linux-kernel - Re: [PATCH tty-next v3 1/6] serial: 8250: Adjust the timeout for FIFO mode

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Message-ID: <837a7ecd-be29-4865-9543-cb6f7e7e46e7@kernel.org>
Date: Wed, 30 Oct 2024 07:05:58 +0100
From: Jiri Slaby <jirislaby@...nel.org>
To: John Ogness <john.ogness@...utronix.de>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: Petr Mladek <pmladek@...e.com>,
 Sergey Senozhatsky <senozhatsky@...omium.org>,
 Steven Rostedt <rostedt@...dmis.org>, Thomas Gleixner <tglx@...utronix.de>,
 Esben Haabendal <esben@...nix.com>, linux-serial@...r.kernel.org,
 linux-kernel@...r.kernel.org,
 Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
 Rengarajan S <rengarajan.s@...rochip.com>,
 Jeff Johnson <quic_jjohnson@...cinc.com>,
 Serge Semin <fancer.lancer@...il.com>,
 Lino Sanfilippo <l.sanfilippo@...bus.com>,
 Wander Lairson Costa <wander@...hat.com>
Subject: Re: [PATCH tty-next v3 1/6] serial: 8250: Adjust the timeout for FIFO
 mode

On 25. 10. 24, 12:57, John Ogness wrote:
> After a console has fed a line into TX, it uses wait_for_xmitr()
> to wait until the data has been sent out before returning to the
> printk code. However, wait_for_xmitr() will timeout after 10ms,
> regardless if the data has been transmitted or not.
> 
> For single bytes, this timeout is sufficient even at very slow
> baud rates, such as 1200bps. However, when FIFO mode is used,
> there may be 64 bytes pushed into the FIFO at once. At a baud
> rate of 115200bps, the 10ms timeout is still sufficient.
> However, when using lower baud rates (such as 57600bps), the
> timeout is _not_ sufficient. This causes longer lines to be cut
> off, resulting in lost and horribly misformatted output on the
> console.
> 
> When using FIFO mode, take the number of bytes into account to
> determine an appropriate max timeout. Increasing the timeout
> does not affect performance since ideally the timeout never
> occurs.
> 
> Fixes: 8f3631f0f6eb ("serial/8250: Use fifo in 8250 console driver")
> Signed-off-by: John Ogness <john.ogness@...utronix.de>
> ---
>   drivers/tty/serial/8250/8250_port.c | 24 ++++++++++++++++++++----
>   1 file changed, 20 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
> index 3509af7dc52b..adc48eeeac2b 100644
> --- a/drivers/tty/serial/8250/8250_port.c
> +++ b/drivers/tty/serial/8250/8250_port.c
> @@ -2059,11 +2059,12 @@ static void serial8250_break_ctl(struct uart_port *port, int break_state)
>   	serial8250_rpm_put(up);
>   }
>   
> -static void wait_for_lsr(struct uart_8250_port *up, int bits)
> +/* Returns true if @bits were set, false on timeout. */
> +static bool wait_for_lsr(struct uart_8250_port *up, int bits)
>   {
>   	unsigned int status, tmout = 10000;
>   
> -	/* Wait up to 10ms for the character(s) to be sent. */
> +	/* Wait up to 10ms for the character to be sent. */
>   	for (;;) {
>   		status = serial_lsr_in(up);
>   
> @@ -2074,10 +2075,13 @@ static void wait_for_lsr(struct uart_8250_port *up, int bits)
>   		udelay(1);
>   		touch_nmi_watchdog();
>   	}
> +
> +	return (tmout != 0);
>   }
>   
>   /*
>    *	Wait for transmitter & holding register to empty
> + *	with timeout
>    */
>   static void wait_for_xmitr(struct uart_8250_port *up, int bits)
>   {
> @@ -3306,13 +3310,18 @@ static void serial8250_console_restore(struct uart_8250_port *up)
>   static void serial8250_console_fifo_write(struct uart_8250_port *up,
>   					  const char *s, unsigned int count)
>   {
> -	int i;
>   	const char *end = s + count;
>   	unsigned int fifosize = up->tx_loadsz;
> +	unsigned int tx_count = 0;
>   	bool cr_sent = false;
> +	unsigned int i;
>   
>   	while (s != end) {
> -		wait_for_lsr(up, UART_LSR_THRE);
> +		/* Allow timeout for each byte of a possibly full FIFO. */
> +		for (i = 0; i < fifosize; i++) {
> +			if (wait_for_lsr(up, UART_LSR_THRE))
> +				break;
> +		}

THRE only signals there is a space for one character. Multiplying it 
with fifosize does not make much sense to me. You perhaps want only to 
increase the timeout. Or somehow incorporate port->frame_time into the 
accounting (I am not sure it is available at this point already).

>   		for (i = 0; i < fifosize && s != end; ++i) {
>   			if (*s == '\n' && !cr_sent) {
> @@ -3323,6 +3332,13 @@ static void serial8250_console_fifo_write(struct uart_8250_port *up,
>   				cr_sent = false;
>   			}
>   		}
> +		tx_count = i;
> +	}
> +
> +	/* Allow timeout for each byte written. */
> +	for (i = 0; i < tx_count; i++) {
> +		if (wait_for_lsr(up, UART_LSR_THRE))

This ensures you sent one character from the FIFO. The FIFO still can 
contain plenty of them. Did you want UART_LSR_TEMT?

But what's the purpose of spinning _here_? The kernel can run and FIFO 
too. Without the kernel waiting for the FIFO.

thanks,
-- 
js
suse labs