pipes: add documentation for pipes

Stick with the name "gdb_port" even if this command
can be used for other things(disable, named pipes,
anonymous stdin/out pipe). "port" is correct for
probably more than 90% of use cases, if not more.

Signed-off-by: Øyvind Harboe <oyvind.harboe@zylin.com>

doc/openocd.texi

@@ -1910,12 +1910,29 @@ use the command line @option{-pipe} option.
 
 @deffn {Command} gdb_port [number]
 @cindex GDB server
-Specify or query the first port used for incoming GDB connections.
-The GDB port for the
-first target will be gdb_port, the second target will listen on gdb_port + 1, and so on.
+Normally gdb listens to a TCP/IP port, but GDB can also
+communicate via pipes(stdin/out or named pipes). The name
+"gdb_port" stuck because it covers probably more than 90% of
+the normal use cases.
+
+No arguments reports GDB port. "pipe" means listen to stdin 
+output to stdout, an integer is base port number, "disable"
+disables the gdb server.
+
+When using "pipe", also use log_output to redirect the log
+output to a file so as not to flood the stdin/out pipes.
+
+The -p/--pipe option is deprecated and a warning is printed
+as it is equivalent to passing in -c "gdb_port pipe; log_output openocd.log".
+
+Any other string is interpreted as named pipe to listen to. 
+Output pipe is the same name as input pipe, but with 'o' appended,
+e.g. /var/gdb, /var/gdbo.
+				
+The GDB port for the first target will be the base port, the 
+second target will listen on gdb_port + 1, and so on.
 When not specified during the configuration stage,
 the port @var{number} defaults to 3333.
-When specified as zero, GDB remote access ports are not activated.
 @end deffn
 
 @deffn {Command} tcl_port [number]
@@ -1925,7 +1942,7 @@ output from the Tcl engine.
 Intended as a machine interface.
 When not specified during the configuration stage,
 the port @var{number} defaults to 6666.
-When specified as zero, this port is not activated.
+
 @end deffn
 
 @deffn {Command} telnet_port [number]

src/server/gdb_server.c

@@ -2545,9 +2545,13 @@ static const struct command_registration gdb_command_handlers[] = {
 		.name = "gdb_port",
 		.handler = handle_gdb_port_command,
 		.mode = COMMAND_ANY,
-		.help = "Display or specify base port on which to listen "
-			"for incoming GDB connections.  "
-			"No arguments reports GDB port; zero disables.",
+		.help = "Normally gdb listens to a TCP/IP port. Each subsequent GDB "
+				"server listens for the next port number after the "
+				"base port number specified. "
+				"No arguments reports GDB port. \"pipe\" means listen to stdin "
+				"output to stdout, an integer is base port number, \"disable\" disables "
+				"port. Any other string is are interpreted as named pipe to listen to. "
+				"Output pipe is the same name as input pipe, but with 'o' appended.",
 		.usage = "[port_num]",
 	},
 	{

src/server/tcl_server.c

@@ -183,7 +183,7 @@ static const struct command_registration tcl_command_handlers[] = {
 		.mode = COMMAND_CONFIG,
 		.help = "Specify port on which to listen "
 			"for incoming Tcl syntax.  "
-			"No arguments reports Tcl port; zero disables.",
+			"Read help on 'gdb_port'.",
 		.usage = "[port_num]",
 	},
 	COMMAND_REGISTRATION_DONE

src/server/telnet_server.c

@@ -619,7 +619,7 @@ static const struct command_registration telnet_command_handlers[] = {
 		.mode = COMMAND_ANY,
 		.help = "Specify port on which to listen "
 			"for incoming telnet connections.  "
-			"No arguments reports telnet port; zero disables.",
+			"Read help on 'gdb_port'.",
 		.usage = "[port_num]",
 	},
 	COMMAND_REGISTRATION_DONE