Re: Patch "serial: Store character timing information to uart_port" has been added to the 5.18-stable tree

["Ilpo Järvinen" <ilpo.jarvinen@linux.intel.com>]

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On Sat, 13 Aug 2022, Sasha Levin wrote:

> This is a note to let you know that I've just added the patch titled
> 
>     serial: Store character timing information to uart_port
> 
> to the 5.18-stable tree which can be found at:
>     http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
> 
> The filename of the patch is:
>      serial-store-character-timing-information-to-uart_po.patch
> and it can be found in the queue-5.18 subdirectory.
> 
> If you, or anyone else, feels it should not be added to the stable tree,
> please let <stable@vger.kernel.org> know about it.
>
> commit 5517053a2e0b30a1e35f90504446af4a2c4920e8
> Author: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
> Date:   Mon Apr 25 17:33:58 2022 +0300
> 
>     serial: Store character timing information to uart_port
>     
>     [ Upstream commit 31f6bd7fad3b149a1eb6f67fc2e742e4df369b3d ]
>     
>     Struct uart_port currently stores FIFO timeout. Having character timing
>     information readily available is useful. Even serial core itself
>     determines char_time from port->timeout using inverse calculation.
>     
>     Store frame_time directly into uart_port. Character time is stored in
>     nanoseconds to have reasonable precision with high rates. To avoid
>     overflow, 64-bit math is necessary.
>     
>     It might be possible to determine timeout from frame_time by
>     multiplying it with fifosize as needed but only part of the users seem
>     to be protected by a lock. Thus, this patch does not pursue storing
>     only frame_time in uart_port.
>     
>     Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
>     Link: https://lore.kernel.org/r/20220425143410.12703-2-ilpo.jarvinen@linux.intel.com
>     Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
>     Signed-off-by: Sasha Levin <sashal@kernel.org>
> 
> diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
> index 95d8d1fcd543..6b07b7b41354 100644
> --- a/drivers/tty/serial/serial_core.c
> +++ b/drivers/tty/serial/serial_core.c
> @@ -24,6 +24,7 @@
>  #include <linux/sysrq.h>
>  #include <linux/delay.h>
>  #include <linux/mutex.h>
> +#include <linux/math64.h>
>  #include <linux/security.h>
>  
>  #include <linux/irq.h>
> @@ -333,15 +334,18 @@ void
>  uart_update_timeout(struct uart_port *port, unsigned int cflag,
>  		    unsigned int baud)
>  {
> -	unsigned int size;
> +	unsigned int size = tty_get_frame_size(cflag);
> +	u64 frame_time;
>  
> -	size = tty_get_frame_size(cflag) * port->fifosize;
> +	frame_time = (u64)size * NSEC_PER_SEC;
> +	size *= port->fifosize;
>  
>  	/*
>  	 * Figure the timeout to send the above number of bits.
>  	 * Add .02 seconds of slop
>  	 */
>  	port->timeout = (HZ * size) / baud + HZ/50;
> +	port->frame_time = DIV64_U64_ROUND_UP(frame_time, baud);
>  }
>  EXPORT_SYMBOL(uart_update_timeout);
>  
> @@ -1610,10 +1614,8 @@ static void uart_wait_until_sent(struct tty_struct *tty, int timeout)
>  	 * Note: we have to use pretty tight timings here to satisfy
>  	 * the NIST-PCTS.
>  	 */
> -	char_time = (port->timeout - HZ/50) / port->fifosize;
> -	char_time = char_time / 5;
> -	if (char_time == 0)
> -		char_time = 1;
> +	char_time = max(nsecs_to_jiffies(port->frame_time / 5), 1UL);
> +
>  	if (timeout && timeout < char_time)
>  		char_time = timeout;
>  
> diff --git a/include/linux/serial_core.h b/include/linux/serial_core.h
> index ca57d686d4d1..409573ea5ea4 100644
> --- a/include/linux/serial_core.h
> +++ b/include/linux/serial_core.h
> @@ -232,6 +232,7 @@ struct uart_port {
>  	int			hw_stopped;		/* sw-assisted CTS flow state */
>  	unsigned int		mctrl;			/* current modem ctrl settings */
>  	unsigned int		timeout;		/* character-based timeout */
> +	unsigned int		frame_time;		/* frame timing in ns */
>  	unsigned int		type;			/* port type */
>  	const struct uart_ops	*ops;
>  	unsigned int		custom_divisor;
> 

Why is this change necessary for stable? The change was made simply to 
make some other feature changes following a lot simpler but I don't think 
there is any known bug/issue it would be fixing.

-- 
 i.