[funkzy]

Summary

The DockChannel console drains only while a debug USB host reads the channel. On a Mac Studio M5 Ultra (J775D)
that boots unattended with nothing attached, the TX FIFO stays full. Each console character then spun up to 10000
reads of TX_FREE, every close waited for output that could never drain, and boot stalled. With the new property
apple,tx-never-wait, output that doesn't fit is dropped instead.

What it changes

• With the property, console writes (the driver's and the early console's) send what TX_FREE says fits and drop
  the rest. tty output is sent from the drain timer and dropped after two ticks without room. A close does not
  wait, and tx_empty reports empty.
• Without the property, every other DockChannel Mac behaves as before.

Testing (J775D)

• With the property set and nothing reading, J775D boots unattended to its login (every boot since 2026-10-30).
  With a reader attached, the console carries the whole boot log.
• On this base: builds with no warnings.
• Booted on these bases (2026-11-02): one tree of t6050/base with nvme: apple: Add resident T6050 ANS over APIF #473 and PCI: apple: Add T6050 APIF hosts and native AIC MSI #474 plus the DockChannel, PCIe and
  NVMe PRs, and the J775D board DT with apple,tx-never-wait. Cold boot and warm reboot in both directions: from
  another kernel into this one, ALARM came up on its NVMe root; out of it, a systemctl reboot took 38 s with a 2 s
  stop phase and no stop timeouts. With nothing reading the console, 20 lines to /dev/console took 3 ms. 64 KiB to /dev/ttyDC0 took 140 ms and 20 KERN_ERR lines 5 ms.
  Without the property, the same tree spent about a second per console line once the channel filled, and a reboot
  out of it hung until the watchdog reset the machine.
• checkpatch: clean.

Not tested

• Other DockChannel Macs with the property set (none need it).

Dependencies

None beyond t6050/base. A board whose console is on DockChannel and may boot with nothing reading it should set
the property; the J775D board DT will.

Assisted-by: LLM

[funkzy]

Closing: this change now lives in the funkzy/j775d branch of aurora-silicon/linux-aurora (the J775D series as built and booted, 681521644671), which dj merges directly; no PR flow for it any more.