[PATCH] perf pmu-events: Fix jevents.py type annotations for mypy 2.x

[Lalit Maganti <lalitmaganti@gmail.com>]

Since the build runs mypy whenever it is installed, the perf build fails
with mypy 2.4 on 7 errors in jevents.py, e.g.:

  pmu-events/jevents.py:542: error: Item "None" of "str | None" has no
  attribute "replace"  [union-attr]
  pmu-events/jevents.py:563: error: Item "None" of "BigCString | None"
  has no attribute "offsets"  [union-attr]

Both come from wrong annotations rather than from code that can see
None:
 - unit_to_pmu() is annotated as returning Optional[str] but always
   returns a string, so JsonEvent.pmu is typed as optional where it is
   used as a str.
 - _bcs is set to None before the BigCString class is defined and only
   assigned an instance after it, so mypy types it as BigCString | None.

Correct the return type and declare _bcs with its type instead of the
None placeholder. The generated pmu-events.c and pmu-events-string.c
are unchanged, and mypy 1.9 still passes.

Fixes: b3e9c396d0ac ("perf build: Make mypy build test opt-out (NO_MYPY=1)")
Assisted-by: LLM
Signed-off-by: Lalit Maganti <lalitmaganti@gmail.com>
---
 tools/perf/pmu-events/jevents.py | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/tools/perf/pmu-events/jevents.py b/tools/perf/pmu-events/jevents.py
index cc143f7e7cdf..080e04b51b8c 100755
--- a/tools/perf/pmu-events/jevents.py
+++ b/tools/perf/pmu-events/jevents.py
@@ -35,7 +35,7 @@ _pending_metrics: list["JsonEvent"] = []
 # Name of metrics table to be written out
 _pending_metrics_tblname: Optional[str] = None
 # Global BigCString shared by all structures.
-_bcs = None
+_bcs: "BigCString"
 # Map from the name of a metric group to a description of the group.
 _metricgroups: Dict[str, str] = {}
 # Order specific JsonEvent attributes will be visited.
@@ -262,7 +262,7 @@ class JsonEvent:
         return fixed[name.lower()]
       return event
 
-    def unit_to_pmu(unit: Optional[str]) -> Optional[str]:
+    def unit_to_pmu(unit: Optional[str]) -> str:
       """Convert a JSON Unit to Linux PMU name."""
       if not unit:
         return 'default_core'

base-commit: 1c90f0a3c8a1c4e38f9edd97af9e20848805a7fb
-- 
2.56.0