1 Device Tree Dynamic Object format internals 2 ------------------------------------------- 3 4 The Device Tree for most platforms is a static representation of 5 the hardware capabilities. This is insufficient for platforms 6 that need to dynamically insert Device Tree fragments into the 7 live tree. 8 9 This document explains the the Device Tree object format and 10 modifications made to the Device Tree compiler, which make it possible. 11 12 1. Simplified Problem Definition 13 -------------------------------- 14 15 Assume we have a platform which boots using following simplified Device Tree. 16 17 ---- foo.dts ----------------------------------------------------------------- 18 /* FOO platform */ 19 / { 20 compatible = "corp,foo"; 21 22 /* shared resources */ 23 res: res { 24 }; 25 26 /* On chip peripherals */ 27 ocp: ocp { 28 /* peripherals that are always instantiated */ 29 peripheral1 { ... }; 30 }; 31 }; 32 ---- foo.dts ----------------------------------------------------------------- 33 34 We have a number of peripherals that after probing (using some undefined method) 35 should result in different Device Tree configuration. 36 37 We cannot boot with this static tree because due to the configuration of the 38 foo platform there exist multiple conficting peripherals DT fragments. 39 40 So for the bar peripheral we would have this: 41 42 ---- foo+bar.dts ------------------------------------------------------------- 43 /* FOO platform + bar peripheral */ 44 / { 45 compatible = "corp,foo"; 46 47 /* shared resources */ 48 res: res { 49 }; 50 51 /* On chip peripherals */ 52 ocp: ocp { 53 /* peripherals that are always instantiated */ 54 peripheral1 { ... }; 55 56 /* bar peripheral */ 57 bar { 58 compatible = "corp,bar"; 59 ... /* various properties and child nodes */ 60 }; 61 }; 62 }; 63 ---- foo+bar.dts ------------------------------------------------------------- 64 65 While for the baz peripheral we would have this: 66 67 ---- foo+baz.dts ------------------------------------------------------------- 68 /* FOO platform + baz peripheral */ 69 / { 70 compatible = "corp,foo"; 71 72 /* shared resources */ 73 res: res { 74 /* baz resources */ 75 baz_res: res_baz { ... }; 76 }; 77 78 /* On chip peripherals */ 79 ocp: ocp { 80 /* peripherals that are always instantiated */ 81 peripheral1 { ... }; 82 83 /* baz peripheral */ 84 baz { 85 compatible = "corp,baz"; 86 /* reference to another point in the tree */ 87 ref-to-res = <&baz_res>; 88 ... /* various properties and child nodes */ 89 }; 90 }; 91 }; 92 ---- foo+baz.dts ------------------------------------------------------------- 93 94 We note that the baz case is more complicated, since the baz peripheral needs to 95 reference another node in the DT tree. 96 97 2. Device Tree Object Format Requirements 98 ----------------------------------------- 99 100 Since the Device Tree is used for booting a number of very different hardware 101 platforms it is imperative that we tread very carefully. 102 103 2.a) No changes to the Device Tree binary format for the base tree. We cannot 104 modify the tree format at all and all the information we require should be 105 encoded using Device Tree itself. We can add nodes that can be safely ignored 106 by both bootloaders and the kernel. The plugin dtbs are optionally tagged 107 with a different magic number in the header but otherwise they're simple 108 blobs. 109 110 2.b) Changes to the DTS source format should be absolutely minimal, and should 111 only be needed for the DT fragment definitions, and not the base boot DT. 112 113 2.c) An explicit option should be used to instruct DTC to generate the required 114 information needed for object resolution. Platforms that don't use the 115 dynamic object format can safely ignore it. 116 117 2.d) Finally, DT syntax changes should be kept to a minimum. It should be 118 possible to express everything using the existing DT syntax. 119 120 3. Implementation 121 ----------------- 122 123 The basic unit of addressing in Device Tree is the phandle. Turns out it's 124 relatively simple to extend the way phandles are generated and referenced 125 so that it's possible to dynamically convert symbolic references (labels) 126 to phandle values. This is a valid assumption as long as the author uses 127 reference syntax and does not assign phandle values manually (which might 128 be a problem with decompiled source files). 129 130 We can roughly divide the operation into two steps. 131 132 3.a) Compilation of the base board DTS file using the '-@' option 133 generates a valid DT blob with an added __symbols__ node at the root node, 134 containing a list of all nodes that are marked with a label. 135 136 Using the foo.dts file above the following node will be generated; 137 138 $ dtc -@ -O dtb -o foo.dtb -b 0 foo.dts 139 $ fdtdump foo.dtb 140 ... 141 / { 142 ... 143 res { 144 ... 145 phandle = <0x00000001>; 146 ... 147 }; 148 ocp { 149 ... 150 phandle = <0x00000002>; 151 ... 