Should displayif's modules be extensible builtins, so a .py can override them?

[bdbarnett]

Every module displayif registers uses MP_REGISTER_MODULE, which makes it a
non-extensible builtin. MicroPython resolves those before the filesystem is
consulted at all (py/builtinimport.c):

// An import of a non-extensible built-in will always bypass the
// filesystem. e.g. `import micropython` or `import pyb`. So try and
// match a non-extensible built-ins first.
module_obj = mp_module_get_builtin(level_mod_name, false);
if (module_obj != MP_OBJ_NULL) {
    return module_obj;
}
// Next try the filesystem. Search for a directory or file relative to
// all the locations in sys.path.

So import spibus cannot reach a spibus.py on the device, no matter where it
sits on sys.path — the filesystem root included. MP_REGISTER_EXTENSIBLE_MODULE
reverses that: the filesystem is searched first and the builtin is the fallback.

Affects spibus, i2cbus, qspibus, rgbmatrix, and the notimpl stubs
(picodvi, dotclockframebuffer, mipidsi, i80bus).

Why it might be worth changing

• A user could override a C driver with a Python one to debug it, or ship a
  fixed version, without rebuilding firmware.
• pydevices ships a pure-Python spibus; today it is dead code on any
  firmware carrying displayif, because the builtin always wins.
• The notimpl stubs are the strongest case: their whole job is to raise on
  unsupported ports. A Python implementation on the filesystem cannot currently
  replace one.

Why it might not

• Extensible lookup stats the filesystem on every import of these names, so
  every board pays a small startup cost so that overriding is possible.
• A stray spibus.py in someone's project directory would silently shadow the C
  driver, and the failure would look like a driver bug. Today that cannot
  happen.
• It changes import semantics for every displayif module at once.

No recommendation. Raised while looking for a way to A/B the C spibus against
the Python one; that comparison does not need this — a renamed copy works — so
this is a question about what displayif wants its modules to be, not a
blocker. Brad, 2026-09-10.