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

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Message-ID: <67b8f469-6634-5e58-17cf-f66f7b9a225e@linux.intel.com>
Date: Wed, 17 Apr 2024 11:00:59 +0300 (EEST)
From: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
To: Michael Pratt <mcpratt@...me>
cc: LKML <linux-kernel@...r.kernel.org>, 
    linux-serial <linux-serial@...r.kernel.org>, 
    Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
    Jiri Slaby <jirislaby@...nel.org>, 
    Wander Lairson Costa <wander@...hat.com>, 
    Andy Shevchenko <andriy.shevchenko@...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, 16 Apr 2024, 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.

Did you actually see some perfomance issue with 115000 bps? Or did you 
just make the "better performance" claim up?

> Fixes: 8f3631f0f6eb ("serial/8250: Use fifo in 8250 console driver")
> Signed-off-by: Michael Pratt <mcpratt@...me>
> ---
> V1 -> V2:
>  Use stored values instead of calling uart_fifo_timeout()
>  or checking capability flags.
>  The existence of the timeout value satisfies fifosize > 1.
> 
>  drivers/tty/serial/8250/8250_port.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
> index 5b0cfe6bc98c..cf67911a74f5 100644
> --- a/drivers/tty/serial/8250/8250_port.c
> +++ b/drivers/tty/serial/8250/8250_port.c
> @@ -2066,7 +2066,10 @@ static void 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 for a time relative to buffer size and baud */
> +	if (up->fifo_enable && up->port.timeout)
> +		tmout = jiffies_to_usecs(up->port.timeout);
> +
>  	for (;;) {
>  		status = serial_lsr_in(up);

Hi,

Is this the only reason for adding the timeout field? I think you should 
just refactor the code such that you don't need to recalculate the timeout 
for each characted when writing n characters?

-- 
 i.