Re: [PATCH v3] serial: 8250_dw: Prefer SRBR in bogus RX timeout workaround if available

[Kumar, Udit]

Hi Yicong,

On 9/8/2026 7:45 PM, Yicong Yang wrote:
> The DW uart could get into the cases where a bogus RX timeout
> interrupt is asserted but no available data. This could be
> workaround by doing a bogus read.
> 
> Currently the driver's using the standard RBR (receive buffer
> register) for this bogus read. However the reading of RBR
> in this case is allowed to raise a hardware error if vendor
> choose to implement in this way (our platform). It's also
> allowed to do the bogus read using SRBR (shadow RBR) for
> workaround which won't raise the hardware error. So change
> to use the SRBR to workaround the issue if it's available.
> 

Thanks for the patch. If I understand correctly, the IP you are using
does not allow reading an empty FIFO, so you decided to read the SRBR
instead.

Does this same case applies when you open a port without any data in the
FIFO:
https://elixir.bootlin.com/linux/v7.3-rc3/source/drivers/tty/serial/8250/8250_port.c#L710
?

If yes then I am wondering, how the condition mentioned above is bypassed

Thanks
Udit


> Signed-off-by: Yicong Yang <yang.yicong@xxxxxxxxxxxxx>
> ---
> Change since v1:
> - refine the comments and type per Andy
> Link: https://lore.kernel.org/linux-serial/20260907074035.77796-1-yang.yicong@xxxxxxxxxxxxx/
> 
> Change since v1:
> - simply the workaround path per Ilpo and Andy
> - run ~48k reboot tests and didn't reproduce the issue
> Link: https://lore.kernel.org/linux-serial/20260629075510.32854-1-yang.yicong@xxxxxxxxxxxxx/
> 
>  drivers/tty/serial/8250/8250_dw.c    | 9 ++++++++-
>  drivers/tty/serial/8250/8250_dwlib.c | 5 +++++
>  drivers/tty/serial/8250/8250_dwlib.h | 5 +++++
>  3 files changed, 18 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/tty/serial/8250/8250_dw.c b/drivers/tty/serial/8250/8250_dw.c
> index 51d026f20825..b8a0bca8b536 100644
> --- a/drivers/tty/serial/8250/8250_dw.c
> +++ b/drivers/tty/serial/8250/8250_dw.c
> @@ -439,8 +439,15 @@ static int dw8250_handle_irq(struct uart_port *p)
>  	if (!up->dma && rx_timeout) {
>  		status = serial_lsr_in(up);
>  
> +		/*
> +		 * Do the bogus read from Shadow RBR (SRBR) if provided.
> +		 * Both RBR and SRBR are supported ways to workaround
> +		 * this problem. But read RBR can cause hardware error
> +		 * (PSLVERR) if hardware choose to implement in this way
> +		 * (implement REG_TIMEOUT_WIDTH to 0), whereas SRBR won't.
> +		 */
>  		if (!(status & (UART_LSR_DR | UART_LSR_BI)))
> -			serial_port_in(p, UART_RX);
> +			serial_port_in(p, d->data.srbr);
>  	}
>  
>  	/* Manually stop the Rx DMA transfer when acting as flow controller */
> diff --git a/drivers/tty/serial/8250/8250_dwlib.c b/drivers/tty/serial/8250/8250_dwlib.c
> index 8859e66d2d71..9bb02a4ab11f 100644
> --- a/drivers/tty/serial/8250/8250_dwlib.c
> +++ b/drivers/tty/serial/8250/8250_dwlib.c
> @@ -247,5 +247,10 @@ void dw8250_setup_port(struct uart_port *p)
>  
>  	if (reg & DW_UART_CPR_SIR_MODE)
>  		up->capabilities |= UART_CAP_IRDA;
> +
> +	if (reg & DW_UART_CPR_SHADOW)
> +		pd->srbr = DW_UART_SRBR_0;
> +	else
> +		pd->srbr = UART_RX;
>  }
>  EXPORT_SYMBOL_GPL(dw8250_setup_port);
> diff --git a/drivers/tty/serial/8250/8250_dwlib.h b/drivers/tty/serial/8250/8250_dwlib.h
> index 1fe52332e774..ee7a07fac0f6 100644
> --- a/drivers/tty/serial/8250/8250_dwlib.h
> +++ b/drivers/tty/serial/8250/8250_dwlib.h
> @@ -13,6 +13,7 @@
>  #include "8250.h"
>  
>  /* Offsets for the DesignWare specific registers */
> +#define DW_UART_SRBR_0	0x0c /* Shadow Receive Buffer Register */
>  #define DW_UART_USR	0x1f /* UART Status Register */
>  #define DW_UART_DMASA	0xa8 /* DMA Software Ack */
>  #define DW_UART_TCR	0xac /* Transceiver Control Register (RS485) */
> @@ -90,6 +91,10 @@ struct dw8250_port_data {
>  
>  	/* RS485 variables */
>  	bool			hw_rs485_support;
> +
> +	/* Register offsets */
> +	/* Shadow RBR (if not defined RBR will be used) */
> +	int		srbr;
>  };
>  
>  void dw8250_do_set_termios(struct uart_port *p, struct ktermios *termios, const struct ktermios *old);