1 commit

Author	SHA1	Message	Date
Simon Glass	7703efbc99	schemas: Add schema for U-Boot driver model 'phase tags' ... U-Boot has some particular challenges with device tree and devices: - 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. It is not always possible to handle these tags entirely at build time, since U-Boot proper must have the full device tree, even though we do not want it to process all nodes until after relocation. - 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 - 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 { bootph-all; compatible = "rockchip,rk3399-cru"; reg = <0x0 0xff760000 0x0 0x1000>; rockchip,grf = <&grf>; #clock-cells = <1>; #reset-cells = <1>; ... }; Here the "bootph-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 { bootph-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. For this version, an attempt is made to name the phases in a generic manner. 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 has been applied upstream[3] [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 [3] https://github.com/devicetree-org/dt-schema/commit/63bd847 Signed-off-by: Simon Glass <sjg@chromium.org>	2023-02-14 09:42:14 -07:00