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

[Sasha Levin <sashal@xxxxxxxxxx>]

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@xxxxxxxxxxxxxxx> know about it.



commit 5517053a2e0b30a1e35f90504446af4a2c4920e8
Author: Ilpo Järvinen <ilpo.jarvinen@xxxxxxxxxxxxxxx>
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@xxxxxxxxxxxxxxx>
    Link: https://lore.kernel.org/r/20220425143410.12703-2-ilpo.jarvinen@xxxxxxxxxxxxxxx
    Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

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;