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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. Signed-off-by: Roger Pau Monn=C3=A9 --- There's a possible alternative approach to solve this by moving the actual interrupt processing to a softirq tasklet and disabling the interrupt source until the processing is done, likely unifying the logic with the timer task. However that's a bigger change, and too risky for 4.22 at this point. --- xen/drivers/char/ns16550.c | 33 ++++++++++++++++++++++++++++++++- 1 file changed, 32 insertions(+), 1 deletion(-) diff --git a/xen/drivers/char/ns16550.c b/xen/drivers/char/ns16550.c index 878da27f2ef8..008f673f52ee 100644 --- a/xen/drivers/char/ns16550.c +++ b/xen/drivers/char/ns16550.c @@ -62,6 +62,7 @@ static struct ns16550 { #endif unsigned int timeout_ms; bool intr_works; + bool force_polling; bool dw_usr_bsy; #ifdef NS16550_PCI /* PCI card parameters. */ @@ -190,12 +191,41 @@ static void cf_check ns16550_interrupt(int irq, void = *dev_id) { struct serial_port *port =3D dev_id; struct ns16550 *uart =3D port->uart; + /* Set quite arbitrarily as 4x the time to drain the TX or fill RX FIF= Os. */ + const s_time_t timeout =3D NOW() + min(MILLISECS(uart->timeout_ms * 4), + MILLISECS(5)); + + if ( uart->force_polling ) + return; =20 uart->intr_works =3D 1; =20 while ( !(ns_read_reg(uart, UART_IIR) & UART_IIR_NOINT) ) { u8 lsr =3D ns_read_reg(uart, UART_LSR); + s_time_t now =3D NOW(); + + /* Break out of the loop if spending too much time. */ + if ( now > timeout ) + { + struct irq_desc *desc =3D irq_to_desc(irq); + + /* Disable the interrupt source - it's never shared. */ + spin_lock_irq(&desc->lock); + desc->status |=3D IRQ_DISABLED; + if ( desc->handler->disable ) + desc->handler->disable(desc); + spin_unlock_irq(&desc->lock); + + /* Disable interrupt generation on the device and arm the time= r. */ + uart->force_polling =3D true; + ns_write_reg(uart, UART_IER, 0); + set_timer(&uart->timer, now + MILLISECS(uart->timeout_ms)); + printk(XENLOG_WARNING + "uart interrupt taking too long, switched to polling\n"= ); + + return; + } =20 if ( (lsr & uart->lsr_mask) =3D=3D uart->lsr_mask ) serial_tx_interrupt(port); @@ -223,7 +253,7 @@ static void cf_check __ns16550_poll(const struct cpu_us= er_regs *regs) struct ns16550 *uart =3D port->uart; const struct cpu_user_regs *old_regs; =20 - if ( uart->intr_works ) + if ( uart->intr_works && !uart->force_polling ) return; /* Interrupts work - no more polling */ =20 /* Mimic interrupt context. */ @@ -313,6 +343,7 @@ static void ns16550_setup_preirq(struct ns16550 *uart) unsigned int divisor; =20 uart->intr_works =3D 0; + uart->force_polling =3D false; =20 pci_serial_early_init(uart); =20 --=20 2.53.0