linux-kernel - Re: [PATCH v1 RESEND] serial: 8250: Set fifo timeout with uart_fifo

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Date: Thu, 11 Jan 2024 15:23:39 +0530
From: VAMSHI GAJJELA <vamshigajjela@...gle.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>, 
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Subject: Re: [PATCH v1 RESEND] serial: 8250: Set fifo timeout with uart_fifo_timeout()

On Thu, Jan 11, 2024 at 5:57 AM Michael Pratt <mcpratt@...me> 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 for all frames
> in the FIFO device to flush before writing data again
> using the now stored frame_time value and size of the buffer.
>
> Fix this by using the new function to calculate the timeout
> whenever the buffer is larger than 1 byte (unknown port default).
>
> Tested on a MIPS device (ar934x) at baud rates 625, 9600, 115200.
>
> Signed-off-by: Michael Pratt <mcpratt@...me>
> ---
> v1 thread: https://lore.kernel.org/linux-serial/20231125063552.517-1-mcpratt@pm.me/
>
>  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 8ca061d3bbb9..777b61a79c5e 100644
> --- a/drivers/tty/serial/8250/8250_port.c
> +++ b/drivers/tty/serial/8250/8250_port.c
> @@ -2076,7 +2076,10 @@ 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->port.fifosize > 1)
> +               tmout = jiffies_to_usecs(uart_fifo_timeout(&up->port));
> +
Michael, I have copied you onto a patch addressing the similar issue, where
it was considered to avoid the computation of timeout on call to wait_for_lsr.
>         for (;;) {
>                 status = serial_lsr_in(up);
>
> --
> 2.30.2
>
>
>