sandbox: Improve sandbox serial port keyboard interface

Implements the tstc() interface for the serial driver.  Multiplexing
the console between the serial port and a keyboard uses a polling
method of checking if characters are available; this means that the
serial console must be non-blocking when attempting to read
characters.

Signed-off-by: Taylor Hutt <thutt@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>

arch/sandbox/cpu/os.c

@@ -44,6 +44,14 @@ ssize_t os_read(int fd, void *buf, size_t count)
 	return read(fd, buf, count);
 }
 
+ssize_t os_read_no_block(int fd, void *buf, size_t count)
+{
+	const int flags = fcntl(fd, F_GETFL, 0);
+
+	fcntl(fd, F_SETFL, flags | O_NONBLOCK);
+	return os_read(fd, buf, count);
+}
+
 ssize_t os_write(int fd, const void *buf, size_t count)
 {
 	return write(fd, buf, count);

drivers/serial/sandbox.c

@@ -30,6 +30,19 @@
 #include <serial.h>
 #include <linux/compiler.h>
 
+/*
+ *
+ *   serial_buf: A buffer that holds keyboard characters for the
+ *		 Sandbox U-boot.
+ *
+ * invariants:
+ *   serial_buf_write		 == serial_buf_read -> empty buffer
+ *   (serial_buf_write + 1) % 16 == serial_buf_read -> full buffer
+ */
+static char serial_buf[16];
+static unsigned int serial_buf_write;
+static unsigned int serial_buf_read;
+
 static int sandbox_serial_init(void)
 {
 	os_tty_raw(0);
@@ -50,18 +63,37 @@ static void sandbox_serial_puts(const char *str)
 	os_write(1, str, strlen(str));
 }
 
-static int sandbox_serial_getc(void)
+static unsigned int increment_buffer_index(unsigned int index)
 {
-	char buf;
-	ssize_t count;
-
-	count = os_read(0, &buf, 1);
-	return count == 1 ? buf : 0;
+	return (index + 1) % ARRAY_SIZE(serial_buf);
 }
 
 static int sandbox_serial_tstc(void)
 {
-	return 0;
+	const unsigned int next_index =
+		increment_buffer_index(serial_buf_write);
+	ssize_t count;
+
+	os_usleep(100);
+	if (next_index == serial_buf_read)
+		return 1;	/* buffer full */
+
+	count = os_read_no_block(0, &serial_buf[serial_buf_write], 1);
+	if (count == 1)
+		serial_buf_write = next_index;
+	return serial_buf_write != serial_buf_read;
+}
+
+static int sandbox_serial_getc(void)
+{
+	int result;
+
+	while (!sandbox_serial_tstc())
+		;	/* buffer empty */
+
+	result = serial_buf[serial_buf_read];
+	serial_buf_read = increment_buffer_index(serial_buf_read);
+	return result;
 }
 
 static struct serial_device sandbox_serial_drv = {

include/os.h

@@ -39,6 +39,16 @@ struct sandbox_state;
  */
 ssize_t os_read(int fd, void *buf, size_t count);
 
+/**
+ * Access to the OS read() system call with non-blocking access
+ *
+ * \param fd	File descriptor as returned by os_open()
+ * \param buf	Buffer to place data
+ * \param count	Number of bytes to read
+ * \return number of bytes read, or -1 on error
+ */
+ssize_t os_read_no_block(int fd, void *buf, size_t count);
+
 /**
  * Access to the OS write() system call
  *