svn commit: r253161 - head/sys/dev/uart

Marcel Moolenaar marcel at
Wed Jul 10 20:28:37 UTC 2013

On Jul 10, 2013, at 11:09 AM, John Baldwin <jhb at> wrote:

> On Wednesday, July 10, 2013 2:05:43 pm John Baldwin wrote:
>> On Wednesday, July 10, 2013 1:42:20 pm Marcel Moolenaar wrote:
>>> Author: marcel
>>> Date: Wed Jul 10 17:42:20 2013
>>> New Revision: 253161
>>> URL:
>>> Log:
>>>  Protect against broken hardware. In this particular case, protect against
>>>  H/W not de-asserting the interrupt at all. On x86, and because of the
>>>  following conditions, this results in a hard hang with interrupts disabled:
>>>  1.  The uart(4) driver uses a spin lock to protect against concurrent
>>>      access to the H/W. Spin locks disable and restore interrupts.
>>>  2.  Restoring the interrupt on x86 always writes the flags register. Even
>>>      if we're restoring the interrupt from disabled to disabled.
>>>  3.  The x86 CPU has a short window in which interrupts are enabled when the
>>>      flags register is written.
>> Do you have proof of this?

No. I only have proof of a hard hang during auto configuration that
cannot be fixed in any other way than not to setup the interrupt at

Marcel Moolenaar
marcel at

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