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Configuration data, which cannot be self discovered in run-time, has to be supplied from external source. The concept of a flattened device tree is a platform and architecture independent approach for resolving such problems. The idea is inherited from Open Firmware IEEE 1275 device-tree notion, and has been successfully adopted by the embedded industry. The scheme works in the following way:
The fdt layer allows any platform code in the kernel to retrieve information about hardware resources from a unified origin, which brings advantages to the embedded applications (eliminates hard-coded configuration approach, enforces code to be data driven and extensible) leading to easier porting and maintenance.
|Device tree source (DTS)|
|The device tree source is a text file which describes hardware resources of a computer system in a human-readable form, with certain hierarchical structure (a tree). The default location for DTS files in the FreeBSD source repository is sys/dts directory.|
|Device tree blob (DTB)|
|The textual device tree description (DTS file) is first converted (compiled) into a binary object (the device tree blob) i.e. the DTB, which is handed over to the final consumer (typically kernel) for parsing and processing of its contents.|
|Device tree compiler (DTC)|
|A utility program executed on the host, which transforms (compiles) a textual description of a device tree (DTS) into a binary object (DTB).|
|Device tree bindings|
|While the device tree textual description and the binary object are media to convey the hardware configuration information, an actual meaning and interpretation of the contents are defined by the device tree bindings. They are certain conventions describing definitions (encoding) of particular nodes in a device tree and their properties, allowed values, ranges and so on. Such reference conventions were provided by the legacy Open Firmware bindings, further supplemented by the ePAPR specification.|
This creates the user space dtc compiler and enables fdt support in loader(8).
|makeoptions DTS+=<board name>.dts|
|Specifies device tree source (DTS) files for a given kernel. The indicated DTS files will be converted (compiled) into a binary form along with building the kernel itself. Any DTS file names not written as an absolute path must be specified relative to the default location of DTS sources i.e., sys/dts.|
|makeoptions DTSO+=<overlay name>.dtso|
|Specifies device tree source overlay (DTSO) files for a given kernel. Overlay files will be built with the kernel as with the makeoption DTS described above. Overlay files specified as relative paths will be relative to the default location of DTS overlays for the platform being built i.e., sys/dts/arm/overlays.|
|The primary option for enabling fdt support in the kernel. It covers all low-level and infrastructure parts of fdt kernel support, which primarily are the fdtbus(4) and simplebus(4) drivers, as well as helper routines and libraries.|
|makeoptions FDT_DTS_FILE=<board name>.dts|
|Specifies a preferred (default) device tree source (DTS) file for a given kernel. It will be built along with the kernel as if it were supplied via the makeoption DTS described above. This makeoption is not mandatory unless FDT_DTB_STATIC is also defined (see below).|
|Typically, the device tree blob (DTB) is a stand-alone file, physically separate from the kernel, but this option lets statically embed a DTB file into a kernel image. Note that when this is specified the FDT_DTS_FILE makeoption becomes mandatory (as there needs to be a DTS file specified in order to embed it into the kernel image).|
Power.org Standard for Embedded Power Architecture Platform Requirements (ePAPR).
|FDT (4)||March 28, 2019|
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|“||Ken Thompson has an automobile which he helped design. Unlike most automobiles, it has neither speedometer, nor gas gauge, nor any of the other numerous idiot lights which plague the modern driver. Rather, if the driver makes a mistake, a giant “?” lights up in the center of the dashboard. “The experienced driver,” says Thompson, “will usually know what's wrong.”||”|