Re: [PATCH v4 2/8] tty: serial: Add uart_fifo_timeout_ms()

[Ilpo Järvinen]

On Mon, 10 Jun 2024, Douglas Anderson wrote:

> The current uart_fifo_timeout() returns jiffies, which is not always
> the most convenient for callers. Add a variant uart_fifo_timeout_ms()
> that returns the timeout in milliseconds.
> 
> NOTES:
> - msecs_to_jiffies() rounds up, unlike nsecs_to_jiffies(). This is
>   because msecs_to_jiffies() is actually intended for device drivers
>   to calculate timeout value. This means we don't need to take the max
>   of the timeout and "1" since the timeout will always be > 0 ms (we
>   add 20 ms of slop).
> - uart_fifo_timeout_ms() returns "unsigned int" but we leave
>   uart_fifo_timeout() returning "unsigned long". This matches the
>   types of msecs_to_jiffies().
> 
> Suggested-by: Ilpo Järvinen <ilpo.jarvinen@xxxxxxxxxxxxxxx>
> Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
> ---
> 
> Changes in v4:
> - New
> 
>  include/linux/serial_core.h | 15 +++++++++++----
>  1 file changed, 11 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/serial_core.h b/include/linux/serial_core.h
> index 8cb65f50e830..97968acfd564 100644
> --- a/include/linux/serial_core.h
> +++ b/include/linux/serial_core.h
> @@ -889,14 +889,21 @@ unsigned int uart_get_divisor(struct uart_port *port, unsigned int baud);
>  /*
>   * Calculates FIFO drain time.
>   */
> -static inline unsigned long uart_fifo_timeout(struct uart_port *port)
> +static inline unsigned int uart_fifo_timeout_ms(struct uart_port *port)
>  {
>  	u64 fifo_timeout = (u64)READ_ONCE(port->frame_time) * port->fifosize;
> +	unsigned int fifo_timeout_ms = div_u64(fifo_timeout, NSEC_PER_MSEC);
>  
> -	/* Add .02 seconds of slop */
> -	fifo_timeout += 20 * NSEC_PER_MSEC;
> +	/*
> +	 * Add .02 seconds of slop. This also helps account for the fact that
> +	 * when we converted from ns to ms that we didn't round up.
> +	 */
> +	return fifo_timeout_ms + 20;
> +}
>  
> -	return max(nsecs_to_jiffies(fifo_timeout), 1UL);
> +static inline unsigned long uart_fifo_timeout(struct uart_port *port)
> +{
> +	return msecs_to_jiffies(uart_fifo_timeout_ms(port));
>  }

Hi,

This is definitely towards the right direction! However, it now does 
double conversion, first div_u64() and then msecs_to_jiffies(). Perhaps it 
would be better to retain the nsecs version (maybe rename it to _ns for 
consistency) and add _ms variant that does the nsec -> msec conversion.

-- 
 i.