Re: [PATCH] 8250 UART backup timer

[H. Peter Anvin]

Alex Williamson wrote:
>    The patch below works around a minor bug found in the UART of the
> remote management card used in many HP ia64 and parisc servers (aka the
> Diva UARTs).  The problem is that the UART does not reassert the THRE
> interrupt if it has been previously cleared and the IIR THRI bit is
> re-enabled.  This can produce a very annoying failure mode when used as
> a serial console, allowing a boot/reboot to hang indefinitely until an
> RX interrupt kicks it into working again (ie. an unattended reboot could
> stall).
>
>    To solve this problem, a backup timer is introduced that runs
> alongside the standard interrupt driven mechanism.  This timer wakes up
> periodically, checks for a hang condition and gets characters moving
> again.  This backup mechanism is only enabled if the UART is detected as
> having this problem, so systems without these UARTs will have no
> additional overhead.
>
>    This version of the patch incorporates previous comments from Pavel
> and removes races in the bug detection code.  The test is now done
> before the irq linking to prevent races with interrupt handler clearing
> the THRE interrupt.  Short delays and syncs are also added to ensure the
> device is able to update register state before the result is tested.
> Comments?  Thanks,

I have seen this same bug in soft UART IP from "a major vendor."

	-hpa
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