[PATCH] serial: mxc: Speed-up character transmission

[Lothar Waßmann]

Hi,

On Mon,  9 Jan 2023 15:59:25 +0100 Loic Poulain wrote:
> Instead of waiting for empty FIFO condition before writing a
> character, wait for non-full FIFO condition.
> 
> This helps in saving several tens of milliseconds during boot
> (depending verbosity).
> 
> Signed-off-by: Loic Poulain <loic.poulain at linaro.org>
> ---
>  drivers/serial/serial_mxc.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/serial/serial_mxc.c b/drivers/serial/serial_mxc.c
> index 82c0d84628..f8c49865be 100644
> --- a/drivers/serial/serial_mxc.c
> +++ b/drivers/serial/serial_mxc.c
> @@ -223,11 +223,11 @@ static void mxc_serial_putc(const char c)
>  	if (c == '\n')
>  		serial_putc('\r');
>  
> -	writel(c, &mxc_base->txd);
> -
>  	/* wait for transmitter to be ready */
> -	while (!(readl(&mxc_base->ts) & UTS_TXEMPTY))
> +	while (readl(&mxc_base->ts) & UTS_TXFULL)
>  		schedule();
> +
> +	writel(c, &mxc_base->txd);
>  }
>  
>  /* Test whether a character is in the RX buffer */
> @@ -311,7 +311,7 @@ static int mxc_serial_putc(struct udevice *dev, const char ch)
>  	struct mxc_serial_plat *plat = dev_get_plat(dev);
>  	struct mxc_uart *const uart = plat->reg;
>  
> -	if (!(readl(&uart->ts) & UTS_TXEMPTY))
> +	if (readl(&uart->ts) & UTS_TXFULL)
>  		return -EAGAIN;
>  
>  	writel(ch, &uart->txd);
>
A previous attempt to do this in:
|commit c7878a0483c59c48a730123bc0f4659fd40921bf
|Author: Johannes Schneider <johannes.schneider at leica-geosystems.com>
|Date:   Tue Sep 6 14:15:04 2022 +0200
|
|    serial: mxc: have putc use the TXFIFO

has been reverted in:
|commit fc1c1760de38823edbdc2cdd9606dff938a07f6e
|Author: Fabio Estevam <festevam at denx.de>
|Date:   Tue Nov 8 08:39:33 2022 -0300
|
|    Revert "serial: mxc: have putc use the TXFIFO"
|    
|    This reverts commit c7878a0483c59c48a730123bc0f4659fd40921bf.
|    
|    Since commit c7878a0483c5 ("serial: mxc: have putc use the TXFIFO"),
|    serial console corruption can be seen when priting inside board_init().
|    
|    Revert it to avoid the regression.
|    
|    Reported-by: Tim Harvey <tharvey at gateworks.com>
|    Signed-off-by: Fabio Estevam <festevam at denx.de>
|    Acked-by: Tim Harvey <tharvey at gateworks.com>


Lothar Waßmann