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

[Saravana Kannan <saravanak@google.com>]

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

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)
-- 
2.44.0.rc0.258.g7320e95886-goog