linux-kernel - Re: [PATCH v2 3/3] serial: 8250: Set fifo timeout using uart_fifo

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Date: Tue, 16 Apr 2024 21:57:39 +0300
From: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To: Michael Pratt <mcpratt@...me>
Cc: linux-kernel@...r.kernel.org, linux-serial@...r.kernel.org,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jirislaby@...nel.org>,
	Wander Lairson Costa <wander@...hat.com>,
	Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>,
	Vamshi Gajjela <vamshigajjela@...gle.com>
Subject: Re: [PATCH v2 3/3] serial: 8250: Set fifo timeout using
 uart_fifo_timeout()

On Tue, Apr 16, 2024 at 06:34:10PM +0000, Michael Pratt wrote:
> Commit 8f3631f0f6eb ("serial/8250: Use fifo in 8250 console driver")
> reworked functions for basic 8250 and 16550 type serial devices
> in order to enable and use the internal fifo device for buffering,
> however the default timeout of 10 ms remained, which is proving
> to be insufficient for low baud rates like 9600, causing data overrun.
> 
> Unforunately, that commit was written and accepted just before commit
> 31f6bd7fad3b ("serial: Store character timing information to uart_port")
> which introduced the frame_time member of the uart_port struct
> in order to store the amount of time it takes to send one uart frame
> relative to the baud rate and other serial port configuration,
> and commit f9008285bb69 ("serial: Drop timeout from uart_port")
> which established function uart_fifo_timeout() in order to
> calculate a reasonable timeout to wait until flushing
> the fifo device before writing data again when partially filled,
> using the now stored frame_time value and size of the buffer.
> 
> Fix this by using the stored timeout value made with this new function
> to calculate the timeout for the fifo device, when enabled, and when
> the buffer is larger than 1 byte (unknown port default).
> 
> The previous 2 commits add the struct members used here
> in order to store the values, so that the calculations
> are offloaded from the functions that are called
> during a write operation for better performance.
> 
> Tested on a MIPS device (ar934x) at baud rates 625, 9600, 115200.

..

>  	unsigned int status, tmout = 10000;
>  
> -	/* Wait up to 10ms for the character(s) to be sent. */
> +	/* Wait for a time relative to buffer size and baud */
> +	if (up->fifo_enable && up->port.timeout)
> +		tmout = jiffies_to_usecs(up->port.timeout);

Why do we still use that default? Can't we calculate timeout even for\
FIFO-less / FIFO-disabled devices?

-- 
With Best Regards,
Andy Shevchenko