Add an RFC for removing upwards propagation of clock domains

text/0059-no-domain-upwards-propagation.md

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+- Start Date: 2024-03-25
+- RFC PR: [amaranth-lang/rfcs#59](https://github.com/amaranth-lang/rfcs/pull/59)
+- Amaranth Issue: [amaranth-lang/amaranth#1242](https://github.com/amaranth-lang/amaranth/issues/1242)
+
+# Get rid of upwards propagation of clock domains
+
+## Summary
+[summary]: #summary
+
+Upwards propagation of clock domains is removed, `local=True` effectively becomes the default.
+
+## Motivation
+[motivation]: #motivation
+
+Currently, a clock domain created anywhere in the hierarchy is, by default, propagated everywhere within that hierarchy. The other option is marking a clock domain with `local=True`, which restricts propagation to only downwards from the point of definition.
+
+The default behavior constitutes the ultimate implicit spooky action at a distance, making it hard to reason about clock domain origins in complex Amaranth design. It is also rarely used. This dangerous default should be changed to something more reasonable.
+
+## Guide and reference-level explanation
+[explanation]: #explanation
+
+In version 0.5, upwards domain propagation becomes deprecated. Any use of a domain that wouldn't be valid without upwards propagation (ie. wouldn't be valid if the domain were `local=True`) triggers a deprecation warning.
+
+In version 0.6, upwards domain propagation is removed entirely. The `local` argument to `ClockDomain` constructor becomes redundant and is deprecated.
+
+In version 0.7, the `local` argument is removed.
+
+## Drawbacks
+[drawbacks]: #drawbacks
+
+A little bit of churn for designs with clock generator modules.
+
+When propagating a domain upwards is actually desired (eg. for clock generator modules), a workaround is now necessary to route the domain to the toplevel. The simplest way is to set an eg. `cd_sync` attribute on the module in the constructor, then do `m.domains.sync = clkgen.cd_sync` in the top-level. This has the slight disadvantage that the clock generator module cannot decide on the attributes of the clock domain (clock polarity, reset asyncness) based on the platform.
+
+## Rationale and alternatives
+[rationale-and-alternatives]: #rationale-and-alternatives
+
+The core proposal is straightforward. The main question is how to handle the usecases currently served by purposeful use of upwards domain propagation. There are three alternatives:
+
+1. None. Clock generator modules must export their domains through some other existing Amaranth functionality. This is the option proposed above.
+2. Keep the current mechanism as-is, but make it opt-in, such by requiring `ClockDomain(global=True)`.
+3. Add a mechanism to explicitly import a domain from a submodule. A draft is proposed here, for discussion purposes:
+
+   ```py
+   m.submodules.clkgen = ClockGenerator(...)
+   m.domains.sync = ImportedDomain("clkgen", "sync") # Imports a domain defined in the submodule "clkgen" named "sync".
+   ```
+
+We feel that 1. is the best option to take, as introducing a new mechanism when a full redesign of the system is pending is likely to just result in another thing that will have to be deprecated later.
+
+## Prior art
+[prior-art]: #prior-art
+
+None.
+
+## Unresolved questions
+[unresolved-questions]: #unresolved-questions
+
+None.
+
+## Future possibilities
+[future-possibilities]: #future-possibilities
+
+The concept of "private" clock domains has been proposed: clock domains that don't propagate at all, including downwards.
+
+It's clear that clock domain overhaul with a larger scope needs to be done. This RFC is (among other things) intended to enable such overhaul, by removing a feature likely to make it infeasible.