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

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Message-ID: <Zxug3qF9KUOn4VaM@smile.fi.intel.com>
Date: Fri, 25 Oct 2024 16:45:02 +0300
From: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To: John Ogness <john.ogness@...utronix.de>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jirislaby@...nel.org>, 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,
	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 Fri, Oct 25, 2024 at 01:03:23PM +0206, 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,

printk here is a function reference or module?
For the latter I would use the filename to be sure it's clear,
like printk.c. For the former (and it seems you know that)
we may use printk().

> 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

maximum
(in order not to mix with max() function)

> does not affect performance since ideally the timeout never
> occurs.

...

>  /*
>   *	Wait for transmitter & holding register to empty
> + *	with timeout

Can you fix the style while at it?

>   */

 /* Wait for transmitter & holding register to empty with timeout */

...

>  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. */

Does the one-line comment style in this file use periods? If not, drop,
otherwise apply it to the above proposal.

> +		for (i = 0; i < fifosize; i++) {
> +			if (wait_for_lsr(up, UART_LSR_THRE))
> +				break;
> +		}

> +	}
> +
> +	/* Allow timeout for each byte written. */
> +	for (i = 0; i < tx_count; i++) {
> +		if (wait_for_lsr(up, UART_LSR_THRE))
> +			break;

This effectively repeats the above. Even for the fix case I would still add
a new helper to deduplicate.

>  	}
>  }

-- 
With Best Regards,
Andy Shevchenko