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Devicetree: Kernel Internals and Practical Troubleshooting

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Devicetree: Kernel Internals and Practical Troubleshooting devicetree: kernel internals and practical troubleshooting There have been many presentations on what a devicetree looks like and how to create a devicetree. This talk instead examines how the Linux kernel uses a devicetree. Topics include the kernel devicetree framework, device creation, resource allocation, driver binding, and connecting objects. Troubleshooting will consider initialization, allocation, and binding ordering; kernel configuration; and driver problems. Frank Rowand, Sony Mobile Communications October 15, 2014 141010_2019 CAUTION The material covered in this presentation is kernel version specific and is actively evolving Most information describes 3.15-rc1 – 3.16-rc7 In cases where arch specific code is involved, I will be looking at arch/arm/ Chapter 1 Device tree what is device tree? “A device tree is a tree data structure with nodes that describe the devices in a system. Each node has property/value pairs that describe the characteristics of the device being represented. Each node has exactly one parent except for the root node, which has no parent.” (ePAPR v1.1) what is device tree? “A device tree is a tree data structure with nodes that describe the devices in a system. Each node has property/value pairs that describe the characteristics of the device being represented. Each node has exactly one parent except for the root node, which has no parent.” (ePAPR v1.1) A device tree describes hardware that can not be located by probing. DT data life cycle (source) .dts .dts - device tree source file / { /* incomplete .dts example */ model = "Qualcomm APQ8074 Dragonboard"; compatible = "qcom,apq8074-dragonboard"; interrupt-parent = <&intc>; soc: soc { ranges; compatible = "simple-bus"; intc: interrupt-controller@f9000000 { compatible = "qcom,msm-qgic2"; interrupt-controller; reg = <0xf9000000 0x1000>, <0xf9002000 0x1000>; console: serial@f991e000 { compatible = "qcom,msm-uartdm-v1.4", "qcom,msm-uartdm"; reg = <0xf991e000 0x1000>; interrupts = <0 108 0x0>; }; }; }; .dts - device tree source file Thomas Pettazzoni's ELC 2014 talk “Device Tree For Dummies” is an excellent introduction to - device tree source - boot loader mechanisms - much more! http://elinux.org/images/f/f9/ Petazzoni-device-tree-dummies_0.pdf .dts - device tree source file Thomas Pettazzoni's ELC 2014 talk “Device Tree For Dummies” is an excellent introduction to - device tree source - boot loader mechanisms - much more! http://elinux.org/images/f/f9/ Petazzoni-device-tree-dummies_0.pdf .dts - device tree source file / { /* incomplete .dts example */ <--- root node model = "Qualcomm APQ8074 Dragonboard"; <--- property compatible = "qcom,apq8074-dragonboard"; <--- property interrupt-parent = <&intc>; <--- property, phandle soc: soc { <--- node ranges; <--- property compatible = "simple-bus"; <--- property intc: interrupt-controller@f9000000 { <--- node, phandle compatible = "qcom,msm-qgic2"; <--- property interrupt-controller; <--- property reg = <0xf9000000 0x1000>, <--- property <0xf9002000 0x1000>; serial@f991e000 { <--- node compatible = "qcom,msm-uartdm-v1.4", "qcom,msm-uartdm"; reg = <0xf991e000 0x1000>; <--- property interrupts = <0 108 0x0>; <--- property }; }; }; Key vocabulary nodes - the tree structure - contain properties and other nodes properties - data values providing information about a node node '/': property 'compatible' - will be used to match a machine_desc entry other nodes: property 'compatible' - will be used to match a driver DT data life cycle (source) (compiler) (binary blob) .dts dtc .dtb DT data life cycle (source) (compiler) (binary blob) .dts dtc .dtb Binary Blob format A “flat” format Access via serial scan and offsets Binary Blob format info struct fdt_header offsets to blocks (free space) section sizes memory reservation block {address, size} tuples (free space) structure block nested nodes - name embedded properties nested in nodes - values embedded (free space) - names are offsets in 'strings' strings block property names - null terminated strings - concatenated (free space) DT data life cycle (source) (compiler) (binary blob) .dts dtc .dtb boot dtb boot vmlinux loader: image: dtb' dtb FDT memory: (flattened device tree) DT data life cycle (source) (compiler) (binary blob) .dts dtc .dtb boot dtb boot vmlinux loader: image: dtb' dtb FDT memory: (flattened device linux tree) kernel Flattened Device Tree format A “flat” format. Access via serial scan and offsets using fdt_*() functions. Flattened Device Tree format A “flat” format. Access via serial scan and offsets using fdt_*() functions. DT data life cycle (source) (compiler) (binary blob) .dts dtc .dtb boot dtb boot vmlinux loader: image: dtb' dtb FDT memory: (flattened