Hi Conon, On Wed, Feb 14, 2024 at 10:49 AM Conor Dooley <conor@xxxxxxxxxx> wrote: > > 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. > > I don't think this sentence makes sense, or at least it is not easy to > parse. It implies that it can "point to" other properties too I don't see how this sentence implies this. But open to suggestions on how to reword it. I don't want to drop this line entirely though because I'm trying to make it clear that this doesn't make a device (that's not previously a supplier) into a supplier. It only down grades an existing supplier to a post device initialization supplier. > - but > that's not the case. It is only valid to "point to" these suppliers. > I'd drop this entirely. > > > + > > + 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. > > "it would be safe to assume" seems odd wording to me - I feel like the > default is stronger than "safe to assume". I'd just drop the "would be > safe to assume and replace with "is assumed". Sounds good. > > > + > > + 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. > > Otherwise, I think this is a great description, describing the use case > well :) Thanks! I always spend more time writing documentation and commit text than the time I spend writing code. > > > + > > +select: true > > +properties: > > + post-init-supplier: [Merging your other email here] > Also, this should likely be pluralised, to match "clocks" "resets" > "interrupts" etc. Good point. Done. > > + # One or more suppliers can be marked as post initialization supplier > > + description: > > + List of phandles to suppliers that are not needed for initializing or > > + resuming this device. > > + $ref: /schemas/types.yaml#/definitions/phandle-array > > + items: > > + maxItems: 1 > > Rob's bot rightfully complains here about invalid syntax. I added these two lines based on Rob's feedback. Is the indentation that's wrong? Yeah, I'm trying to run the dts checker, but I haven't be able to get it to work on my end. See my email to Rob on the v1 series about this. $ make DT_CHECKER_FLAGS=-m dt_binding_check The best I could get out of it is a bunch of error reports on other files and then: ... <snip>/Documentation/devicetree/bindings/post-init-suppliers.yaml: ignoring, error parsing file ... I also tried to use DT_SCHEMA_FILES so I can only test this one file, but that wasn't working either: $ make DT_CHECKER_FLAGS=-m dt_binding_check DT_SCHEMA_FILES=Documentation/devicetree/bindings/post-init-suppliers.yaml or $ make DT_CHECKER_FLAGS=-m dt_binding_check DT_SCHEMA_FILES=<path to the .patch file> Results in this error early on in the output: ... usage: yamllint [-h] [-] [-c CONFIG_FILE | -d CONFIG_DATA] [--list-files] [-f {parsable,standard,colored,github,auto}] [-s] [--no-warnings] [-v] [FILE_OR_DIR ...] yamllint: error: one of the arguments FILE_OR_DIR - is required ... /mnt/android/linus-tree/Documentation/devicetree/bindings/post-init-suppliers.yaml: ignoring, error parsing file ... > What you > actually want to enforce here is any number of device phandles, but > these phandles all contain only the label and no indices etc, right? Correct. > > > + > > +examples: > > + - | > > + gcc: clock-controller@1000 { > > + compatible = "vendor,soc4-gcc", "vendor,soc1-gcc"; > > + reg = <0x1000 0x80>; > > + clocks = <&dispcc 0x1> > > This clearly was never tested, Rob's bot warnings aside. You're missing > a ; at EOL here and with the other clock below. Yup. I'm unable to get the test to run. Thanks, Saravana
