Fix intermittent suspend hang on T2 MacBooks (force synchronous device suspend)

[andyhoops]

Refs: #1840

Problem

On T2 MacBooks, systemctl suspend / lid-close intermittently hard-hangs the
machine the instant it enters suspend — black screen, dead keyboard/trackpad/
Touch Bar, no wake; only a forced power-off recovers. This is the core of the
widely-reported #1840 across many T2 models (13"/15"/16", 2018–2020).

Root cause

An asynchronous device-suspend race. The kernel default
(/sys/power/pm_async=1) suspends devices in parallel; on T2 hardware this
races and deadlocks. pm_trace localises the hang to the async suspend core
(drivers/base/power/main.c), not to any single driver — which is why
unloading individual drivers (brcmfmac, apple-bce) and PCI D3cold tweaks don't
help. Forcing synchronous device suspend (pm_async=0) eliminates the race.

The existing install/config/hardware/apple/fix-suspend-nvme.sh does not cover
these machines: it excludes 16,x/15,x models and targets a discrete NVMe at PCI
0000:01:00.0, whereas 16,2-class storage is the T2-bridged ANS2 controller at
e6:00.0. This fix is independent of that one.

Fix

Add install/config/hardware/apple/fix-suspend-async.sh: on T2 Macs
(lspci 106b:180[12]), install + enable a oneshot service that writes
0 to /sys/power/pm_async at boot. Mirrors the structure of the existing
fix-suspend-nvme.sh. No change to mem_sleep / logind.conf / sleep.conf /
bootloader cmdline is required.

Testing

• MacBookPro16,2, Omarchy 3.8.2, linux-t2 7.0.12: 5/5 clean suspend/resume
  cycles with the fix vs 5/5 hard hangs without (same deep mode). Lid
  close/reopen confirmed. Persists across reboot.
• Low risk: serial device suspend is marginally slower (sub-second) and is a
  no-op on hardware that wasn't racing.

Changes

1. install/config/hardware/apple/fix-suspend-async.sh (new)
2. install/config/all.sh — add one line after the fix-suspend-nvme.sh entry:
   run_logged $OMARCHY_INSTALL/config/hardware/apple/fix-suspend-async.sh
3. migrations/<unix-timestamp>.sh (new) — sources the script so existing
   installs get it on omarchy update.

Note for reviewers

Gated to T2 Macs (where it's proven). pm_async=0 is harmless on non-racing
hardware, so the gate could be widened to all Apple MacBooks (#1840 includes a
couple of non-T2 reports) if preferred. Co-testers from #1840 on 13"/15" models
would help confirm breadth.

[Copilot]

Pull request overview

This PR addresses intermittent hard hangs during suspend on T2 MacBooks by forcing synchronous device suspend (setting /sys/power/pm_async=0) via a boot-time systemd oneshot service, and ensures existing installations receive the fix via a migration.

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Changes:

• Add a T2-gated hardware script that installs/enables a systemd oneshot service to set pm_async=0 at boot.
• Wire the new script into the standard install/config stage.
• Add a migration to apply the same fix to existing installs on update.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
migrations/1782742549.sh	Runs the new Apple T2 suspend fix for existing installs via migration.
install/config/hardware/apple/fix-suspend-async.sh	New T2-detection script that installs/enables a oneshot systemd unit to set pm_async=0.
install/config/all.sh	Invokes the new Apple suspend async fix during install/config.

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[andyhoops]

Done in 0fe6d3f — migration now leads with the echo like the others

[andyhoops]

Added ConditionPathExists=/sys/power/pm_async in 0fe6d3f, thanks.