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Subject: Re: svn commit: r253161 - head/sys/dev/uart
From: Marcel Moolenaar <marcel@xcllnt.net>
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Date: Wed, 10 Jul 2013 12:59:40 -0700
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References: <201307101742.r6AHgKOm024113@svn.freebsd.org>
 <201307101405.44158.jhb@freebsd.org> <201307101409.42228.jhb@freebsd.org>
To: John Baldwin <jhb@FreeBSD.org>
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 Marcel Moolenaar <marcel@freebsd.org>, src-committers@freebsd.org
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On Jul 10, 2013, at 11:09 AM, John Baldwin <jhb@FreeBSD.org> 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: http://svnweb.freebsd.org/changeset/base/253161
>>>=20
>>> 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.
>>=20
>> 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
all.

--=20
Marcel Moolenaar
marcel@xcllnt.net