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

[John Baldwin]

On Wednesday, July 10, 2013 3:59:40 pm Marcel Moolenaar wrote:
> 
> On Jul 10, 2013, at 11:09 AM, John Baldwin <jhb at 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
> >>> 
> >>> 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
> all.

Ok.  I think what is happening is that you are just spinning in the interrupt
handler forever due to the hardware being stuck in that case in the old code.
I assume you tried just using the count first but it still hung?  (Perhaps the
interrupt was for a PCI device and level-triggered and so it kept reasserting
anyway?)

I think your change is correct for a uart that is stuck in this way regardless,
I just think the hang isn't related to weirdness with x86 temporarily
re-enabling interrupts.

-- 
John Baldwin