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diff --git a/Documentation/devicetree/of_unittest.rst b/Documentation/devicetree/of_unittest.rst new file mode 100644 index 000000000..dea05214f --- /dev/null +++ b/Documentation/devicetree/of_unittest.rst @@ -0,0 +1,205 @@ +.. SPDX-License-Identifier: GPL-2.0 + +================================== +Open Firmware Device Tree Unittest +================================== + +Author: Gaurav Minocha <gaurav.minocha.os@gmail.com> + +1. Introduction +=============== + +This document explains how the test data required for executing OF unittest +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.rst +(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/unittest-data/testcases.dts) contains +the test data required for executing the unit tests automated in +drivers/of/unittest.c. Currently, following Device Tree Source Include files +(.dtsi) are included in testcases.dts:: + + drivers/of/unittest-data/tests-interrupts.dtsi + drivers/of/unittest-data/tests-platform.dtsi + drivers/of/unittest-data/tests-phandle.dtsi + drivers/of/unittest-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 (testcases.dts) into a binary blob +(testcases.dtb), also referred as flattened DT. + +After that, using the following rule the binary blob above is wrapped as an +assembly file (testcases.dtb.S):: + + $(obj)/%.dtb.S: $(obj)/%.dtb + $(call cmd, dt_S_dtb) + +The assembly file is compiled into an object file (testcases.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; + ... + }; + +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 + + +Before executing OF unittest, 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_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 + + +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() 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 :) |