PATCH: TIOCMBIS/TIOCMBIC not working when using usbser.sys

["Carlo B." <carlo.bramini@gmail.com>]

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I'm sending here the patch as you requested.
I hope that you will find it useful.

Sincerely,

Carlo Bramini.

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From 753b491fd70d337b6f21de577e833a85b19a8e7c Mon Sep 17 00:00:00 2001
From: Carlo Bramini <30959007+carlo-bramini@users.noreply.github.com>
Date: Fri, 11 Nov 2022 11:15:27 +0100
Subject: [PATCH] [PATCH] Cygwin: fix TIOCMBIS/TIOCMBIC not working when using
 usbser.sys

Into winsup/cygwin/fhandler/serial.cc, the function fhandler_serial::switch_modem_lines() is called when TIOCMBIS/TIOCMBIC are used into an ioctl() call.
This function uses EscapeCommFunction() for setting and resetting RTS and DTR signals of a serial port.
Unfortunately, this function does not work on USB CDC devices.
This is not a true bug of a CYGWIN but an issue of the usbser.sys driver from Microsoft, from Windows 2000 to the latest Windows 11.
Both 32bit and 64bit versions of the operating system are affected.
Actually, I tested EscapeCommFunction() also when using a real UART, based on the traditional 16550 driver and it works fine.
Using thirdy party CDC drivers, like the one provided by FTDI for their USB bridge chips, probably also works.
However, it is also possible to drive the RTS/DTR signals by writing their state with SetCommState(), which proved to be working fine all types of connection.
This is also a better solution for handling these signals since RTS/DTR can be set at the same time rather than having two separate calls with a visible delay between them.
---
 winsup/cygwin/fhandler/serial.cc | 67 +++++++++++++-------------------
 1 file changed, 28 insertions(+), 39 deletions(-)

diff --git a/winsup/cygwin/fhandler/serial.cc b/winsup/cygwin/fhandler/serial.cc
index 174a57a43..58a8df677 100644
--- a/winsup/cygwin/fhandler/serial.cc
+++ b/winsup/cygwin/fhandler/serial.cc
@@ -391,50 +391,39 @@ fhandler_serial::tcflow (int action)
 int
 fhandler_serial::switch_modem_lines (int set, int clr)
 {
-  int res = 0;
-
-  if (set & TIOCM_RTS)
-    {
-      if (EscapeCommFunction (get_handle (), SETRTS))
-	rts = TIOCM_RTS;
-      else
-	{
-	  __seterrno ();
-	  res = -1;
-	}
-    }
-  else if (clr & TIOCM_RTS)
-    {
-      if (EscapeCommFunction (get_handle (), CLRRTS))
-	rts = 0;
-      else
-	{
-	  __seterrno ();
-	  res = -1;
-	}
-    }
-  if (set & TIOCM_DTR)
+  if ((set & (TIOCM_RTS | TIOCM_DTR)) || (clr & (TIOCM_RTS | TIOCM_DTR)))
     {
-      if (EscapeCommFunction (get_handle (), SETDTR))
-	rts = TIOCM_DTR;
+      DCB dcb;
+
+      memset(&dcb,0,sizeof(dcb));
+      dcb.DCBlength = sizeof(dcb);
+
+      if (!GetCommState(get_handle(), &dcb))
+        {
+          __seterrno ();
+          return -1;
+        }
+
+      if (set & TIOCM_RTS)
+        dcb.fRtsControl = RTS_CONTROL_ENABLE;
       else
-	{
-	  __seterrno ();
-	  res = -1;
-	}
-    }
-  else if (clr & TIOCM_DTR)
-    {
-      if (EscapeCommFunction (get_handle (), CLRDTR))
-	rts = 0;
+      if (clr & TIOCM_RTS)
+        dcb.fRtsControl = RTS_CONTROL_DISABLE;
+
+      if (set & TIOCM_DTR)
+        dcb.fRtsControl = DTR_CONTROL_ENABLE;
       else
-	{
-	  __seterrno ();
-	  res = -1;
-	}
+      if (clr & TIOCM_DTR)
+        dcb.fRtsControl = DTR_CONTROL_DISABLE;
+
+      if (!SetCommState(get_handle(), &dcb))
+        {
+          __seterrno ();
+          return -1;
+        }
     }
 
-  return res;
+  return 0;
 }
 
 /* ioctl: */
-- 
2.38.1.windows.1