From: Saravana Kannan <saravanak@google.com>
To: Rob Herring <robh@kernel.org>
Cc: devicetree-spec@vger.kernel.org, kernel-team@android.com
Subject: Re: [PATCH v2] schemas: Add schema for post-init-providers
Date: Thu, 7 Mar 2024 22:58:22 -0800 [thread overview]
Message-ID: <CAGETcx8fdJWE1BT-OCXQwVyDURSeJfXvvNS6fWfWG9z-MNZykA@mail.gmail.com> (raw)
In-Reply-To: <20240308065633.2180623-1-saravanak@google.com>
On Thu, Mar 7, 2024 at 10:56 PM Saravana Kannan <saravanak@google.com> 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 <saravanak@google.com>
> ---
> 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 <saravanak@google.com>
> +
> +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
>
prev parent reply other threads:[~2024-03-08 6:59 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-03-08 6:56 [PATCH v2] schemas: Add schema for post-init-providers Saravana Kannan
2024-03-08 6:58 ` Saravana Kannan [this message]
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