# HG changeset patch
# User Mychaela Falconia <falcon@freecalypso.org>
# Date 1581488602 0
# Node ID 9d9c241f2c84860c553ca857ca80dc6f5994b3b6
# Parent  d5abcbbf7432f751deecdd4055b1325b9d6a4c15
libserial-newlnx experimental change to set ASYNC_LOW_LATENCY

diff -r d5abcbbf7432 -r 9d9c241f2c84 libserial-newlnx/openport.c
--- a/libserial-newlnx/openport.c	Tue Feb 11 21:27:40 2020 +0000
+++ b/libserial-newlnx/openport.c	Wed Feb 12 06:23:22 2020 +0000
@@ -5,6 +5,7 @@
 #include <sys/types.h>
 #include <sys/file.h>
 #include <sys/ioctl.h>
+#include <linux/serial.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <unistd.h>
@@ -14,11 +15,25 @@
 open_serial_port(ttyport)
 	char *ttyport;
 {
+	struct serial_struct kernel_serial_settings;
+
 	target_fd = open(ttyport, O_RDWR|O_NONBLOCK);
 	if (target_fd < 0) {
 		perror(ttyport);
 		exit(1);
 	}
 	ioctl(target_fd, TIOCEXCL);
+	/*
+	 * It appears that some miscreants have modified recent Linux kernel
+	 * versions (newer than linux-4.4.14 of Slackware 14.2) to insert
+	 * a delay of 10 ms before select system call returns when serial
+	 * input is ready; this Linux kernel misbehaviour majorly slows down
+	 * many of our operations.  It appears that we need to set the
+	 * ASYNC_LOW_LATENCY flag in TIOCSSERIAL in order to get the old
+	 * sane behaviour back.
+	 */
+	ioctl(target_fd, TIOCGSERIAL, &kernel_serial_settings);
+	kernel_serial_settings.flags |= ASYNC_LOW_LATENCY;
+	ioctl(target_fd, TIOCSSERIAL, &kernel_serial_settings);
 	return 0;
 }