Re: [PATCH 1/3] serial: 8250_dw: Keep the BUSY-safe divisor hook when DLF is present

["Ilpo Järvinen" <ilpo.jarvinen@linux.intel.com>]

On Mon, 5 Oct 2026, Vinicius Pedrosa wrote:

> dw8250_probe() installs dw8250_set_divisor(), which brackets the
> DLAB/DLL/DLH sequence with dw8250_idle_enter()/dw8250_idle_exit() so that
> a BUSY UART cannot drop the divisor LCR write. dw8250_setup_port() runs
> afterwards and replaces that hook with the dwlib one whenever it detects
> a DLF register. The dwlib hook programs the divisor through
> serial8250_do_set_divisor() without entering idle.

Yeah, non-dwlib code should own the hook if it sets one.

> When termios changes while the UART is transmitting, the LCR|DLAB write
> is ignored and dw8250_check_lcr() retries it between
> dw8250_idle_enter() and dw8250_idle_exit(). dw8250_idle_exit() restores
> IER while DLAB is still set, so the value lands in DLH. The real IER
> keeps the zero written by dw8250_idle_enter(), and the port gets no
> further TX or RX interrupts while up->ier says they're enabled.
> 
> Program DLF from dw8250_set_divisor() inside the idle section and keep
> that hook after dw8250_setup_port().
> 
> Observed on an Allwinner A733 (Radxa Cubie A7Z): the first interactive
> tcsetattr() during console output left hardware IER at 0 and up->ier at
> 0x07. The shell never printed its prompt or read input. With this change
> alone the same boot reached the prompt and echoed input, and hardware
> IER matched up->ier afterwards.
> 
> Fixes: a7b9ce39fbe4 ("serial: 8250_dw: Ensure BUSY is deasserted")
> Cc: stable@vger.kernel.org
> Signed-off-by: Vinicius Pedrosa <vinicius.eduardo.pedrosa@gmail.com>
> ---
> The A733 has no real DLF. Offset 0xc0 is an RS485 control register that
> dw8250_setup_port() mistakes for one (patch 3), which is how this path
> was reached. The new DLF write hasn't been tested on a UART with a real
> DLF register.
> 
>  drivers/tty/serial/8250/8250_dw.c | 12 +++++++++++-
>  1 file changed, 11 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/tty/serial/8250/8250_dw.c b/drivers/tty/serial/8250/8250_dw.c
> index 0ab558230370..ba414306c98a 100644
> --- a/drivers/tty/serial/8250/8250_dw.c
> +++ b/drivers/tty/serial/8250/8250_dw.c
> @@ -189,6 +189,7 @@ static int dw8250_idle_enter(struct uart_port *p)
>  static void dw8250_set_divisor(struct uart_port *p, unsigned int baud,
>  			       unsigned int quot, unsigned int quot_frac)
>  {
> +	struct dw8250_data *d = to_dw8250_data(p->private_data);
>  	struct uart_8250_port *up = up_to_u8250p(p);
>  	int ret;
>  
> @@ -196,6 +197,9 @@ static void dw8250_set_divisor(struct uart_port *p, unsigned int baud,
>  	if (ret < 0)
>  		return;
>  
> +	if (d->data.dlf_size)
> +		dw8250_writel_ext(p, DW_UART_DLF, quot_frac);

It might be better to add __dw8250_set_divisor() to dwlib to keep the DLF 
setting code in one place. If the dwlib side does d->data.dlf_size check, 
8250_dw side doesn't need to care about DLF stuff but can just 
unconditionally call __dw8250_set_divisor().

> +
>  	serial_port_out(p, UART_LCR, up->lcr | UART_LCR_DLAB);
>  	if (!(serial_port_in(p, UART_LCR) & UART_LCR_DLAB))
>  		goto idle_failed;
> @@ -650,7 +654,6 @@ static int dw8250_probe(struct platform_device *pdev)
>  
>  	p->set_ldisc	= dw8250_set_ldisc;
>  	p->set_termios	= dw8250_set_termios;
> -	p->set_divisor	= dw8250_set_divisor;
>  
>  	data = devm_kzalloc(dev, sizeof(*data), GFP_KERNEL);
>  	if (!data)
> @@ -767,6 +770,13 @@ static int dw8250_probe(struct platform_device *pdev)
>  	if (!data->skip_autocfg)
>  		dw8250_setup_port(p);
>  
> +	/*
> +	 * dw8250_setup_port() installs the library divisor hook when it finds
> +	 * DLF. Keep the BUSY-safe hook instead: it also programs DLF, and it
> +	 * must own DLAB so that a BUSY UART never drops the divisor LCR write.
> +	 */
> +	p->set_divisor = dw8250_set_divisor;
> +
>  	/* If we have a valid fifosize, try hooking up DMA */
>  	if (p->fifosize) {
>  		data->data.dma.rxconf.src_maxburst = p->fifosize / 4;
> 

It might be safer approach to NULL check .set_divisor in dwlib before 
setting it as ordering requirements like this are easy to miss.

-- 
 i.