arch/sim: Add PTY mode for simulated UART

Documentation/platforms/sim/sim/boards/sim/index.rst

@@ -2173,10 +2173,23 @@ nxscope
 
 Configuration demonstrating NxScope stream over simulated UART interface.
 
-The simulated UART must be created on host before running NuttX::
+If ``CONFIG_SIM_UART_PTY`` is disabled, the simulated UART peer must be
+created on the host before running NuttX::
 
   socat PTY,link=/dev/ttySIM0 PTY,link=/dev/ttyNX0
 
+In that mode ``CONFIG_SIM_UART0_NAME`` is both the NuttX device name and
+the host path that the sim UART backend opens.  This works when the host
+path is stable, but it is inconvenient for automated tests and generated
+PTYs: the peer device must exist before the UART is opened, and changing
+the host path requires changing the NuttX configuration.
+
+If ``CONFIG_SIM_UART_PTY`` is enabled, NuttX creates the host
+pseudoterminal when the simulated UART is opened and prints the host
+PTY slave path.  The configured ``SIM_UARTx_NAME`` remains the NuttX
+device name, while the host PTY path is allocated at runtime and can be
+passed to the external simulator or test program.
+
 See :doc:`/applications/examples/nxscope/index` and
 :doc:`/applications/logging/nxscope/index` for more details.
 

arch/sim/Kconfig

@@ -774,6 +774,11 @@ config SIM_UART_NUMBER
 		Anything sent to the one of these ports will be relayed
 		automatically to the other, and vice-versa.
 
+		If SIM_UART_PTY is enabled, NuttX creates the simulated
+		host pseudoterminal itself.  In that mode SIM_UARTx_NAME is
+		only the NuttX device name, and the host pseudoterminal path
+		is printed when the port is opened.
+
 config SIM_UART_BUFFER_SIZE
 	int "UART buffer size"
 	default 256
@@ -784,49 +789,80 @@ config SIM_UART_BUFFER_SIZE
 
 		Note that all ports will have the same buffer size.
 
+config SIM_UART_PTY
+	bool "Create host pseudoterminals for simulated UART ports"
+	default n
+	depends on HOST_LINUX && SIM_UART_NUMBER >= 1
+	---help---
+		Open a new host pseudoterminal for each simulated UART
+		instead of opening an existing host serial device.  The
+		UART remains registered in NuttX using SIM_UARTx_NAME, and
+		the host PTY slave path is printed when the port is opened.
+
+		The existing behavior requires the host endpoint named by
+		SIM_UARTx_NAME to exist before the simulated UART is opened.
+		This makes automated tests awkward when the peer endpoint is
+		a generated PTY with a host-dependent path, because the host
+		path must be prepared up front and encoded in the NuttX
+		configuration.  With this option enabled, the sim UART
+		backend owns the host PTY allocation and reports the actual
+		slave path at runtime, while keeping the NuttX device name
+		stable for applications.
+
+		If disabled, SIM UART keeps the existing behavior and opens
+		the host device path specified by SIM_UARTx_NAME directly.
+
 config SIM_UART0_NAME
 	string "UART port 0 name"
 	default "/dev/ttySIM0"
 	depends on SIM_UART_NUMBER >= 1
 	---help---
 		This is the name of the simulated UART port.
-		The port will be mounted in NuttX under this name.
+		The port will be registered in NuttX under this name.
 
-		A UART port must also exist on the host system
-		with the exact same name specified here.
+		If SIM_UART_PTY is disabled, a UART port must also exist on
+		the host system with the exact same name specified here.  If
+		SIM_UART_PTY is enabled, the host pseudoterminal is allocated
+		automatically and printed when this port is opened.
 
 config SIM_UART1_NAME
 	string "UART port 1 name"
 	default "/dev/ttySIM1"
 	depends on SIM_UART_NUMBER >= 2
 	---help---
 		This is the name of the simulated UART port.
-		The port will be mounted in NuttX under this name.
+		The port will be registered in NuttX under this name.
 
-		A UART port must also exist on the host system
-		with the exact same name specified here.
+		If SIM_UART_PTY is disabled, a UART port must also exist on
+		the host system with the exact same name specified here.  If
+		SIM_UART_PTY is enabled, the host pseudoterminal is allocated
+		automatically and printed when this port is opened.
 
 config SIM_UART2_NAME
 	string "UART port 2 name"
 	default "/dev/ttySIM2"
 	depends on SIM_UART_NUMBER >= 3
 	---help---
 		This is the name of the simulated UART port.
-		The port will be mounted in NuttX under this name.
+		The port will be registered in NuttX under this name.
 
-		A UART port must also exist on the host system
-		with the exact same name specified here.
+		If SIM_UART_PTY is disabled, a UART port must also exist on
+		the host system with the exact same name specified here.  If
+		SIM_UART_PTY is enabled, the host pseudoterminal is allocated
+		automatically and printed when this port is opened.
 
 config SIM_UART3_NAME
 	string "UART port 3 name"
 	default "/dev/ttySIM3"
 	depends on SIM_UART_NUMBER >= 4
 	---help---
 		This is the name of the simulated UART port.
-		The port will be mounted in NuttX under this name.
+		The port will be registered in NuttX under this name.
 
