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At least on the Lenovo ThinkSystem SR630 V4 the flow > >> control of the serial-over-lan emulated UART seems to be broken, as it > >> doesn't set the NOINT bit consistently. The Transmitter Holding Register > >> Empty in LSR also seems to not be properly signaled, as even with it set > >> writes to the transmit register take ~6ms. This leads to the watchdog > >> triggering very easily on such system. > >> > >> Introduce an upper bound on the execution time of ns16550_interrupt(), this > >> is currently set as 4x the polling interval, which is calculated as the > >> time to fill RX FIFO and/or empty TX FIFO. The current maximum is 5ms. > >> Once the timeout triggers the interrupt is disabled and the uart is > >> switched to polling mode. > > > > Don't you mmiss Fixes: tag? > > Fixes: "SoL on Lenovo ThinkSystem SR630 V4" > > you mean? I think there's nothing wrong with our pre-existing code, and > the changes here instead are a workaround for some (apparently) badly > implemented SoL. It was on purpose that no Fixes tag was provided. Xen code would be fine with well-behaved uarts, however most of the serial-over-lan emulated ones are not well behaved it seems. There's a possible issue with the unbounded loop in ns16550_interrupt() as it's relying solely on hardware register values to terminate, which again would be OK if hardware was correctly implemented. I don't think this warrants a Fixes tag. Thanks, Roger.