[maxdavidson]

What and why

On a Mac over HDMI to a 5.1 TV, receiver or soundbar (or an iPhone on a multichannel route), a game that writes six channels plays in stereo. GetActiveAudioDeviceChannelCountForOutput reported two channels on every route, and every six-channel output was folded to stereo before Core Audio.

The change:

• Channel count: the host reports the route's count. It is six when the Mac's output device has a stream format of six or more channels, or when the iOS session's output does. Game threads read it from a cache (dm_audio_route) because a Core Audio read can need more stack than a game thread has.
• Six-channel outputs: on a six-channel route, a six-channel AudioOut, and the in-process renderer's DeviceSink, go to the AudioUnit as they are, with the input layout MPEG_5_1_A. Core Audio then maps them onto the device's own layout. That matters on HDMI, whose order puts the LFE before the center.
• Stereo routes: the host downmixes a six-channel output with the renderer's default coefficients. Outputs of other channel counts go as the guest writes them.
• Route changes: if the route changes while an output plays (HDMI to the speakers or back), the outputs whose channel count no longer fits get a new unit. This goes through the media services reset path. On iOS the session asks for the new route's channels first.
• Test aids: DM_AUDIO_DEVICE (Mac, diagnostic) pins the output to a device by name. t_audio_live and dm/tools/audio_channel_check.py check the channels on a loopback device.

This overlaps #1381, the opt-in Spatial Audio through macOS's renderer. Both PRs hand six channels on in hle_audio.c and hle_audren.c. This PR is the default path for six-channel devices and iOS, and #1381 is the opt-in system spatializer for headphones. Whichever lands second adapts to the other.

Reference entry: Stereo only on a 5.1 system.

Checked

• Tests:
  • make -C dm/host/tests GAME=010015100B514000 run passes. That includes t_audio_device, which uses mock units and covers:
    • six channels with the layout set;
    • the downmix values and their clamp;
    • a route change in both directions;
    • a failed new unit, made again at the next change;
    • stereo and mono outputs left as they are.
  • t_audio_device passed 20 runs in a row.
  • It also includes t_audren_layout: six channels in sink order, and a mono sink on the front pair.
  • make -C dm/gfx/tests GAME=010015100B514000 run passes.
  • dm/tools/tests: test_docs, test_env_switches, test_pr_areas and test_audio_channel_check pass, and so does env_switches.py --check.
  • Lint: lint_python reports 0 new. lint_c.py tidy (LLVM 23.1.4) reports nothing in the files this change touches; its seven findings above the baseline are in five dm/gfx/tests files that are the same on main.
  • hle_audio.c, hle_audren.c and front_ios.m compile for iOS 26.
  • On loopback devices (BlackHole 16ch and 2ch, audio_channel_check.py, test tones captured in-process): each tone on its own channel, the others more than 100 dB down. The downmix came within 0.05 dB of the host's coefficients. With BlackHole 16ch set to HDMI's layout, the center and the LFE land on HDMI's channels 4 and 3.
  • Switching while playing: t_audio_live moved the output between the two devices mid-run. Each device carried the 5.1 signal or the downmix right after the switch.
• Games run: none yet. The iPhone route-change path has not run on a device either. Spatial Audio over AirPods is not part of this change.
• Devices and OS: MacBook Pro M4 Pro, macOS 27.2. By ear over HDMI to an LG TV, passed over eARC to a Sonos system: t_audio_live played a spoken channel name on each channel, and each came from the right speaker.
• regress.py: not run yet; the report will be added here. regress.py impact names Hades, Breath of the Wild, Pokémon Scarlet and Tears of the Kingdom by their usage manifests.

Docs

• The finding is in its docs/reference/audio.md entry, and python3 dm/tools/docs_symptoms.py was run (the symptom index)
• No game files, keys or data extracted from a game in the change or the attachments

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