1# SPDX-License-Identifier: BSD-2-Clause 2# Copyright 2022 Google LLC 3%YAML 1.2 4--- 5$id: http://devicetree.org/schemas/bootph.yaml# 6$schema: http://devicetree.org/meta-schemas/core.yaml# 7 8title: Boot-phase-specific device nodes 9 10maintainers: 11 - Simon Glass <sjg@chromium.org> 12 13description: | 14 Some programs run in memory-constrained environments yet want to make use 15 of device tree. 16 17 The full device tree is often quite large relative to the available memory 18 of a boot phase, so cannot fit into every phase of the boot process. Even 19 when memory is not a problem, some phases may wish to limit which device 20 nodes are present, so as to reduce execution time. 21 22 This binding supports adding tags to device tree nodes to allow them to be 23 marked according to the phases where they should be included. 24 25 Without any tags, nodes are included only in the final phase, where all 26 memory is available. Any untagged nodes are dropped from previous phases 27 and are ignored before the final phase is reached. 28 29 The build process produces a separate executable for each phase. It can 30 use fdtgrep to drop any nodes which are not needed for a particular build. 31 For example, the pre-sram build will drop any nodes which are not marked 32 with bootph-pre-sram or bootph-all tags. 33 34 Note that phase builds may drop the tags, since they have served their 35 purpose by that point. So when looking at phase-specific device tree files 36 you may not see these tags. 37 38 Multiple tags can be used in the same node. 39 40 Tags in a child node are implied to be present in all parent nodes as well. 41 This is important, since some missing properties (such as "ranges", or 42 "compatible") can cause the child node to be ignored or incorrectly 43 parsed. 44 45 That said, at present, fdtgrep applies tags only to the node they are 46 added to, not to any parents. This means U-Boot device tree files often 47 add the same tag to parent nodes, rather than relying on tooling to do 48 this. This is a limitation of fdtgrep and it will be addressed so that 49 'Linux DTs' do not need to do this. 50 51 The available tags are described as properties below, in order of phase 52 execution. 53 54select: true 55 56properties: 57 bootph-pre-sram: 58 type: boolean 59 description: 60 Enable this node when SRAM is not available. This phase must set up 61 some SRAM or cache-as-RAM so it can obtain data/BSS space to use 62 during execution. 63 64 bootph-verify: 65 type: boolean 66 description: 67 Enable this node in the verification step, which decides which of the 68 available images should be run next. 69 70 bootph-pre-ram: 71 type: boolean 72 description: 73 Enable this node in the phase that sets up SDRAM. 74 75 bootph-some-ram: 76 type: boolean 77 description: 78 Enable this node in the phase that is run after SDRAM is working but 79 before all of it is available. Some RAM is available but it is limited 80 (e.g. it may be split into two pieces by the location of the running 81 program) because the program code is not yet relocated out of the way. 82 83 bootph-all: 84 type: boolean 85 description: 86 Include this node in all phases (for U-Boot see enum u_boot_phase). 87 88additionalProperties: true 89