[Bug 194979] uart_intr leads to "bad stray interrupt" under pressure

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=194979

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A commit references this bug:

Author: marius
Date: Fri Jul 24 17:01:18 UTC 2015
New revision: 285843
URL: https://svnweb.freebsd.org/changeset/base/285843

Log:
  - Since r253161, uart_intr() abuses FILTER_SCHEDULE_THREAD for signaling
    uart_bus_attach() during its test that 20 iterations weren't sufficient
    for clearing all pending interrupts, assuming this means that hardware
    is broken and doesn't deassert interrupts. However, under pressure, 20
    iterations also can be insufficient for clearing all pending interrupts,
    leading to a panic as intr_event_handle() tries to schedule an interrupt
    handler not registered. Solve this by introducing a flag that is set in
    test mode and otherwise restores pre-r253161 behavior of uart_intr(). The
    approach of additionally registering uart_intr() as handler as suggested
    in PR 194979 is not taken as that in turn would abuse special pccard and
    pccbb handling code of intr_event_handle(). [1]
  - Const'ify uart_driver_name.
  - Fix some minor style bugs.

  PR:        194979 [1]
  Reviewed by:    marcel (earlier version)
  MFC after:    3 days

Changes:
  head/sys/dev/uart/uart_bus.h
  head/sys/dev/uart/uart_core.c

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