serial: core: introduce uart_port_tx_flags()

And an enum with a flag: UART_TX_NOSTOP. To NOT call
__port->ops->stop_tx() when the circular buffer is empty. mxs-uart needs
this (see the next patch).

Signed-off-by: "Jiri Slaby (SUSE)" <jirislaby@kernel.org>
Cc: stable <stable@kernel.org>
Tested-by: Emil Kronborg <emil.kronborg@protonmail.com>
Link: https://lore.kernel.org/r/20240201105557.28043-1-jirislaby@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

include/linux/serial_core.h

@@ -748,8 +748,17 @@ struct uart_driver {
 
 void uart_write_wakeup(struct uart_port *port);
 
-#define __uart_port_tx(uport, ch, tx_ready, put_char, tx_done, for_test, \
- for_post) \
+/**
+ * enum UART_TX_FLAGS -- flags for uart_port_tx_flags()
+ *
+ * @UART_TX_NOSTOP: don't call port->ops->stop_tx() on empty buffer
+ */
+enum UART_TX_FLAGS {
+ UART_TX_NOSTOP = BIT(0),
+};
+
+#define __uart_port_tx(uport, ch, flags, tx_ready, put_char, tx_done, \
+ for_test, for_post) \
 ({ \
  struct uart_port *__port = (uport); \
  struct circ_buf *xmit = &__port->state->xmit; \
@@ -777,7 +786,7 @@ void uart_write_wakeup(struct uart_port *port);
  if (pending < WAKEUP_CHARS) { \
  uart_write_wakeup(__port); \
  \
- if (pending == 0)  \
+ if (!((flags) & UART_TX_NOSTOP) && pending == 0) \
  __port->ops->stop_tx(__port); \
  } \
  \
@@ -812,7 +821,7 @@ void uart_write_wakeup(struct uart_port *port);
  */
 #define uart_port_tx_limited(port, ch, count, tx_ready, put_char, tx_done) ({ \
  unsigned int __count = (count); \
- __uart_port_tx(port, ch, tx_ready, put_char, tx_done, __count,  \
+ __uart_port_tx(port, ch, 0, tx_ready, put_char, tx_done, __count, \
  __count--); \
 })
 
@@ -826,8 +835,21 @@ void uart_write_wakeup(struct uart_port *port);
  * See uart_port_tx_limited() for more details.
  */
 #define uart_port_tx(port, ch, tx_ready, put_char) \
- __uart_port_tx(port, ch, tx_ready, put_char, ({}), true, ({}))
+ __uart_port_tx(port, ch, 0, tx_ready, put_char, ({}), true, ({}))
+
 
+/**
+ * uart_port_tx_flags -- transmit helper for uart_port with flags
+ * @port: uart port
+ * @ch: variable to store a character to be written to the HW
+ * @flags: %UART_TX_NOSTOP or similar
+ * @tx_ready: can HW accept more data function
+ * @put_char: function to write a character
+ *
+ * See uart_port_tx_limited() for more details.
+ */
+#define uart_port_tx_flags(port, ch, flags, tx_ready, put_char) \
+ __uart_port_tx(port, ch, flags, tx_ready, put_char, ({}), true, ({}))
 /*
  * Baud rate helpers.
  */