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

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

[Yicong Yang]

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.

Signed-off-by: Yicong Yang <yang.yicong@picoheart.com>
---
Change since v1:
- refine the comments and type per Andy
Link: https://lore.kernel.org/linux-serial/20260907074035.77796-1-yang.yicong@picoheart.com/

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@picoheart.com/

 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);
-- 
2.50.1 (Apple Git-155)

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

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:

Pre-existing issues:
- [High] Calling pm_runtime_enable() after serial port registration leads to circular dependencies and blocked tasks during device probe.
--

commit fbed7674eea9b0cab135230c5f02f518f9590574
Author: Yicong Yang <yang.yicong@picoheart.com>

serial: 8250_dw: Prefer SRBR in bogus RX timeout workaround if available

This commit updates the DesignWare 8250 UART driver to use the Shadow 
Receive Buffer Register (SRBR) instead of the standard RBR for bogus 
reads during RX timeouts. This avoids potential hardware errors on 
platforms that do not support standard RBR reads in this state.

Link: https://lore.kernel.org/linux-serial/20260907074035.77796-1-yang.yicong@picoheart.com/
Link: https://lore.kernel.org/linux-serial/20260629075510.32854-1-yang.yicong@picoheart.com/

> diff --git a/drivers/tty/serial/8250/8250_dw.c b/drivers/tty/serial/8250/8250_dw.c
> index 51d026f20825a..b8a0bca8b536f 100644
> --- a/drivers/tty/serial/8250/8250_dw.c
> +++ b/drivers/tty/serial/8250/8250_dw.c

[Severity: High]
This isn't a bug introduced by this patch, but while reviewing the changes 
to this file, I noticed a pre-existing issue regarding runtime PM 
initialization in the probe sequence.

Does dw8250_probe() register the serial port before enabling runtime PM?

When serial8250_register_8250_port() is called, it synchronously invokes
uart_add_one_port(). This can trigger the dw8250_do_pm() callback:

dw8250_do_pm() {
    if (!state)
        pm_runtime_get_sync(port->dev);
    ...
}

If this callback fires before pm_runtime_enable() is executed at the end 
of dw8250_probe(), it will attempt to use runtime PM APIs on an 
uninitialized device.

Could this circular dependency lead to blocked tasks and hung worker 
threads during device probe?

[ ... ]

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260908141524.20302-1-yang.yicong@picoheart.com?part=1

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

[Andy Shevchenko]

On Tue, Sep 08, 2026 at 10:15:24PM +0800, 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.

Okay now.
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>

-- 
With Best Regards,
Andy Shevchenko