On Mon, Feb 12, 2024 at 01:31:44PM -0800, Saravana Kannan wrote: > The post-init-supplier property can be used to break a dependency cycle by > marking some supplier(s) as a post device initialization supplier(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> > --- > .../bindings/post-init-supplier.yaml | 101 ++++++++++++++++++ > MAINTAINERS | 13 +-- > 2 files changed, 108 insertions(+), 6 deletions(-) > create mode 100644 Documentation/devicetree/bindings/post-init-supplier.yaml > > diff --git a/Documentation/devicetree/bindings/post-init-supplier.yaml b/Documentation/devicetree/bindings/post-init-supplier.yaml > new file mode 100644 > index 000000000000..aab75b667259 > --- /dev/null > +++ b/Documentation/devicetree/bindings/post-init-supplier.yaml > @@ -0,0 +1,101 @@ > +# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause) > +# Copyright (c) 2020, Google LLC. All rights reserved. > +%YAML 1.2 > +--- > +$id: http://devicetree.org/schemas/post-init-supplier.yaml# > +$schema: http://devicetree.org/meta-schemas/core.yaml# > + > +title: Post device initialization supplier > + > +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 is meaningful only when pointing to direct suppliers > + of a device that are pointed to by other properties in the device. > + > + A device can list its suppliers in devicetree using one or more of the > + standard devicetree bindings. By default, it would be safe to assume the > + supplier 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 supplier (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 supplier 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 supplier(s) is not needed for initializing the > + device that lists this property. > + > + In the example above, Z would list X as a post-init-supplier and the > + initialization dependency would become X -> Y -> Z -/-> X. So the best order > + to initialize them become clear: Z, Y and then X. > + > +select: true blank line > +properties: > + post-init-supplier: 'supply' is already used for regulators. Let's make it 'post-init-providers'. Rob
