drivers: auxdisplay: hd44780: implement busy-flag polling mechanism

HD44780 controller can indicate via busy flag whether it's finished
processing current command. This allows for faster communication between
MCU and the HD44780, as the MCU doesn't have to wait for fixed extra
period of time in case the current command is processed earlier.
Implement this functionality.

Signed-off-by: Jan Kuliga <jtkuliga@gmail.com>

drivers/auxdisplay/auxdisplay_hd44780.c

@@ -88,17 +88,75 @@ static void hd44780_pulse_enable_line(const struct device *dev)
 	k_sleep(K_NSEC(config->enable_line_fall_delay));
 }
 
-static void auxdisplay_hd44780_command(const struct device *dev, bool rs, uint8_t cmd,
-				       uint8_t mode)
+static inline void hd44780_set_rs_rw_lines(const struct device *dev, bool rs, bool rw)
+{
+	const struct auxdisplay_hd44780_config *const config = dev->config;
+
+	gpio_pin_set_dt(&config->rs_gpio, rs);
+	if (config->rw_gpio.port) {
+		gpio_pin_set_dt(&config->rw_gpio, rw);
+	}
+
+	k_sleep(K_NSEC(config->rs_line_delay));
+}
+
+static int hd44780_db_gpios_configure(const struct device *dev, uint8_t lsb_line,
+				      gpio_flags_t flags)
 {
+	const struct auxdisplay_hd44780_config *config = dev->config;
+	int rc;
+
+	for (int line = 7; line >= lsb_line; --line) {
+		rc = gpio_pin_configure_dt(&config->db_gpios[line], flags);
+		if (rc < 0) {
+			return rc;
+		}
+	}
+
+	return 0;
+}
+
+static void auxdisplay_hd44780_command(const struct device *dev, bool rs,
+				       uint8_t cmd, uint8_t mode)
+{
+	int rc;
 	const struct auxdisplay_hd44780_config *config = dev->config;
 	int8_t i = 7;
 	const int lsb_line = (mode == AUXDISPLAY_HD44780_MODE_8_BIT) ? 0 : 4;
 	int ncommands = (mode == AUXDISPLAY_HD44780_MODE_4_BIT) ? 2 : 1;
+	const bool check_busy_flag = (!config->rw_gpio.port ||
+				mode == AUXDISPLAY_HD44780_MODE_4_BIT_ONCE) ? false : true;
 
-	gpio_pin_set_dt(&config->rs_gpio, rs);
-	k_sleep(K_NSEC(config->rs_line_delay));
+	if (check_busy_flag) {
+		bool busy;
+
+		rc = hd44780_db_gpios_configure(dev, lsb_line, GPIO_INPUT | GPIO_PULL_DOWN);
+		if (rc < 0) {
+			LOG_ERR("configuration of db-gpios as inputs failed: %d", rc);
+			return;
+		}
+
+		hd44780_set_rs_rw_lines(dev, 0, 1);
+		do {
+			hd44780_pulse_enable_line(dev);
 
+			/* We don't care about the other pins. */
+			busy = gpio_pin_get_dt(&config->db_gpios[7]);
+
+			if (config->capabilities.mode == AUXDISPLAY_HD44780_MODE_4_BIT) {
+				/* In this mode we have to initiate two separate readbacks. */
+				hd44780_pulse_enable_line(dev);
+			}
+		} while (busy);
+
+		rc = hd44780_db_gpios_configure(dev, lsb_line, GPIO_OUTPUT);
+		if (rc < 0) {
+			LOG_ERR("configuration of db-gpios as outputs failed: %d", rc);
+			return;
+		}
+	}
+
+	hd44780_set_rs_rw_lines(dev, rs, 0);
 	while (ncommands--) {
 		for (int line = 7; line >= lsb_line; --line) {
 			gpio_pin_set_dt(&config->db_gpios[line], ((cmd & BIT(i)) ? 1 : 0));
@@ -107,6 +165,12 @@ static void auxdisplay_hd44780_command(const struct device *dev, bool rs, uint8_
 
 		hd44780_pulse_enable_line(dev);
 	}
+
+	if (!check_busy_flag) {
+		/* Sleep for a max execution time for a given instruction. */
+		uint16_t cmd_delay_us = (cmd == AUXDISPLAY_HD44780_CMD_CLEAR) ? 1520 : 37;
+		k_sleep(K_USEC(cmd_delay_us));
+	}
 }
 
 static void hd44780_ic_initialize(const struct device *dev)
@@ -179,17 +243,6 @@ static int auxdisplay_hd44780_init(const struct device *dev)
 		return rc;
 	}
 
-	if (config->rw_gpio.port) {
-		rc = gpio_pin_configure_dt(&config->rw_gpio, GPIO_OUTPUT);
-
-		if (rc < 0) {
-			LOG_ERR("Configuration of RW GPIO failed: %d", rc);
-			return rc;
-		}
-
-		gpio_pin_set_dt(&config->rw_gpio, 0);
-	}
-
 	rc = gpio_pin_configure_dt(&config->e_gpio, GPIO_OUTPUT);
 
 	if (rc < 0) {
@@ -222,6 +275,15 @@ static int auxdisplay_hd44780_init(const struct device *dev)
 		++i;
 	}
 
+	if (config->rw_gpio.port) {
+		rc = gpio_pin_configure_dt(&config->rw_gpio, GPIO_OUTPUT);
+
+		if (rc < 0) {
+			LOG_ERR("Configuration of RW GPIO failed: %d", rc);
+			return rc;
+		}
+	}
+
 	if (config->backlight_gpio.port) {
 		rc = gpio_pin_configure_dt(&config->backlight_gpio, GPIO_OUTPUT);