linux-kernel - Re: [PATCH v2 1/3] serial: core: Store fifo timeout again

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Date: Wed, 17 Apr 2024 10:52:48 +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 1/3] serial: core: Store fifo timeout again

On Tue, 16 Apr 2024, Michael Pratt wrote:

> This is a partial revert of Commit f9008285bb69
> ("serial: Drop timeout from uart_port").
> 
> In order to prevent having to calculate a timeout
> for the fifo device during a write operation, if enabled,
> calculate it ahead of time and store the value of the timeout
> in a struct member of uart_port.

Hi,

Why is calculating during write bad/wrong, you don't give any real 
justification for this change? You're talking about "low rates" in cover 
letter, which makes it even more confusing because writes occur very 
infrequently so a few math operations are nothing to be concerned of (in 
timescales UARTs operate in, no math penalty is really worth even 
discussing IMO).

-- 
 i.

> Signed-off-by: Michael Pratt <mcpratt@...me>
> ---
> V1 -> V2: new commit
> 
>  drivers/tty/serial/serial_core.c | 3 +++
>  include/linux/serial_core.h      | 1 +
>  2 files changed, 4 insertions(+)
> 
> diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
> index ff85ebd3a007..9b3176d684a4 100644
> --- a/drivers/tty/serial/serial_core.c
> +++ b/drivers/tty/serial/serial_core.c
> @@ -414,6 +414,9 @@ uart_update_timeout(struct uart_port *port, unsigned int cflag,
>  
>  	temp *= NSEC_PER_SEC;
>  	port->frame_time = (unsigned int)DIV64_U64_ROUND_UP(temp, baud);
> +
> +	if (port->fifosize > 1)
> +		port->timeout = uart_fifo_timeout(port);
>  }
>  EXPORT_SYMBOL(uart_update_timeout);
>  
> diff --git a/include/linux/serial_core.h b/include/linux/serial_core.h
> index 0a0f6e21d40e..c6422021152f 100644
> --- a/include/linux/serial_core.h
> +++ b/include/linux/serial_core.h
> @@ -561,6 +561,7 @@ struct uart_port {
>  
>  	bool			hw_stopped;		/* sw-assisted CTS flow state */
>  	unsigned int		mctrl;			/* current modem ctrl settings */
> +	unsigned long		timeout;		/* character-based timeout */
>  	unsigned int		frame_time;		/* frame timing in ns */
>  	unsigned int		type;			/* port type */
>  	const struct uart_ops	*ops;
>