On Wed, 28 Jan 2026, Andy Shevchenko wrote: > On Wed, Jan 28, 2026 at 12:53:00PM +0200, Ilpo Järvinen wrote: > > When DW UART is !uart_16550_compatible, it can indicate BUSY at any > > point (when under constant Rx pressure) unless a complex sequence of > > steps is performed. Any LCR write can run a foul with the condition > > that prevents writing LCR while the UART is BUSY, which triggers > > BUSY_DETECT interrupt that seems unmaskable using IER bits. > > > > Normal flow is that dw8250_handle_irq() handles BUSY_DETECT condition > > by reading USR register. This BUSY feature, however, breaks the > > assumptions made in serial8250_do_shutdown(), which runs > > synchronize_irq() after clearing IER and assumes no interrupts can > > occur after that point but then proceeds to update LCR, which on DW > > UART can trigger an interrupt. > > > > If serial8250_do_shutdown() releases the interrupt handler before the > > handler has run and processed the BUSY_DETECT condition by read the USR > > register, the IRQ is not deasserted resulting in interrupt storm that > > triggers "irq x: nobody cared" warning leading to disabling the IRQ. > > > > Add late synchronize_irq() into serial8250_do_shutdown() to ensure > > BUSY_DETECT from DW UART is handled before port's interrupt handler is > > released. Alternative would be to add DW UART specific shutdown > > function but it would mostly duplicate the generic code and the extra > > synchronize_irq() seems pretty harmless in serial8250_do_shutdown(). > > Dunno if the triggered interrupt may lead to a new DMA transfers (since > this is generic 8520 code...) in some cases. If BUSY_DETECT interrupt fires (like I'd expect), the handler won't start anything but exits early from dw8250_handle_irq(). > Anyway I've just sent a patch > that is Cc'ed to you to prevent that from happening. Not sure if it needs > to be incorporated into your series or should have a Fixes tag. > > Reviewed-by: Andy Shevchenko -- i.