device linux tree) kernel expanded DT Expanded format A “computer-sciency” data structure Plan is to change implementation soon - interfaces to access data should not change - implications for DT usage should not change Expanded format A “computer-sciency” data structure Exact details not too interesting unless you are maintaining the DT internals, so I will mostly not leave the slides up long enough for you to read the details. If you are interested, read the slides at home. I will be trying to emphasize concepts instead of details. Expanded format A tree data structure Access and modified via tree operations using of_*() functions Access all nodes via a linked list Created during boot Nodes and properties can be added or deleted after boot Expanded format A tree data structure Access and modified via tree operations using of_*() functions Access all nodes via a linked list Created during boot Nodes and properties can be added or deleted after boot Expanded format tree of struct device_node struct device_node { const char *name; const char *type; phandle phandle; const char *full_name; struct property *properties; struct property *deadprops; struct device_node *parent; struct device_node *child; struct device_node *sibling; struct device_node *next; struct device_node *allnext; struct kobject kobj; unsigned long _flags; void *data; #if defined(CONFIG_SPARC) ... #endif }; Expanded format tree of struct device_node struct device_node { struct property *properties; struct device_node *parent; struct device_node *child; struct device_node *sibling; struct device_node *allnext; }; Expanded format tree of struct device_node Tree pointers child pointer sibling pointer Expanded format tree of struct device_node of_allnodes child sibling Expanded format tree of struct device_node Tree pointers child pointer sibling pointer Used to find node by tree search of_find_node_by_path() Expanded format tree of struct device_node Global linked list pointer allnext pointer removal targeted for 3.19 - API: unchanged - implementation: depth first traversal of tree Expanded format tree of struct device_node child sibling allnext allnext linked list Follows a depth first traversal of the tree After boot: YES After dynamic node addition: NO To be safe, think of allnext as a randomly ordered linked list of all nodes. allnext linked list - internal usage Common pattern: of_find_by_XXX(struct device node *from, …) { np = from ? from->allnext : of_allnodes; for (; np; np = np->allnext) … If 'from' is NULL then search from of_allnodes, else search from a specific starting point allnext linked list - internal usage Common pattern: of_find_by_XXX(struct device node *from, …) { np = from ? from->allnext : of_allnodes; for (; np; np = np->allnext) … If 'from' is NULL then search from of_allnodes, else search from a specific starting point allnext linked list - internal usage Find nodes by attribute struct device_node { const char *name; const char *type; phandle phandle; const char *full_name; allnext linked list - internal usage Find nodes by attribute of_find_node_by_name (*from, …) of_find_node_by_type (*from, …) of_find_node_by_phandle (handle) handle is unique, so *from not needed of_find_node_with_property (*from, …) traverse allnext and properties allnext linked list - internal usage Find nodes by attribute of_find_node_by_name (*from, …) of_find_node_by_type (*from, …) of_find_node_by_phandle (handle) handle is unique, so *from not needed of_find_node_with_property (*from, …) traverse allnext and properties allnext linked list Properties 'name' and 'device_type' are special. memory { device_type = "memory"; reg = <0 0>; }; In addition to existing on the node's properties list, these properties are hoisted into: device_node.name device_node.type Expanded format tree of struct device_node parent pointer Expanded format tree of struct device_node parent child sibling allnext Expanded format tree of struct device_node properties pointer struct property { char *name; int length; void *value; struct property *next; unsigned long _flags; unsigned int unique_id; struct bin_attribute attr; }; Expanded format tree of struct device_node child sibling properties next Chapter 2 Matching boot customization options to the device tree Kernel boot Linux kernel - machine_desc A struct machine_desc describes boot customizations for a specific device tree A struct machine_desc may be used for several different device trees Linux kernel - machine_desc A struct machine_desc describes boot customizations for a specific device tree A struct machine_desc may be used for several different device trees machine_desc struct machine_desc { unsigned int nr; /* architecture */ const char *name; /* architecture */ unsigned long atag_offset; char *dt_compat; /* 'compatible' strings */ unsigned int nr_irqs; phys_addr_t dma_zone_size; ... enum reboot_mode
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