+U-Boot Custodians too
On Thu, 29 Sept 2022 at 13:58, Simon Glass <s...@chromium.org> wrote: > > 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 <s...@chromium.org> > --- > > dtschema/schemas/u-boot.yaml | 48 ++++++++++++++++++++++++++++++++++++ > 1 file changed, 48 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..6c5c820 > --- /dev/null > +++ b/dtschema/schemas/u-boot.yaml > @@ -0,0 +1,48 @@ > +# 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 <s...@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. > + > + Note that xPL builds also drop the tags, since they have server their > + purpose by that point. So when looking at xPL device tree files you > will > + not see these tags. The means, for example, that ofnode_pre_reloc() > + returns true always 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 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 > -- > 2.38.0.rc1.362.ged0d419d3c-goog >