Re: [PATCH] serial: uartps: Fix stuck ISR if RX disabled with non-empty FIFO

[Michal Simek <michal.simek@xilinx.com>]

+ Shubhrajyoti

On 06. 02. 19 15:08, Anssi Hannula wrote:
> If RX is disabled while there are still unprocessed bytes in RX FIFO,
> cdns_uart_handle_rx() called from interrupt handler will get stuck in
> the receive loop as read bytes will not get removed from the RX FIFO
> and CDNS_UART_SR_RXEMPTY bit will never get set.
> 
> Avoid the stuck handler by checking first if RX is disabled. port->lock
> protects against race with RX-disabling functions.
> 
> This HW behavior was mentioned by Nathan Rossi in 43e98facc4a3 ("tty:
> xuartps: Fix RX hang, and TX corruption in termios call") which fixed a
> similar issue in cdns_uart_set_termios().
> The behavior can also be easily verified by e.g. setting
> CDNS_UART_CR_RX_DIS at the beginning of cdns_uart_handle_rx() - the
> following loop will then get stuck.
> 
> Resetting the FIFO using RXRST would not set RXEMPTY either so simply
> issuing a reset after RX-disable would not work.
> 
> I observe this frequently on a ZynqMP board during heavy RX load at 1M
> baudrate when the reader process exits and thus RX gets disabled.
> 
> Fixes: 61ec9016988f ("tty/serial: add support for Xilinx PS UART")
> Signed-off-by: Anssi Hannula <anssi.hannula@bitwise.fi>
> Cc: stable@vger.kernel.org
> ---
>  drivers/tty/serial/xilinx_uartps.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/drivers/tty/serial/xilinx_uartps.c b/drivers/tty/serial/xilinx_uartps.c
> index 094f2958cb2b..f0c4f59d9314 100644
> --- a/drivers/tty/serial/xilinx_uartps.c
> +++ b/drivers/tty/serial/xilinx_uartps.c
> @@ -219,6 +219,13 @@ static void cdns_uart_handle_rx(void *dev_id, unsigned int isrstatus)
>  
>  	is_rxbs_support = cdns_uart->quirks & CDNS_UART_RXBS_SUPPORT;
>  
> +	/*
> +	 * RXEMPTY will never be set if RX is disabled as read bytes
> +	 * will not be removed from the FIFO
> +	 */
> +	if (readl(port->membase + CDNS_UART_CR) & CDNS_UART_CR_RX_DIS)
> +		return;
> +
>  	while ((readl(port->membase + CDNS_UART_SR) &
>  		CDNS_UART_SR_RXEMPTY) != CDNS_UART_SR_RXEMPTY) {
>  		if (is_rxbs_support)
> 


Please review.
M