Re: [PATCH RESEND 15/16] serial: msm: wait for TX empty before reinit

[Stephan Gerhold <stephan.gerhold@linaro.org>]

On Mon, Jun 01, 2026 at 06:12:56PM +1000, Sam Day via B4 Relay wrote:
> From: Sam Day <me@samcday.com>
> 
> At least on UARTDM 1.3, I was noticing the early UART debug banner
> getting corrupted. It turns out this is because U-Boot was
> re-initializing the UARTDM block and writing to it before it had
> finished shifting out the FIFO from the previous bootloader. Waiting for
> TX_EMPTY in the status register consistently fixes the issue.
> 
> Signed-off-by: Sam Day <me@samcday.com>
> ---
>  drivers/serial/serial_msm.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/drivers/serial/serial_msm.c b/drivers/serial/serial_msm.c
> index 10948e2aede..db085d68a0d 100644
> --- a/drivers/serial/serial_msm.c
> +++ b/drivers/serial/serial_msm.c
> @@ -351,6 +351,8 @@ static inline void _debug_uart_init(void)
>  	 *   - HMIBSC: GCC_BLSP1_UART1_APPS_CLK
>  	 */
>  	//apq8016_clk_init_uart(0x1800000, <uart_clk_id>);
> +	while (!(readl(init_serial_data.base + UARTDM_SR) & UARTDM_SR_TX_EMPTY))
> +		;

I think you need to add some timeout here. There is some funky SoC where
the UART is left in some half-disabled state and this bit will never
become unset. Maybe it was MSM8909, I don't remember exactly. I have
similar code in TF-A and there I had to add the timeout to prevent the
device from getting completely stuck during boot sometimes because of
this loop:

https://github.com/ARM-software/arm-trusted-firmware/blob/da738d5eae93af342fdc4995dd3c05acb4c9d757/plat/qti/bear/msm8916/aarch32/uartdm_console.S#L68-L81

Thanks,
Stephan