tty: serial: introduce transmit helper generators

Many serial drivers do the same thing:
* send x_char if set
* keep sending from the xmit circular buffer until either
  - the loop reaches the end of the xmit buffer
  - TX is stopped
  - HW fifo is full
* check for pending characters and:
  - wake up tty writers to fill for more data into xmit buffer
  - stop TX if there is nothing in the xmit buffer

The only differences are:
* how to write the character to the HW fifo
* the check of the end condition:
  - is the HW fifo full?
  - is limit of the written characters reached?

So unify the above into two helper generators:
* DEFINE_UART_PORT_TX_HELPER_LIMITED() -- it performs the above taking
  the written characters limit into account, and
* DEFINE_UART_PORT_TX_HELPER() -- the same as above, except it only
  checks the HW readiness, not the characters limit.

The HW specific operations (as stated as "differences" above) are passed
as arguments to the macros. They are:
* tx_ready() -- returns true if HW can accept more data.
* put_char() -- write a character to the device.
* tx_done() -- when the write loop is done, perform arbitrary action
  before potential invocation of ops->stop_tx() happens.

Note that the above macros are generators. This means the code is
generated in place and the above 3 arguments are "inlined". I.e. no
added penalty by generating call instructions for every single
character. Nor any indirect calls. (As in previous versions of this
patchset.)

Signed-off-by: Jiri Slaby <jslaby@suse.cz>

Documentation/driver-api/serial/driver.rst

@@ -78,6 +78,9 @@ Other functions
            uart_get_lsr_info uart_handle_dcd_change uart_handle_cts_change
            uart_try_toggle_sysrq uart_get_console
 
+.. kernel-doc:: include/linux/serial_core.h
+   :identifiers: DEFINE_UART_PORT_TX_HELPER_LIMITED DEFINE_UART_PORT_TX_HELPER
+
 Other notes
 -----------
 

include/linux/serial_core.h

@@ -638,6 +638,92 @@ struct uart_driver {
 
 void uart_write_wakeup(struct uart_port *port);
 
+#define __DEFINE_UART_PORT_TX_HELPER(name, port, ch, tx_ready, put_char,  \
+		tx_done, for_test, for_post, ...)			  \
+unsigned int name(struct uart_port *port __VA_OPT__(,) __VA_ARGS__)	  \
+{									  \
+	struct circ_buf *xmit = &port->state->xmit;			  \
+	unsigned int pending;						  \
+	u8 ch;								  \
+									  \
+	for (; (for_test) && (tx_ready); (for_post), port->icount.tx++) { \
+		if (port->x_char) {					  \
+			ch = port->x_char;				  \
+			(put_char);					  \
+			port->x_char = 0;				  \
+			continue;					  \
+		}							  \
+									  \
+		if (uart_circ_empty(xmit) || uart_tx_stopped(port))	  \
+			break;						  \
+									  \
+		ch = xmit->buf[xmit->tail];				  \
+		(put_char);						  \
+		xmit->tail = (xmit->tail + 1) % UART_XMIT_SIZE;		  \
+	}								  \
+									  \
+	(tx_done);							  \
+									  \
+	pending = uart_circ_chars_pending(xmit);			  \
+	if (pending < WAKEUP_CHARS) {					  \
+		uart_write_wakeup(port);				  \
+									  \
+		if (pending == 0)					  \
+			port->ops->stop_tx(port);			  \
+	}								  \
+									  \
+	return pending;							  \
+}
+
+/**
+ * DEFINE_UART_PORT_TX_HELPER_LIMITED -- generate transmit helper for uart_port
+ *	with count limiting
+ * @name: name of the helper to generate
+ * @port: name of variable holding uart_port
+ * @ch: name of variable holding a character to write
+ * @tx_ready: can HW accept more data function
+ * @put_char: function to write a character
+ * @tx_done: function to call after the loop is done
+ *
+ * This macro generates a function @name. The generated function is meant as a
+ * helper to transmit characters from the xmit buffer to the hardware using
+ * @put_char(). It does so until count (passed to @name) characters are sent
+ * and while @tx_ready() still returns non-zero (if non-NULL).
+ *
+ * The generated function returns the number of characters in the xmit buffer
+ * when done.
+ *
+ * The functions in parameters shall be designed as follows:
+ *  * **tx_ready(port):** the function shall return true if the HW can accept
+ *    more data to be sent. This function can be %NULL, which means the HW is
+ *    always ready.
+ *  * **put_char(port, ch):** the function shall write @ch to the device.
+ *  * **tx_done(port):** when the write loop is done, this function can
+ *    perform arbitrary action before potential invocation of ops->stop_tx()
+ *    happens. This function can be %NULL.
+ *
+ * For all of them, @port->lock is held, interrupts are locally disabled and
+ * the functions must not sleep.
+ */
+#define DEFINE_UART_PORT_TX_HELPER_LIMITED(name, port, ch, tx_ready, put_char, \
+		                tx_done)				       \
+	__DEFINE_UART_PORT_TX_HELPER(name, port, ch, tx_ready, put_char,       \
+			tx_done, count, count--, unsigned int count)
+
+/**
+ * DEFINE_UART_PORT_TX_HELPER -- generate transmit helper for uart_port
+ * @name: name of the helper to generate
+ * @port: name of variable holding uart_port
+ * @ch: name of variable holding a character to write
+ * @tx_ready: can HW accept more data function
+ * @put_char: function to write a character
+ *
+ * See DEFINE_UART_PORT_TX_HELPER_LIMITED() for more details.
+ */
+#define DEFINE_UART_PORT_TX_HELPER(name, port, ch, tx_ready, put_char)	 \
+	__DEFINE_UART_PORT_TX_HELPER(name, port, ch, tx_ready, put_char, \
+			({}), true, ({}))
+
 /*
  * Baud rate helpers.
  */