-		A UART port must also exist on the host system
-		with the exact same name specified here.
+		If SIM_UART_PTY is disabled, a UART port must also exist on
+		the host system with the exact same name specified here.  If
+		SIM_UART_PTY is enabled, the host pseudoterminal is allocated
+		automatically and printed when this port is opened.
 
 config SIM_RAM_UART
 	bool "SIM RAM UART Device"

arch/sim/src/sim/posix/sim_hostuart.c

@@ -24,6 +24,7 @@
  * Included Files
  ****************************************************************************/
 
+#include <sys/ioctl.h>
 #include <fcntl.h>
 #include <stdbool.h>
 #include <unistd.h>
@@ -130,6 +131,58 @@ int host_uart_open(const char *pathname)
   return fd;
 }
 
+/****************************************************************************
+ * Name: host_uart_openpty
+ ****************************************************************************/
+
+int host_uart_openpty(const char *name)
+{
+#ifdef CONFIG_HOST_LINUX
+  unsigned int ptyno;
+  int lock = 0;
+  int oflags;
+  int ret;
+  int fd;
+
+  oflags = O_RDWR | O_NOCTTY | O_NONBLOCK;
+#ifdef O_CLOEXEC
+  oflags |= O_CLOEXEC;
+#endif
+  fd = open("/dev/ptmx", oflags);
+  if (fd < 0)
+    {
+      return -errno;
+    }
+
+  ret = ioctl(fd, TIOCGPTN, &ptyno);
+  if (ret < 0)
+    {
+      ret = -errno;
+      goto errout;
+    }
+
+  ret = ioctl(fd, TIOCSPTLCK, &lock);
+  if (ret < 0)
+    {
+      ret = -errno;
+      goto errout;
+    }
+
+  setrawmode(fd);
+
+  printf("%s connected to pseudotty: /dev/pts/%u\n", name, ptyno);
+
+  return fd;
+
+errout:
+  close(fd);
+  return ret;
+#else
+  (void)name;
+  return -ENOSYS;
+#endif
+}
+
 /****************************************************************************
  * Name: host_uart_close
  ****************************************************************************/
@@ -229,7 +282,7 @@ bool host_uart_checkin(int fd)
 
   pfd.fd     = fd;
   pfd.events = POLLIN;
-  return poll(&pfd, 1, 0) == 1;
+  return poll(&pfd, 1, 0) == 1 && (pfd.revents & POLLIN) != 0;
 }
 
 /****************************************************************************
@@ -242,7 +295,7 @@ bool host_uart_checkout(int fd)
 
   pfd.fd     = fd;
   pfd.events = POLLOUT;
-  return poll(&pfd, 1, 0) == 1;
+  return poll(&pfd, 1, 0) == 1 && (pfd.revents & POLLOUT) != 0;
 }
 
 /****************************************************************************

arch/sim/src/sim/sim_internal.h

@@ -284,6 +284,7 @@ void sim_uartinit(void);
 
 void host_uart_start(void);
 int  host_uart_open(const char *pathname);
+int  host_uart_openpty(const char *name);
 void host_uart_close(int fd);
 int  host_uart_puts(int fd, const char *buf, size_t size);
 int  host_uart_gets(int fd, char *buf, size_t size);

arch/sim/src/sim/sim_uart.c

@@ -307,17 +307,26 @@ static void uart_nputs(int fd, const char *buf, size_t size)
  * Name: tty_setup
  *
  * Description:
- *   Configure the UART baud, bits, parity, fifos, etc. This
- *   method is called the first time that the serial port is
- *   opened.
+ *   Open and configure the host endpoint for the simulated UART.
+ *   This method is called the first time that the serial port is opened.
  *
  ****************************************************************************/
 
 static int tty_setup(struct uart_dev_s *dev)
 {
   struct tty_priv_s *priv = dev->priv;
 
-  priv->fd = host_uart_open(priv->path);
+#ifdef CONFIG_SIM_UART_PTY
+  if (!dev->isconsole)
+    {
+      priv->fd = host_uart_openpty(priv->path);
+    }
+  else
+#endif
+    {
+      priv->fd = host_uart_open(priv->path);
+    }
+
   return priv->fd;
 }
 

arch/sim/src/sim/win/sim_hostuart.c

@@ -74,6 +74,16 @@ int host_uart_open(const char *pathname)
   return -ENOSYS;
 }
 
+/****************************************************************************
+ * Name: host_uart_openpty
+ ****************************************************************************/
+
+int host_uart_openpty(const char *name)
+{
+  (void)name;
+  return -ENOSYS;
+}
+
 /****************************************************************************
  * Name: host_uart_close
  ****************************************************************************/