tty: implement TCFLSH to flush buffers

This is probably incomplete. Is locking needed? Should we notify the
next writer (if any) that the port is writable when flushing the output?

Change-Id: I2566e2d036a61af4819894a44f57603179aa27df
Reviewed-on: https://review.haiku-os.org/c/haiku/+/2516
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>

headers/os/drivers/tty/tty_module.h

@@ -39,6 +39,7 @@ typedef bool (*tty_service_func)(struct tty *tty, uint32 op, void *buffer,
 #define	TTYSETDTR		6	/* bool dataTerminalReady */
 #define TTYSETRTS		7	/* bool requestToSend */
 #define	TTYGETSIGNALS	8	/* call tty_hardware_signal for all bits */
+#define TTYFLUSH		9	/* clear input and/or output buffers */
 
 typedef struct tty_module_info tty_module_info;
 

src/add-ons/kernel/drivers/ports/pc_serial/SerialDevice.cpp

@@ -359,6 +359,19 @@ SerialDevice::Service(struct tty *tty, uint32 op, void *buffer, size_t length)
 
 			return true;
 		}
+		case TTYFLUSH:
+		{
+			int directions = *(int*)buffer;
+			uint8 mask = 0;
+			if (directions & TCIFLUSH)
+				mask |= FCR_RX_RST;
+			if (directions & TCOFLUSH)
+				mask |= FCR_TX_RST;
+			// These bits clear themselves when flushing is complete
+			OrReg8(FCR, mask);
+
+			return true;
+		}
 		default:
 			return false;
 	}

src/add-ons/kernel/drivers/ports/usb_serial/SerialDevice.cpp

@@ -274,6 +274,7 @@ SerialDevice::Service(struct tty *tty, uint32 op, void *buffer, size_t length)
 		case TTYOSTART:
 		case TTYOSYNC:
 		case TTYSETBREAK:
+		case TTYFLUSH:
 			TRACE("TTY other\n");
 			return true;
 	}

src/add-ons/kernel/generic/tty/tty.cpp

@@ -1810,6 +1810,28 @@ tty_control(tty_cookie* cookie, uint32 op, void* buffer, size_t length)
 
 			return B_ERROR;
 		}
+
+		case TCFLSH:
+		{
+			int value;
+			if (user_memcpy(&value, buffer, sizeof(value)) != B_OK)
+				return B_BAD_ADDRESS;
+			if (value & TCOFLUSH) {
+				struct tty* otherTTY = cookie->other_tty;
+				if (otherTTY->open_count <= 0)
+					return B_ERROR;
+
+				clear_line_buffer(otherTTY->input_buffer);
+			}
+
+			if (value & TCIFLUSH)
+				clear_line_buffer(tty->input_buffer);
+
+			if (tty->service_func(tty, TTYFLUSH, &value, sizeof(value)))
+				return B_OK;
+
+			return B_ERROR;
+		}
 	}
 
 	TRACE(("tty: unsupported opcode %lu\n", op));