[newlib-cygwin] Cygwin: fix TIOCMBIS/TIOCMBIC not working when using usbser.sys

[Corinna Vinschen]

https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;h=2dab880c963ce0204c3513d664f610b587a3e6a6

commit 2dab880c963ce0204c3513d664f610b587a3e6a6
Author: Carlo Bramini <30959007+carlo-bramini@users.noreply.github.com>
Date:   Fri Nov 11 11:15:27 2022 +0100

    Cygwin: fix TIOCMBIS/TIOCMBIC not working when using usbser.sys
    
    In winsup/cygwin/fhandler/serial.cc, the function
    fhandler_serial::switch_modem_lines() is called when TIOCMBIS/TIOCMBIC
    are used in 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 CYGWIN but an issue of the usbser.sys driver,
    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.

Diff:
---
 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: */