mirror of
https://github.com/AsahiLinux/u-boot
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89 lines
3.1 KiB
YAML
89 lines
3.1 KiB
YAML
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# SPDX-License-Identifier: BSD-2-Clause
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# Copyright 2022 Google LLC
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%YAML 1.2
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---
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$id: http://devicetree.org/schemas/bootph.yaml#
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$schema: http://devicetree.org/meta-schemas/core.yaml#
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title: Boot-phase-specific device nodes
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maintainers:
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- Simon Glass <sjg@chromium.org>
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description: |
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Some programs run in memory-constrained environments yet want to make use
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of device tree.
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The full device tree is often quite large relative to the available memory
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of a boot phase, so cannot fit into every phase of the boot process. Even
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when memory is not a problem, some phases may wish to limit which device
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nodes are present, so as to reduce execution time.
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This binding supports adding tags to device tree nodes to allow them to be
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marked according to the phases where they should be included.
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Without any tags, nodes are included only in the final phase, where all
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memory is available. Any untagged nodes are dropped from previous phases
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and are ignored before the final phase is reached.
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The build process produces a separate executable for each phase. It can
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use fdtgrep to drop any nodes which are not needed for a particular build.
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For example, the pre-sram build will drop any nodes which are not marked
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with bootph-pre-sram or bootph-all tags.
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Note that phase builds may drop the tags, since they have served their
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purpose by that point. So when looking at phase-specific device tree files
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you may not see these tags.
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Multiple tags can be used in the same node.
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Tags in a child node are implied to be present in all parent nodes as well.
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This is important, since some missing properties (such as "ranges", or
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"compatible") can cause the child node to be ignored or incorrectly
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parsed.
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That said, at present, fdtgrep applies tags only to the node they are
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added to, not to any parents. This means U-Boot device tree files often
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add the same tag to parent nodes, rather than relying on tooling to do
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this. This is a limitation of fdtgrep and it will be addressed so that
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'Linux DTs' do not need to do this.
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The available tags are described as properties below, in order of phase
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execution.
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select: true
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properties:
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bootph-pre-sram:
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type: boolean
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description:
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Enable this node when SRAM is not available. This phase must set up
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some SRAM or cache-as-RAM so it can obtain data/BSS space to use
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during execution.
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bootph-verify:
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type: boolean
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description:
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Enable this node in the verification step, which decides which of the
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available images should be run next.
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bootph-pre-ram:
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type: boolean
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description:
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Enable this node in the phase that sets up SDRAM.
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bootph-some-ram:
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type: boolean
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description:
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Enable this node in the phase that is run after SDRAM is working but
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before all of it is available. Some RAM is available but it is limited
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(e.g. it may be split into two pieces by the location of the running
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program) because the program code is not yet relocated out of the way.
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bootph-all:
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type: boolean
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description:
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Include this node in all phases (for U-Boot see enum u_boot_phase).
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additionalProperties: true
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