docs(plex): the transcode comments say the CPU-decode route

backend/app/plex/models.py

@@ -227,16 +227,17 @@ class PlexItem(Base, TimestampMixin, UpdatedAtMixin):
     playable: Mapped[str | None] = mapped_column(String(12), index=True)
     probed_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True))
     # Transcode HW-capability facet (the playback-class filter). NULL for non-transcode items; for a
-    # `playable="transcode"` item: True = the prod iGPU can hardware-transcode it (codec is HW-decodable
-    # AND the source is 8-bit SDR), False = software libx264 only (AV1/XviD, or a 10-bit/HDR source that
-    # breaks the VAAPI nv12 pipeline — see plex/stream.py). Derived by `plex.sync.transcode_hw_capable`:
-    # set codec-only at sync (bit depth unknown from the cheap listing), then refined by the bit-depth
-    # backfill (`_probe_media_batch`) which downgrades a 10-bit/HDR HEVC to False. The one filterable
-    # home of the invariant `_want_hw`'s source gate computes at play time — kept in sync via the shared
-    # classifier so the catalog facet and the runtime encoder choice never disagree by codec.
+    # `playable="transcode"` item: True = the prod iGPU ENCODES it (a HW-decodable 8-bit SDR source, a
+    # true-HDR one through the OpenCL tonemap, or an 8-bit source in a codec the CPU decodes for it —
+    # VC-1, XviD, DivX), False = software libx264 (AV1, or a 10-bit SDR source — see plex/stream.py).
+    # Derived by `plex.sync.transcode_hw_capable`: set codec-only at sync (bit depth unknown from the
+    # cheap listing), then refined by the bit-depth backfill (`_probe_media_batch`), which downgrades a
+    # 10-bit SDR HEVC to False. The filterable twin of the rule `_select_encode` applies at play time;
+    # the two share their codec sets, not their code, so a change to one tree is a change to both.
     transcode_hw: Mapped[bool | None] = mapped_column(Boolean, index=True)
     # Video bit depth (8/10/12…) from the Plex media Stream, filled by the bit-depth backfill. NULL =
-    # not yet probed. Surfaced on the info page (e.g. "HEVC · 10-bit") and feeds `transcode_hw`.
+    # not yet probed. Surfaced on the info page (e.g. "HEVC · 10-bit") and feeds `transcode_hw` (a
+    # 10-bit SDR source drops to software; an HDR one keeps the GPU through the tonemap).
     video_bit_depth: Mapped[int | None] = mapped_column(Integer)
     # Watermark for the bit-depth backfill: when this item's media Stream was last probed for
     # bit-depth/HDR. NULL = never (the backfill's work-list). Only HW-decodable transcode items are

backend/app/plex/stream.py

@@ -4,9 +4,10 @@ Playback is from the LOCAL physical file. Browser-compatible files (playable="di
 raw with HTTP range requests. Everything else that is h264 video (playable="remux") is remuxed on
 the fly to HLS with **video stream-copy** (cheap, I/O-bound — no video re-encode) and audio to AAC
 when needed. Non-h264 video (playable="transcode": HEVC/VP9/AV1/MPEG-2/XviD…) needs a full re-encode
-to H.264: on the prod host that runs on the Intel iGPU via VAAPI (`h264_vaapi`, near-zero CPU), with
-software libx264 as the automatic fallback where the render node is absent or the source codec isn't
-hardware-decodable. Same HLS session/seek machinery either way — only the video encoder differs.
+to H.264: on the prod host that is encoded on the Intel iGPU via VAAPI (`h264_vaapi`) — decoded there
+too when it can, else decoded on the CPU and uploaded (VC-1, XviD, DivX) — with software libx264 as
+the fallback where the render node is absent, for a source neither GPU route takes (AV1, 10-bit SDR),
+and when a GPU session produces nothing. Same HLS session/seek machinery either way.
 
 Seek model (Jellyfin-style): one ffmpeg session per item, started at a given offset via `-ss`
 (fast keyframe seek). A seek beyond the generated region restarts the session at the new offset,
@@ -501,16 +502,17 @@ def _want_hw(db: DbSession, item: PlexItem, transcode: bool, src: Path) -> bool:
     return _select_encode(db, item, transcode, src)[0]
 
 
-# CPU-nice increment for the SOFTWARE transcode child. The Plex ffmpeg is a Popen in the long-lived API
-# process (which must NOT itself be niced), so — unlike the download worker's _deprioritize (run once in
-# its own main()) — we lower priority per-child via preexec_fn in the fork. The GPU path needs none of
-# this. Reuses the download child's nice default so all co-located re-encoders share one handicap.
+# CPU-nice increment for a transcode child that works the CPU: a software encode, or a CPU decode
+# feeding the GPU encoder. The Plex ffmpeg is a Popen in the long-lived API process (which must NOT
+# itself be niced), so — unlike the download worker's _deprioritize (run once in its own main()) — we
+# lower priority per-child via preexec_fn in the fork. The full-GPU path needs none of this. Reuses
+# the download child's nice default so all co-located re-encoders share one handicap.
 _SW_TRANSCODE_NICE = settings.download_child_nice
 
 
 def _sw_child_preexec() -> None:
-    """Runs in the forked child (POSIX) before exec: renice this software-transcode ffmpeg down so it
-    can't starve the co-located API / Postgres / playback. Best-effort — a failed renice must not fail
+    """Runs in the forked child (POSIX) before exec: renice this CPU-bound transcode ffmpeg down so
+    it can't starve the co-located API / Postgres / playback. Best-effort — a failed renice must not fail
     playback."""
     try:
         os.nice(_SW_TRANSCODE_NICE)