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kern/57598: Writing to uart4 on Beaglebone Black hangs the kernel

>Number:         57598
>Category:       kern
>Synopsis:       Writing to uart4 on Beaglebone Black hangs the kernel
>Confidential:   no
>Severity:       critical
>Priority:       high
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sat Sep 02 22:15:01 +0000 2023
>Originator:     Brook Milligan
>Release:        NetBSD 10.99.7
NetBSD armv7 10.99.7 NetBSD 10.99.7 (GENERIC) #0: Wed Aug 30 15:02:40 MDT 2023  brook@...:/.../NetBSD/evbarm-earmv7hf-beaglebone/obj/sys/arch/evbarm/compile/GENERIC evbarm
The Beaglebone Black has 6 uarts, one of which (uart0) is used for the serial console.  Another, uart4, is used by the comms cape, but is not enabled by the default device tree.  Enabling it with a device tree overlay causes the kernel to configure it as com1.  However, any write to the device will immediately hang the kernel; it does not respond to keystrokes on the console, will not enter ddb, and will not respond to pings.
(i) Boot a Beaglebone Black with a device tree that enables uart4.  One is available at

(ii) Write a character to com1, e.g., with "cu -l /dev/dty01".

No fix is known.

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