Re: [PATCH] TTY: fix UV serial console regression

[Greg KH]

On Thu, Jan 12, 2012 at 10:55:15PM +0100, Jiri Slaby wrote:
> Commit 74c2107759d (serial: Use block_til_ready helper) and its fixup
> 3f582b8c110 (serial: fix termios settings in open) introduced a
> regression on UV systems. The serial eventually freezes while being
> used. It's completely unpredictable and sometimes needs a heap of
> traffic to happen first.
> 
> To reproduce this, yast installation was used as it turned out to be
> pretty reliable in reproducing. Especially during installation process
> where one doesn't have an SSH daemon running. And no monitor as the HW
> is completely headless. So this was fun to find. Given the machine
> doesn't boot on vanilla before 2.6.36 final. (And the commits above
> are older.)
> 
> Unless there is some bad race in the code, the hardware seems to be
> pretty broken. Otherwise pure MSR read should not cause such a bug,
> or?
> 
> So to prevent the bug, revert to the old behavior. I.e. read modem
> status only if we really have to -- for non-CLOCAL set serials.
> Non-CLOCAL works on this hardware OK, I tried. See? I don't.
> 
> And document that shit.

Thanks for tracking this down, I'll queue it up and get it to Linus
after 3.3-rc1 is out.

greg k-h
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