On Wed, Mar 6, 2024 at 1:17 PM Simon Glass <sjg@xxxxxxxxxxxx> wrote: > > Hi Saravana, > > On Wed, 6 Mar 2024 at 13:30, Saravana Kannan <saravanak@xxxxxxxxxx> 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 > > Please can you add hyphens to avoid confusion? I believe this should > be 'post-device-initialization' throughout. There is no 'post' device. There is no 'post-device-initialization' thing either. Shrug. > > 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> > > --- > > 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..92eb9a0 > > --- /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. > > +%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@xxxxxxxxxx> > > + > > +description: | > > + This property is used to indicate that the device(s) pointed to by the > > Should this be 'device node' instead of 'device'? > > > + 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). > > How about '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. > > This seems strange to me. It seems like something that could be > implemented. That said, I understand that such an implementation could > become painful. It has been implemented. And cycle detection has been implemented. And yet we are still here needing something more. > > + 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: > > What is 'init'? It seems to mean when Linux probes the device. In > U-Boot, devices all are bound at the start, but only probed (lazilly) > when used. For some device types there is then an 'init' step which > actually starts using the device, before which no hardware is > accessed. So the use of the word 'init' seems a bit vague to me. > > In general this binding seems liable to be specific to the OS being > used. It also seems to be a hint, rather than something that must be > parsed and used. However I suppose adding the suffix '-hint' would > just confuse things. > > How about 'secondary-providers' or 'minor-providers' or > 'delayed-providers' or 'deferred-providers'? > > Separately/alternatively I wonder if the target node is always going > to be inited later, for any device that uses it. If so, perhaps a > property in the target node would be better, something like: > > dispcc: clock-controller@2000 { > ... > minor-provider; That doesn't work if a node is 2 or more providers and only some of them are optional/post-init/deferred. Rob
