Re: [PATCH v2] schemas: Add schema for post-init-providers

On Thu, Mar 7, 2024 at 10:56 PM Saravana Kannan <[email protected]> wrote:
>
> 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 <[email protected]>
> ---
>  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.

Sigh. Forgot to update this.

Rob, can you please just change this to 2024 on your end?

Thanks,
Saravana

> +%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 <[email protected]>
> +
> +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
>