linux-kernel - Re: [PATCH] serial: uartps: Wait for tx

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Date:   Wed, 08 Apr 2020 17:50:32 +0200
From:   Maarten Brock <m.brock@...mierlo.com>
To:     Raviteja Narayanam <raviteja.narayanam@...inx.com>
Cc:     linux-serial@...r.kernel.org, gregkh@...uxfoundation.org,
        jslaby@...e.com, michal.simek@...inx.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        git@...inx.com, linux-serial-owner@...r.kernel.org
Subject: Re: [PATCH] serial: uartps: Wait for tx_empty in console setup

On 2020-04-07 18:53, Raviteja Narayanam wrote:
> On some platforms, the log is corrupted while console is being
> registered. It is observed that when set_termios is called, there
> are still some bytes in the FIFO to be transmitted.
> 
> So, wait for tx_empty inside cdns_uart_console_setup before
> calling set_termios.
> 
> Signed-off-by: Raviteja Narayanam <raviteja.narayanam@...inx.com>
> ---
>  drivers/tty/serial/xilinx_uartps.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/drivers/tty/serial/xilinx_uartps.c
> b/drivers/tty/serial/xilinx_uartps.c
> index 6b26f76..23468ff 100644
> --- a/drivers/tty/serial/xilinx_uartps.c
> +++ b/drivers/tty/serial/xilinx_uartps.c
> @@ -1260,6 +1260,8 @@ static int cdns_uart_console_setup(struct
> console *co, char *options)
>  	int bits = 8;
>  	int parity = 'n';
>  	int flow = 'n';
> +	unsigned long time_out = jiffies + usecs_to_jiffies(TX_TIMEOUT);
> +	int status;
> 
>  	if (!port->membase) {
>  		pr_debug("console on " CDNS_UART_TTY_NAME "%i not present\n",
> @@ -1270,6 +1272,14 @@ static int cdns_uart_console_setup(struct
> console *co, char *options)
>  	if (options)
>  		uart_parse_options(options, &baud, &parity, &bits, &flow);
> 
> +	/* Wait for tx_empty before setting up the console */
> +	while (time_before(jiffies, time_out)) {
> +		status = cdns_uart_tx_empty(port);
> +		if (status == TIOCSER_TEMT)
> +			break;
> +		cpu_relax();
> +	}
> +
>  	return uart_set_options(port, co, baud, parity, bits, flow);
>  }
>  #endif /* CONFIG_SERIAL_XILINX_PS_UART_CONSOLE */

You could do without the status variable. You could even combine the
while and if conditions.

And while you're at it, you might as well also rewrite the lines
1236-1238 to also use cdns_uart_tx_empty() for clarity.

Maarten