[PATCH v3 1/2] serial: mxc: Wait for TX completion before reset

[Pali Rohár]

On Tuesday 10 January 2023 20:24:06 Loic Poulain wrote:
> The u-boot console may show some corrupted characters when
> printing in board_init() due to reset of the UART (probe)
> before the TX FIFO has been completely drained.
> 
> To fix this issue, and in case UART is still running, we now
> try to flush the FIFO before proceeding to UART reinitialization.
> For this we're waiting for Transmitter Complete bit, indicating
> that the FIFO and the shift register are empty.
> 
> flushing has a 4ms timeout guard, which is normally more than
> enough to consume the FIFO @ low baudrate (9600bps).
> 
> Signed-off-by: Loic Poulain <loic.poulain at linaro.org>
> Tested-by: Lothar Waßmann <LW at KARO-electronics.de>

Hello! Last time when I looked at this driver I was in impression that
also _mxc_serial_setbrg() function requires calling some flush function
at the beginning. Could you please check if it is needed or not? I'm
really not sure.

> ---
>  v2: Add this commit to the series
>  v3: Fix typo & reordering commits for good bisectability 
> 
>  drivers/serial/serial_mxc.c | 24 +++++++++++++++++++++++-
>  1 file changed, 23 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/serial/serial_mxc.c b/drivers/serial/serial_mxc.c
> index 82c0d84628..3c89781f2c 100644
> --- a/drivers/serial/serial_mxc.c
> +++ b/drivers/serial/serial_mxc.c
> @@ -13,6 +13,7 @@
>  #include <dm/platform_data/serial_mxc.h>
>  #include <serial.h>
>  #include <linux/compiler.h>
> +#include <linux/delay.h>
>  
>  /* UART Control Register Bit Fields.*/
>  #define URXD_CHARRDY	(1<<15)
> @@ -144,8 +145,22 @@ struct mxc_uart {
>  	u32 ts;
>  };
>  
> +static void _mxc_serial_flush(struct mxc_uart *base)
> +{
> +	unsigned int timeout = 4000;
> +
> +	if (!(readl(&base->cr1) & UCR1_UARTEN) ||
> +	    !(readl(&base->cr2) & UCR2_TXEN))
> +		return;
> +
> +	while (!(readl(&base->sr2) & USR2_TXDC) && --timeout)
> +		udelay(1);
> +}
> +
>  static void _mxc_serial_init(struct mxc_uart *base, int use_dte)
>  {
> +	_mxc_serial_flush(base);
> +
>  	writel(0, &base->cr1);
>  	writel(0, &base->cr2);
>  
> @@ -252,10 +267,17 @@ static int mxc_serial_init(void)
>  	return 0;
>  }
>  
> +static int mxc_serial_stop(void)
> +{
> +	_mxc_serial_flush(mxc_base);
> +
> +	return 0;
> +}
> +
>  static struct serial_device mxc_serial_drv = {
>  	.name	= "mxc_serial",
>  	.start	= mxc_serial_init,
> -	.stop	= NULL,
> +	.stop	= mxc_serial_stop,
>  	.setbrg	= mxc_serial_setbrg,
>  	.putc	= mxc_serial_putc,
>  	.puts	= default_serial_puts,

Anyway, this code touches _only_ non-DM version of the driver. So same
fix should be implemented also for DM version because non-DM is now
legacy and will be removed in the future from U-Boot. Please look at the
DM version too.

> -- 
> 2.34.1
>