[PATCH 3/3] serial: serial_msm: Delay initialization to let pins stabilize

[Ramon Fried]

On Wed, Jul 14, 2021 at 11:56 AM Stephan Gerhold <stephan at gerhold.net> wrote:
>
> For some reason, the DragonBoard 410c aborts autoboot immediately if
> U-Boot is started without LK. It looks like it picks up a single broken
> character via serial and therefore believes a key was pressed to abort
> autoboot.
>
> After some debugging, it seems like adding some delay after pinctrl
> setup but before UART initialization fixes the issue. It's also worth
> mentioning that unlike when booting from LK, the pinctrl setup is
> actually necessary when booting U-Boot without LK since UART is broken
> if the pinctrl line is removed.
>
> I suspect that reconfiguring the pins might take some time to stabilize
> and if the UART controller is enabled too quickly it will pick up some
> random noise. Adding a few milliseconds of delay fixes the issue and
> shouldn't have any other negative side effects.
>
> 3ms seems to be the minimum delay required in my tests, use 5ms instead
> just to be sure.
>
> Signed-off-by: Stephan Gerhold <stephan at gerhold.net>
> ---
>
>  drivers/serial/serial_msm.c | 4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/drivers/serial/serial_msm.c b/drivers/serial/serial_msm.c
> index d8dd5c1104..ac4d0824b9 100644
> --- a/drivers/serial/serial_msm.c
> +++ b/drivers/serial/serial_msm.c
> @@ -18,6 +18,7 @@
>  #include <asm/global_data.h>
>  #include <asm/io.h>
>  #include <linux/compiler.h>
> +#include <linux/delay.h>
>  #include <dm/pinctrl.h>
>
>  /* Serial registers - this driver works in uartdm mode*/
> @@ -193,6 +194,9 @@ static int msm_uart_clk_init(struct udevice *dev)
>
>  static void uart_dm_init(struct msm_serial_data *priv)
>  {
> +       /* Delay initialization for a bit to let pins stabilize if necessary */
> +       mdelay(5);
> +
>         writel(priv->clk_bit_rate, priv->base + UARTDM_CSR);
>         writel(0x0, priv->base + UARTDM_MR1);
>         writel(MSM_BOOT_UART_DM_8_N_1_MODE, priv->base + UARTDM_MR2);
> --
> 2.32.0
>
Eh...this is not a nice fix, perhaps just clearing the RX buffer after
initialization will work better ?