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