The post-init-providers property can be used to break a dependency cycle by marking some provider(s) as a post-device-initialization provider(s). This allows an OS to do a better job at ordering initialization and suspend/resume of the devices in a dependency cycle. Signed-off-by: Saravana Kannan <saravanak@xxxxxxxxxx> --- dtschema/schemas/post-init-providers.yaml | 105 ++++++++++++++++++++++ 1 file changed, 105 insertions(+) create mode 100644 dtschema/schemas/post-init-providers.yaml diff --git a/dtschema/schemas/post-init-providers.yaml b/dtschema/schemas/post-init-providers.yaml new file mode 100644 index 0000000..a329776 --- /dev/null +++ b/dtschema/schemas/post-init-providers.yaml @@ -0,0 +1,105 @@ +# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause) +# Copyright (c) 2020, Google LLC. All rights reserved. +%YAML 1.2 +--- +$id: http://devicetree.org/schemas/post-init-providers.yaml# +$schema: http://devicetree.org/meta-schemas/core.yaml# + +title: Post-device-initialization providers + +maintainers: + - Saravana Kannan <saravanak@xxxxxxxxxx> + +description: | + This property is used to indicate that the device(s) pointed to by the + property are not needed for the initialization of the device that lists this + property. This property does not make a device (that's previously not a + provider) into a provider. It simply downgrades an existing provider to a + post-device-initialization provider. + + A device can list its providers in devicetree using one or more of the + standard devicetree bindings. By default, it is assumed that the provider + device can be initialized before the consumer device is initialized. + + However, that assumption cannot be made when there are cyclic dependencies + between devices. Since each device is a provider (directly or indirectly) of + the others in the cycle, there is no guaranteed safe order for initializing + the devices in a cycle. We can try to initialize them in an arbitrary order + and eventually successfully initialize all of them, but that doesn't always + work well. + + For example, say, + * The device tree has the following cyclic dependency X -> Y -> Z -> X (where + -> denotes "depends on"). + * But X is not needed to fully initialize Z (X might be needed only when a + specific functionality is requested after initialization of Z). + + If all the other -> are mandatory initialization dependencies, then trying to + initialize the devices in a loop (or arbitrarily) will always eventually end + up with the devices being initialized in the order Z, Y and X. + + However, if Y is an optional provider for X (where X provides limited + functionality when Y is not initialized and providing its services), then + trying to initialize the devices in a loop (or arbitrarily) could end up with + the devices being initialized in the following order: + + * Z, Y and X - All devices provide full functionality + * Z, X and Y - X provides partial functionality + * X, Z and Y - X provides partial functionality + + However, we always want to initialize the devices in the order Z, Y and X + since that provides the full functionality without interruptions. + + One alternate option that might be suggested is to have the driver for X + notice that Y became available at a later point and adjust the functionality + it provides. However, other userspace applications could have started using X + with the limited functionality before Y was available and it might not be + possible to transparently transition X or the users of X to full + functionality while X is in use. + + Similarly, when it comes to suspend (resume) ordering, it's unclear which + device in a dependency cycle needs to be suspended/resumed first and trying + arbitrary orders can result in system crashes or instability. + + Explicitly calling out which link in a cycle needs to be broken when + determining the order, simplifies things a lot, improves efficiency, makes + the behavior more deterministic and maximizes the functionality that can be + provided without interruption. + + This property is used to provide this additional information between devices + in a cycle by telling which provider(s) is not needed for initializing the + device that lists this property. + + In the example above, Z would list X as a post-init-providers and the + initialization dependency would become X -> Y -> Z -/-> X. So the best order + to initialize them becomes clear: Z, Y and then X. + +select: true + +properties: + post-init-providers: + # One or more providers can be marked as post initialization provider + description: + List of phandles to providers that are not needed for initializing or + resuming this device. + $ref: /schemas/types.yaml#/definitions/phandle-array + items: + maxItems: 1 + +additionalProperties: true + +examples: + - | + gcc: clock-controller@1000 { + compatible = "vendor,soc4-gcc", "vendor,soc1-gcc"; + reg = <0x1000 0x80>; + clocks = <&dispcc 0x1>; + #clock-cells = <1>; + post-init-providers = <&dispcc>; + }; + dispcc: clock-controller@2000 { + compatible = "vendor,soc4-dispcc", "vendor,soc1-dispcc"; + reg = <0x2000 0x80>; + clocks = <&gcc 0xdd>; + #clock-cells = <1>; + }; -- 2.44.0.278.ge034bb2e1d-goog
