drm/apple: don't latch ->crashed on a poweroff swap timeout

[haripako]

iomfb_poweroff() waits 50 ms for the poweroff clear swap and, on timeout, sets
dcp->crashed and returns without logging anything.

That flag is permanent and fatal. dcp_crtc_atomic_check() opens with
if (dcp->crashed) return -EINVAL; and nothing ever clears it, so from that
moment every atomic commit for the device is rejected and the display does not
come back until reboot.

It is not a crash. After the timeout the DCP keeps working — it renegotiates the
link, publishes its mode list and asserts HPD (dcp_hotplug() connected:1 nr_modes:23). ->crashed is otherwise set only by dcp_rtk_crashed(), the
RTKit crash callback, which also logs. RTKit reported no crash in any of these
runs.

This keeps the control flow identical, leaves ->crashed alone and emits a
dev_warn so the timeout is visible at all.

How it was found

Observed on a MacBook Pro 13" M1 (j293) driving DisplayPort alt mode over USB-C.
Cold plug works; the first hot unplug kills the external output for the rest of
the boot. The compositor logs failed to commit: Invalid argument for all 23
modes the monitor offers, from 3440x1440 down to 640x480, and nothing in the
kernel log explains it — of the three -EINVAL exits in
dcp_crtc_atomic_check() only two log a dev_err, and the silent one is this.

Confirmed with kprobes on each of the three exits: 110 hits on the ->crashed
branch, all returning -22, with zero "DCP has crashed" messages. The timeout is
intermittent — it fired on 2 of 6 unplug cycles — which is why the failure looked
erratic.

With this applied, 6 unplug/replug cycles completed 8 modesets with no failures,
including the two cycles where the timeout did fire.

Full traces and the validation run are at
https://github.com/haripako/dp-altmode (validacion/).

Notes

• Tested on linux-asahi 7.1.6 (asahi-7.1.6-1); the patch applies unchanged to
  asahi as of today, where the code is still present.
• iomfb_template.c is included once per IOMFB version, so this covers both the
  v12_3 and v13_3 paths.
• Reaching DisplayPort alt mode on M1 at all needed an out-of-tree device tree
  and a backport of the fairydust cd321x HPD forwarding. This fix is
  independent of both — it is in the poweroff path and applies to any CRTC
  being disabled, including the internal panel, though that path was not
  reproduced here.

[HexCodeYT]

Tested this on a MacBook Air (M1, 2020), J313 / T8103, running 7.1.12-asahi-fairydust with this PR's commit 6b70d02bcb5758a625d8bcedbff340cf544a4496 built into appledrm.ko with matching vermagic.

This is DisplayPort Alt Mode over USB-C. Before applying the patch, a failed hot-unplug could leave the external display unusable until reboot.

With the patch applied, repeated unplug/replug cycles recover without reboot. In the same boot I observed:

• iomfb_poweroff_v13_3: timed out waiting for the poweroff clear swap; continuing: 3 times
• dcp_hotplug() connected:1 valid_mode:0 nr_modes:28: 6 times
• FIFO error interrupt: 5 times
• DCP has crashed: 0
• failed to commit / Invalid argument: 0

One captured sequence hit the poweroff clear-swap timeout at 13:59:08, then reconnected successfully with HPD asserted and 28 modes published at 13:59:20. Further hotplug cycles continued to recover.

So this reproduces the exact timeout path on J313 and confirms that not latching dcp->crashed allows subsequent DP modesets/recovery to continue.