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SubjectRe: [PATCH] 8250 UART backup timer
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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