Until recently it has not been possible to add any U-Boot-specific properties to the device tree schema. Now that it is somewhat separated from Linux and people are feeling the enormous pain of the bifurcated schema, it seems like a good time to add this and other U-Boot-specific bindings.
U-Boot has some particular challenges with device tree and devices which are not faced with Linux:
- U-Boot has multiple build phases, such as a Secondary Program Loader (SPL) phase which typically runs in a pre-SDRAM environment where code and data space are limited. In particular, there may not be enough space for the full device tree blob. U-Boot uses various automated techniques to reduce the size from perhaps 40KB to 3KB. - Some U-Boot phases needs to run before the clocks are properly set up, where the CPU may be running very slowly. Therefore it is important to bind only those devices which are actually needed in that phase - Unlike Linux or UEFI, U-Boot uses lazy initialisation for its devices, with 'bind' and 'probe' being separate steps. Even if a device is bound, it is not actually probed until it is used. This is necessary to keep the boot time reasonable, e.g. to under a second
The phases of U-Boot in order are: TPL, VPL, SPL, U-Boot (first pre-relocation, then post-relocation). ALl but the last two are optional.
For the above reasons, U-Boot only includes the full device tree in the final 'U-Boot proper' build. Even then, before relocation U-Boot only processes nodes which are marked as being needed.
For this to work, U-Boot's driver model[1] provides a way to mark device tree nodes as applicable for a particular phase. This works by adding a tag to the node, e.g.:
cru: clock-controller@ff760000 { u-boot,dm-all; compatible = "rockchip,rk3399-cru"; reg = <0x0 0xff760000 0x0 0x1000>; rockchip,grf = <&grf>; #clock-cells = <1>; #reset-cells = <1>; ... };
Here the "u-boot,dm-all" tag indicates that the node must be present in all phases, since the clock driver is required
There has been discussion over the years about whether this could be done in a property instead, e.g.
options { u-boot,dm-all = <&cru> <&gpio_a> ...; ... };
Some problems with this:
- we need to be able to merge several such tags from different .dtsi files since many boards have their own specific requirements - it is hard to find and cross-reference the affected nodes - it is more error-prone - it requires significant tool rework in U-Boot, including fdtgrep and the build system - is harder (slower, more code) to process since it involves scanning another node/property to find out what to do with a particular node - we don't want to add phandle arguments to the above since we are referring, e.g., to the clock device as a whole, not a paricular clock - the of-platdata feature[2], which converts device tree to C for even more constrained environments, would need to become aware of the /options node
There is also the question about whether this needs to be U-Boot-specific, or whether the tags could be generic. From what I can tell, U-Boot is the only bootloader which seriously attempts to use a runtime device tree in all cases. We could perhaps adopt U-Boot's naming for the phases and drop the "u-boot," prefix, but that might cause confusion.
It should also be noted that the approach provided here has stood the test of time, used in U-Boot for 8 years so far.
So add the schema for this. This will allow a major class of schema exceptions to be dropped from the U-Boot source tree.
This being sent to the mailing list since it might attract more review. A PR will be sent when this has had some review. That is why the file path is set up for https://github.com/devicetree-org/dt-schema rather than the Linux kernel.
[1] https://u-boot.readthedocs.io/en/latest/develop/driver-model/index.html [2] https://u-boot.readthedocs.io/en/latest/develop/driver-model/of-plat.html
Signed-off-by: Simon Glass sjg@chromium.org ---
Changes in v2: - Expand docs to include a description of each tag - Fix some typos and unclear wording
dtschema/schemas/u-boot.yaml | 66 ++++++++++++++++++++++++++++++++++++ 1 file changed, 66 insertions(+) create mode 100644 dtschema/schemas/u-boot.yaml
diff --git a/dtschema/schemas/u-boot.yaml b/dtschema/schemas/u-boot.yaml new file mode 100644 index 0000000..0674684 --- /dev/null +++ b/dtschema/schemas/u-boot.yaml @@ -0,0 +1,66 @@ +# SPDX-License-Identifier: BSD-2-Clause +# Copyright 2022 Google LLC +%YAML 1.2 +--- +$id: http://devicetree.org/schemas/serial.yaml# +$schema: http://devicetree.org/meta-schemas/core.yaml# + +title: Bindings required by U-Boot driver model + +maintainers: + - Simon Glass sjg@chromium.org + +patternProperties: + "^u-boot,dm-(tpl|vpl|spl|all|pre-reloc)$": + type: boolean + description: | + The device tree is often quite large (e.g. 40KB) and cannot fit into xPL + phases like SPL, TPL. Even with U-Boot proper, we normally only want to + init a small subset of devices before relocation. + + U-Boot supports adding tags to device tree nodes to allow them to be + marked according to the U-Boot phases where they should be included. + + Without any tags, nodes are included only in U-Boot proper after + relocation. Any untagged nodes are dropped from xPL and are ignored before + relocation in U-Boot proper. + + The xPL builds use fdtgrep drop any nodes which are not needed for that + build. For example, TPL will drop any nodes which are not marked with + u-boot,dm-tpl or u-boot,dm-all tags. + + The available tags are as follows: + + u-boot,dm-all: + Include this node in all phases (see enum u_boot_phase) + + u-boot,dm-pre-reloc: + Enable this node before relocation in U-Boot proper + + u-boot,dm-tpl: + Enable this node in the Tertiary Program Loader (TPL) + + u-boot,dm-vpl: + Enable this node in the Verifying Program Loader (VPL) + + u-boot,dm-spl: + Enable this node in the Secondary Program Loader (SPL) + + Note that xPL builds also drop the tags, since they have served their + purpose by that point. So when looking at xPL device tree files you will + not see these tags. This means, for example, that ofnode_pre_reloc() + always returns true in xPL builds. This is done to save space. + + Multiple tags can be used in the same node. + + One complication is that tags apply only to the node they are added to, + not to any parents. This means that you often need to add the same tag to + parent nodes so that any properties needed by the parent driver are + included. Without that, the parent node may have no properties, or may not + be bound before relocation (meaning that its child will not be bound + either). + + The phases of U-Boot in order are: TPL, VPL, SPL, U-Boot (first + pre-relocation, then post-relocation). ALl but the last two are optional. + +additionalProperties: true