Re: [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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