else it
112 attaches itself as a live device tree.
114 attach_node_and_children() uses of_attach_node() to attach the nodes into the
115 live tree as explained below. To explain the same, the test data tree described
116 in Figure 2 is attached to the live tree described in Figure 1.
122 test-child0 -> test-sibling1 -> test-sibling2 -> test-sibling3 -> null
124 test-child01 null null null
127 Figure 2: Example test data tree to be attached to live tree.
129 According to the scenario above, the live tree is already present so it isn't
130 required to attach the root('/') node. All other nodes are attached by calling
131 of_attach_node() on each node.
133 In the function of_attach_node(), the new node is attached as the child of the
134 given parent in live tree. But, if parent already has a child then the new node
135 replaces the current child and turns it into its sibling. So, when the testcase
136 data node is attached to the live tree above (Figure 1), the final structure is
137 as shown in Figure 3.
141 testcase-data -> child1 -> sibling2 -> sibling3 -> sibling4 -> null
144 | | child31 -> sibling32 -> null
148 | child21 -> sibling22 -> sibling23 -> null
152 child11 -> sibling12 -> sibling13 -> sibling14 -> null
159 -----------------------------------------------------------------------
163 testcase-data -> child1 -> sibling2 -> sibling3 -> sibling4 -> null
165 | (...) (...) (...) null
167 test-sibling3 -> test-sibling2 -> test-sibling1 -> test-child0 -> null
169 null null null test-child01
172 Figure 3: Live device tree structure after attaching the testcase-data.
175 Astute readers would have noticed that test-child0 node becomes the last
176 sibling compared to the earlier structure (Figure 2). After attaching first
177 test-child0 the test-sibling1 is attached that pushes the child node
178 (i.e. test-child0) to become a sibling and makes itself a child node,
181 If a duplicate node is found (i.e. if a node with same full_name property is
182 already present in the live tree), then the node isn't attached rather its
183 properties are updated to the live tree's node by calling the function
184 update_node_properties().
187 2.2. Removing the test data
189 Once the test case execution is complete, selftest_data_remove is called in
190 order to remove the device nodes attached initially (first the leaf nodes are
191 detached and then moving up the parent nodes are removed, and eventually the
192 whole tree). selftest_data_remove() calls detach_node_and_children() that uses
193 of_detach_node() to detach the nodes from the live device tree.
195 To detach a node, of_detach_node() either updates the child pointer of given
196 node's parent to its sibling or attaches the previous sibling to the given
197 node's sibling, as appropriate. That is it :)