152 }; 153 __symbols__ { 154 res="/res"; 155 ocp="/ocp"; 156 }; 157 }; 158 159 Notice that all the nodes that had a label have been recorded, and that 160 phandles have been generated for them. 161 162 This blob can be used to boot the board normally, the __symbols__ node will 163 be safely ignored both by the bootloader and the kernel (the only loss will 164 be a few bytes of memory and disk space). 165 166 We generate a __symbols__ node to record nodes that had labels in the base 167 tree (or subsequent loaded overlays) so that they can be matched up with 168 references made to them in Device Tree objects. 169 170 3.b) The Device Tree fragments must be compiled with the same option but they 171 must also have a tag (/plugin/) that allows undefined references to nodes 172 that are not present at compilation time to be recorded so that the runtime 173 loader can fix them. 174 175 So the bar peripheral's DTS format would be of the form: 176 177 /dts-v1/; 178 /plugin/; /* allow undefined references and record them */ 179 / { 180 .... /* various properties for loader use; i.e. part id etc. */ 181 fragment@0 { 182 target = <&ocp>; 183 __overlay__ { 184 /* bar peripheral */ 185 bar { 186 compatible = "corp,bar"; 187 ... /* various properties and child nodes */ 188 } 189 }; 190 }; 191 }; 192 193 Note that there's a target property that specifies the location where the 194 contents of the overlay node will be placed, and it references the node 195 in the foo.dts file. 196 197 $ dtc -@ -O dtb -o bar.dtbo -b 0 bar.dts 198 $ fdtdump bar.dtbo 199 ... 200 / { 201 ... /* properties */ 202 fragment@0 { 203 target = <0xffffffff>; 204 __overlay__ { 205 bar { 206 compatible = "corp,bar"; 207 ... /* various properties and child nodes */ 208 } 209 }; 210 }; 211 __fixups__ { 212 ocp = "/fragment@0:target:0"; 213 }; 214 }; 215 216 No __symbols__ node has been generated (no label in bar.dts). 217 Note that the target's ocp label is undefined, so the phandle 218 value is filled with the illegal value '0xffffffff', while a __fixups__ 219 node has been generated, which marks the location in the tree where 220 the label lookup should store the runtime phandle value of the ocp node. 221 222 The format of the __fixups__ node entry is 223 224 <label> = "<local-full-path>:<property-name>:<offset>" 225 [, "<local-full-path>:<property-name>:<offset>"...]; 226 227 <label> Is the label we're referring 228 <local-full-path> Is the full path of the node the reference is 229 <property-name> Is the name of the property containing the 230 reference 231 <offset> The offset (in bytes) of where the property's 232 phandle value is located. 233 234 Doing the same with the baz peripheral's DTS format is a little bit more 235 involved, since baz contains references to local labels which require 236 local fixups. 237 238 /dts-v1/; 239 /plugin/; /* allow undefined label references and record them */ 240 / { 241 .... /* various properties for loader use; i.e. part id etc. */ 242 fragment@0 { 243 target = <&res>; 244 __overlay__ { 245 /* baz resources */ 246 baz_res: res_baz { ... }; 247 }; 248 }; 249 fragment@1 { 250 target = <&ocp>; 251 __overlay__ { 252 /* baz peripheral */ 253 baz { 254 compatible = "corp,baz"; 255 /* reference to another point in the tree */ 256 ref-to-res = <&baz_res>; 257 ... /* various properties and child nodes */ 258 } 259 }; 260 }; 261 }; 262 263 Note that &bar_res reference. 264 265 $ dtc -@ -O dtb -o baz.dtbo -b 0 baz.dts 266 $ fdtdump baz.dtbo 267 ... 268 / { 269 ... /* properties */ 270 fragment@0 { 271 target = <0xffffffff>; 272 __overlay__ { 273 res_baz { 274 .... 275 phandle = <0x00000001>; 276 }; 277 }; 278 }; 279 fragment@1 { 280 target = <0xffffffff>; 281 __overlay__ { 282 baz { 283 compatible = "corp,baz"; 284 ... /* various properties and child nodes */ 285 ref-to-res = <0x00000001>; 286 } 287 }; 288 }; 289 __fixups__ { 290 res = "/fragment@0:target:0"; 291 ocp = "/fragment@1:target:0"; 292 }; 293 __local_fixups__ { 294 fragment@1 { 295 __overlay__ { 296 baz { 297 ref-to-res = <0>; 298 }; 299 }; 300 }; 301 }; 302 }; 303 304 This is similar to the bar case, but the reference of a local label by the 305 baz node generates a __local_fixups__ entry that records the place that the 306 local reference is being made. No matter how phandles are allocated from dtc 307 the run time loader must apply an offset to each phandle in every dynamic 308 DT object loaded. The __local_fixups__ node records the offset relative to the 309 start of every local reference within that property so that the loader can apply 310 the offset. 311 312 There is an alternative syntax to the expanded form for overlays with phandle 313 targets which makes the format similar to the one using in .dtsi include files. 314 315 So for the &ocp target example above one can simply write: 316 317 /dts-v1/; 318 /plugin/; 319 &ocp { 320 /* bar peripheral */ 321 bar { 322 compatible = "corp,bar"; 323 ... /* various properties and child nodes */ 324 } 325 }; 326 327 The resulting dtb object is identical. 328
