1 .. SPDX-License-Identifier: GPL-2.0
3 =================================
4 Open Firmware Devicetree Unittest
5 =================================
7 Author: Gaurav Minocha <gaurav.minocha.os@gmail.com>
12 This document explains how the test data required for executing OF unittest
13 is attached to the live tree dynamically, independent of the machine's
16 It is recommended to read the following documents before moving ahead.
18 (1) Documentation/devicetree/usage-model.rst
19 (2) http://www.devicetree.org/Device_Tree_Usage
21 OF Selftest has been designed to test the interface (include/linux/of.h)
22 provided to device driver developers to fetch the device information..etc.
23 from the unflattened device tree data structure. This interface is used by
24 most of the device drivers in various use cases.
27 2. Verbose Output (EXPECT)
28 ==========================
30 If unittest detects a problem it will print a warning or error message to
31 the console. Unittest also triggers warning and error messages from other
32 kernel code as a result of intentionally bad unittest data. This has led
33 to confusion as to whether the triggered messages are an expected result
34 of a test or whether there is a real problem that is independent of unittest.
36 'EXPECT \ : text' (begin) and 'EXPECT / : text' (end) messages have been
37 added to unittest to report that a warning or error is expected. The
38 begin is printed before triggering the warning or error, and the end is
39 printed after triggering the warning or error.
41 The EXPECT messages result in very noisy console messages that are difficult
42 to read. The script scripts/dtc/of_unittest_expect was created to filter
43 this verbosity and highlight mismatches between triggered warnings and
44 errors vs expected warnings and errors. More information is available
45 from 'scripts/dtc/of_unittest_expect --help'.
51 The Device Tree Source file (drivers/of/unittest-data/testcases.dts) contains
52 the test data required for executing the unit tests automated in
53 drivers/of/unittest.c. Currently, following Device Tree Source Include files
54 (.dtsi) are included in testcases.dts::
56 drivers/of/unittest-data/tests-interrupts.dtsi
57 drivers/of/unittest-data/tests-platform.dtsi
58 drivers/of/unittest-data/tests-phandle.dtsi
59 drivers/of/unittest-data/tests-match.dtsi
61 When the kernel is build with OF_SELFTEST enabled, then the following make
64 $(obj)/%.dtb: $(src)/%.dts FORCE
65 $(call if_changed_dep, dtc)
67 is used to compile the DT source file (testcases.dts) into a binary blob
68 (testcases.dtb), also referred as flattened DT.
70 After that, using the following rule the binary blob above is wrapped as an
71 assembly file (testcases.dtb.S)::
73 $(obj)/%.dtb.S: $(obj)/%.dtb
76 The assembly file is compiled into an object file (testcases.dtb.o), and is
77 linked into the kernel image.
80 3.1. Adding the test data
81 -------------------------
83 Un-flattened device tree structure:
85 Un-flattened device tree consists of connected device_node(s) in form of a tree
86 structure described below::
88 // following struct members are used to construct the tree
91 struct device_node *parent;
92 struct device_node *child;
93 struct device_node *sibling;
97 Figure 1, describes a generic structure of machine's un-flattened device tree
98 considering only child and sibling pointers. There exists another pointer,
99 ``*parent``, that is used to traverse the tree in the reverse direction. So, at
100 a particular level the child node and all the sibling nodes will have a parent
101 pointer pointing to a common node (e.g. child1, sibling2, sibling3, sibling4's
102 parent points to root node)::
106 child1 -> sibling2 -> sibling3 -> sibling4 -> null
110 | | child31 -> sibling32 -> null
114 | child21 -> sibling22 -> sibling23 -> null
118 child11 -> sibling12 -> sibling13 -> sibling14 -> null
122 null null child131 -> null
126 Figure 1: Generic structure of un-flattened device tree
129 Before executing OF unittest, it is required to attach the test data to
130 machine's device tree (if present). So, when selftest_data_add() is called,
131 at first it reads the flattened device tree data linked into the kernel image
132 via the following kernel symbols::
134 __dtb_testcases_begin - address marking the start of test data blob
135 __dtb_testcases_end - address marking the end of test data blob
137 Secondly, it calls of_fdt_unflatten_tree() to unflatten the flattened
138 blob. And finally, if the machine's device tree (i.e live tree) is present,
139 then it attaches the unflattened test data tree to the live tree, else it
140 attaches itself as a live device tree.
142 attach_node_and_children() uses of_attach_node() to attach the nodes into the
143 live tree as explained below. To explain the same, the test data tree described
144 in Figure 2 is attached to the live tree described in Figure 1::
150 test-child0 -> test-sibling1 -> test-sibling2 -> test-sibling3 -> null
152 test-child01 null null null
155 Figure 2: Example test data tree to be attached to live tree.
157 According to the scenario above, the live tree is already present so it isn't
158 required to attach the root('/') node. All other nodes are attached by calling
159 of_attach_node() on each node.
161 In the function of_attach_node(), the new node is attached as the child of the
162 given parent in live tree. But, if parent already has a child then the new node
163 replaces the current child and turns it into its sibling. So, when the testcase
164 data node is attached to the live tree above (Figure 1), the final structure is
165 as shown in Figure 3::
169 testcase-data -> child1 -> sibling2 -> sibling3 -> sibling4 -> null
172 | | child31 -> sibling32 -> null
176 | child21 -> sibling22 -> sibling23 -> null
180 child11 -> sibling12 -> sibling13 -> sibling14 -> null
187 -----------------------------------------------------------------------
191 testcase-data -> child1 -> sibling2 -> sibling3 -> sibling4 -> null
193 | (...) (...) (...) null
195 test-sibling3 -> test-sibling2 -> test-sibling1 -> test-child0 -> null
197 null null null test-child01
200 Figure 3: Live device tree structure after attaching the testcase-data.
203 Astute readers would have noticed that test-child0 node becomes the last
204 sibling compared to the earlier structure (Figure 2). After attaching first
205 test-child0 the test-sibling1 is attached that pushes the child node
206 (i.e. test-child0) to become a sibling and makes itself a child node,
209 If a duplicate node is found (i.e. if a node with same full_name property is
210 already present in the live tree), then the node isn't attached rather its
211 properties are updated to the live tree's node by calling the function
212 update_node_properties().
215 3.2. Removing the test data
216 ---------------------------
218 Once the test case execution is complete, selftest_data_remove is called in
219 order to remove the device nodes attached initially (first the leaf nodes are
220 detached and then moving up the parent nodes are removed, and eventually the
221 whole tree). selftest_data_remove() calls detach_node_and_children() that uses
222 of_detach_node() to detach the nodes from the live device tree.
224 To detach a node, of_detach_node() either updates the child pointer of given
225 node's parent to its sibling or attaches the previous sibling to the given
226 node's sibling, as appropriate. That is it :)