[oliverames]

Not fully tested; opt-in. The runs so far show no effect; it is filed for review, and the maintainer may prefer to wait for a measurement.

What and why

Mario Kart World 1.8.0 (#1383's build) on a physical iPhone flips its present interval between 1 and 2 about 350 times in 20 s after late frames (96 flips in one 600-frame window, 258 in the next); cheemu21/plumber-racer-2 notes the game sets interval 2 after a late frame. Each flip re-declares 16.7 or 33.3 ms to the scheduler (set_time_constraint). In one run the process left the performance cores in that window (mean clock 3.5 to 1.6 GHz, 56% to 8% on performance cores, thermal fair); the frame-step comment above rt_period_ns describes the same demotion for a longer hold.

DM_RT60=1 keeps the declared period at the base vsync period (1 / g_hz) whatever the game's interval does. Off by default: with the variable unset the code runs as before.

Checked

• Tests: hle_nvn.c compiles on this base with a generated profile header (clang syntax-only); dm/tools/tests/test_docs.py passes; the new switch is in docs/env_switches.tsv with its meaning (env_switches.py --check still reports DM_NVN_BC_ALIAS, which is missing from the table on main too).
• Games run, with title ID and version: Mario Kart World (0400C3F00006E000) 1.8.0, built from WIP: Mario Kart World 1.8.0 on iOS - rendering, memory and presentation #1383's tree, on the phone: three HDR-off runs of 1,800 to 15,000 frames with and without it.
• Devices and OS: iPhone 19,4 (2 Performance + 4 Efficiency cores), iOS 27.3; Apple-silicon Mac, macOS 27.3
• scripts/regress.py check --games: not run: off by default, so every game runs as before without the variable.
• Measurements before and after: no measurable difference so far: with and without it the CPU stayed healthy in the HDR-off runs and the GPU-time plateau seen in all of them was the same.
• Not checked: whether it prevents the CPU demotion (it did not in an HDR-on run where the demotion appeared with it set). No unit test: the declaration is a scheduler hint with no observable output on the Mac. No Mac run.

Front ends

None: no front end has to act on this.

Docs

• The mechanism is in docs/development.md and the switch in docs/env_switches.tsv
• A chain tool changed: dmsh_chain.py --header was run and dm/gfx/nvn/dmsh_chain.h is part of the change. Not applicable: no chain tool changed.
• No game files, keys or data extracted from a game in the change or the attachments

[oliverames]

Combined into #1713 with the other two pacing switches; same commit, same tests. Closing this one.