Re: [Xen-devel] [PATCH 2/2] xen/ns16550: Fix ISR lockup on Designware 8250 (H5)

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 19.09.17 at 15:38, <awais.masood@xxxxxxxxxx> wrote:
> --- a/xen/drivers/char/ns16550.c
> +++ b/xen/drivers/char/ns16550.c
> @@ -521,6 +521,18 @@ static void ns16550_interrupt(
>              serial_tx_interrupt(port, regs);
>          if ( lsr & UART_LSR_DR )
>              serial_rx_interrupt(port, regs);
> +        if ( uart->dw_usr_bsy &&
> +             (ns_read_reg(uart, UART_IIR) & UART_IIR_BSY) == UART_IIR_BSY )
> +        {
> +            /* If DesignWare 8250 UART became busy again when LCR was written
> +             * earlier, it can raise a "busy detect" again.
> +             * Read the UART Status Register to clear this state or we'll 
> end up
> +             * in an infinte loop because UART_IIR_NOINT is not true.
> +             * Placing this check in setup_preirq after LCR write does not 
> work
> +             * probably due to a delayed interrupt.
> +             */
> +            ns_read_reg(uart, UART_USR);
> +        }

This same code already exists in ns16550_setup_preirq() - please
introduce a helper function. It would also help if you referred to
the commit introducing that other instance of the code, explaining
why what was done there was not enough (because pretty clearly
the author must have assumed that change to be sufficient).

Jan


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