diff mbox series

[v4,3/4] dt-bindings: Add post-init-providers property

Message ID 20240222034624.2970024-4-saravanak@google.com (mailing list archive)
State Handled Elsewhere, archived
Headers show
Series Add post-init-providers binding to improve suspend/resume stability | expand

Commit Message

Saravana Kannan Feb. 22, 2024, 3:46 a.m. UTC
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>
---
 .../bindings/post-init-providers.yaml         | 105 ++++++++++++++++++
 MAINTAINERS                                   |  13 ++-
 2 files changed, 112 insertions(+), 6 deletions(-)
 create mode 100644 Documentation/devicetree/bindings/post-init-providers.yaml

Comments

Rob Herring (Arm) Feb. 22, 2024, 5:23 a.m. UTC | #1
On Wed, 21 Feb 2024 19:46:21 -0800, Saravana Kannan 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>
> ---
>  .../bindings/post-init-providers.yaml         | 105 ++++++++++++++++++
>  MAINTAINERS                                   |  13 ++-
>  2 files changed, 112 insertions(+), 6 deletions(-)
>  create mode 100644 Documentation/devicetree/bindings/post-init-providers.yaml
> 

My bot found errors running 'make DT_CHECKER_FLAGS=-m dt_binding_check'
on your patch (DT_CHECKER_FLAGS is new in v5.13):

yamllint warnings/errors:

dtschema/dtc warnings/errors:
Documentation/devicetree/bindings/post-init-providers.example.dtb: /example-0/clock-controller@1000: failed to match any schema with compatible: ['vendor,soc4-gcc', 'vendor,soc1-gcc']
Documentation/devicetree/bindings/post-init-providers.example.dtb: /example-0/clock-controller@1000: failed to match any schema with compatible: ['vendor,soc4-gcc', 'vendor,soc1-gcc']
Documentation/devicetree/bindings/post-init-providers.example.dtb: /example-0/clock-controller@2000: failed to match any schema with compatible: ['vendor,soc4-dispcc', 'vendor,soc1-dispcc']
Documentation/devicetree/bindings/post-init-providers.example.dtb: /example-0/clock-controller@2000: failed to match any schema with compatible: ['vendor,soc4-dispcc', 'vendor,soc1-dispcc']

doc reference errors (make refcheckdocs):

See https://patchwork.ozlabs.org/project/devicetree-bindings/patch/20240222034624.2970024-4-saravanak@google.com

The base for the series is generally the latest rc1. A different dependency
should be noted in *this* patch.

If you already ran 'make dt_binding_check' and didn't see the above
error(s), then make sure 'yamllint' is installed and dt-schema is up to
date:

pip3 install dtschema --upgrade

Please check and re-submit after running the above command yourself. Note
that DT_SCHEMA_FILES can be set to your schema file to speed up checking
your schema. However, it must be unset to test all examples with your schema.
diff mbox series

Patch

diff --git a/Documentation/devicetree/bindings/post-init-providers.yaml b/Documentation/devicetree/bindings/post-init-providers.yaml
new file mode 100644
index 000000000000..92eb9a027443
--- /dev/null
+++ b/Documentation/devicetree/bindings/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@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 post initialization).
+
+  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>;
+    };
diff --git a/MAINTAINERS b/MAINTAINERS
index 9ed4d3868539..feec740239c9 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -6060,12 +6060,6 @@  S:	Maintained
 F:	drivers/base/devcoredump.c
 F:	include/linux/devcoredump.h
 
-DEVICE DEPENDENCY HELPER SCRIPT
-M:	Saravana Kannan <saravanak@google.com>
-L:	linux-kernel@vger.kernel.org
-S:	Maintained
-F:	scripts/dev-needs.sh
-
 DEVICE DIRECT ACCESS (DAX)
 M:	Dan Williams <dan.j.williams@intel.com>
 M:	Vishal Verma <vishal.l.verma@intel.com>
@@ -8300,6 +8294,13 @@  F:	include/linux/firewire.h
 F:	include/uapi/linux/firewire*.h
 F:	tools/firewire/
 
+FIRMWARE DEVICE LINK (fw_devlink)
+M:	Saravana Kannan <saravanak@google.com>
+L:	linux-kernel@vger.kernel.org
+S:	Maintained
+F:	Documentation/devicetree/bindings/post-init-providers.yaml
+F:	scripts/dev-needs.sh
+
 FIRMWARE FRAMEWORK FOR ARMV8-A
 M:	Sudeep Holla <sudeep.holla@arm.com>
 L:	linux-arm-kernel@lists.infradead.org (moderated for non-subscribers)