drivers: display: mcux_elcdif: rework frame completion logic

Rework the frame completion logic for the eLCDIF. Per the documentation
of the eLCDIF frame completion interrupt, the interrupt will be asserted
at every vertical blanking interval. In order to be sure the eLCDIF has
streamed a full frame we need to:
- wait for the start of a vertical blanking interval
- wait for the next vertical blanking interval, as that indicates a full
  frame was rendered

In order to do this, wait for two frame completion interrupts from the
eLCDIF every time we reload the framebuffer. The first interrupt signals
that our new frame is now being displayed, the second interrupt signals
it is fully rendered and the buffer is no longer needed by the eLCDIF.

Fixes #80590

Signed-off-by: Daniel DeGrasse <[email protected]>

Author: danieldegrasse

drivers/display/Kconfig.mcux_elcdif

@@ -54,18 +54,6 @@ config MCUX_ELCDIF_PXP
 	  display_write is called with a framebuffer equal in size to the
 	  display.
 
-config MCUX_ELCDIF_LP
-	bool "ELCDIF low power"
-	help
-	  This option, when enabled, will enable CUR_FRAME_DONE_IRQ at the display
-	  write function and disable it at the interruption handler for each new frame.
-	  Disabling the interrupt when no new frame needs to be sent gives the CPU the
-	  possibility to enter low-power mode, thus saving energy.
-	  This option, when disabled, CUR_FRAME_DONE_IRQ will be enabled only
-	  once at initialization. This option should be disabled when the application's
-	  frame rate is close to the display's refresh rate to avoid introducing
-	  additional latency caused by frequently enabling and disabling CUR_FRAME_DONE_IRQ.
-
 if MCUX_ELCDIF_PXP
 
 choice MCUX_ELCDIF_PXP_ROTATE_DIRECTION

drivers/display/display_mcux_elcdif.c

@@ -50,7 +50,7 @@ struct mcux_elcdif_data {
 	size_t pixel_bytes;
 	size_t fb_bytes;
 	elcdif_rgb_mode_config_t rgb_mode;
-	struct k_sem sem;
+	struct k_sem vblank_sem;
 	/* Tracks index of next active driver framebuffer */
 	uint8_t next_idx;
 #ifdef CONFIG_MCUX_ELCDIF_PXP
@@ -214,12 +214,19 @@ static int mcux_elcdif_write(const struct device *dev, const uint16_t x, const u
 	/* Update index of active framebuffer */
 	dev_data->next_idx = (dev_data->next_idx + 1) % CONFIG_MCUX_ELCDIF_FB_NUM;
 #endif
-
-	if (IS_ENABLED(CONFIG_MCUX_ELCDIF_LP)) {
-		ELCDIF_EnableInterrupts(config->base, kELCDIF_CurFrameDoneInterruptEnable);
-	}
-	/* Wait for frame send to complete */
-	k_sem_take(&dev_data->sem, K_FOREVER);
+	/*
+	 * In order to be sure we have rendered the framebuffer, we need to
+	 * wait a full vertical blank interval. Enable the ELCDIF
+	 * frame done interrupt (which fires at each vertical blank),
+	 * and wait for the vblank_sem to be posted to twice
+	 */
+	ELCDIF_EnableInterrupts(config->base, kELCDIF_CurFrameDoneInterruptEnable);
+	/* Wait for first vblank */
+	k_sem_take(&dev_data->vblank_sem, K_FOREVER);
+	/* Wait for second vblank */
+	k_sem_take(&dev_data->vblank_sem, K_FOREVER);
+	/* Disable vblank interrupt */
+	ELCDIF_DisableInterrupts(config->base, kELCDIF_CurFrameDoneInterruptEnable);
 	return ret;
 }
 
@@ -309,14 +316,8 @@ static void mcux_elcdif_isr(const struct device *dev)
 
 	status = ELCDIF_GetInterruptStatus(config->base);
 	ELCDIF_ClearInterruptStatus(config->base, status);
-	if (config->base->CUR_BUF == ((uint32_t)dev_data->active_fb)) {
-		if (IS_ENABLED(CONFIG_MCUX_ELCDIF_LP)) {
-			/* Disable frame completion interrupt if Low power mode is activated*/
-			ELCDIF_DisableInterrupts(config->base, kELCDIF_CurFrameDoneInterruptEnable);
-		}
-		/* Post to sem to notify that frame display is complete.*/
-		k_sem_give(&dev_data->sem);
-	}
+	/* Post to sem to notify that vertical blank occurred */
+	k_sem_give(&dev_data->vblank_sem);
 }
 
 static int mcux_elcdif_init(const struct device *dev)
@@ -335,7 +336,7 @@ static int mcux_elcdif_init(const struct device *dev)
 		return err;
 	}
 
-	k_sem_init(&dev_data->sem, 0, 1);
+	k_sem_init(&dev_data->vblank_sem, 0, 1);
 #ifdef CONFIG_MCUX_ELCDIF_PXP
 	k_sem_init(&dev_data->pxp_done, 0, 1);
 	if (!device_is_ready(config->pxp)) {
@@ -352,9 +353,6 @@ static int mcux_elcdif_init(const struct device *dev)
 	dev_data->active_fb = dev_data->fb[0];
 
 	ELCDIF_RgbModeInit(config->base, &dev_data->rgb_mode);
-	if (!IS_ENABLED(CONFIG_MCUX_ELCDIF_LP)) {
-		ELCDIF_EnableInterrupts(config->base, kELCDIF_CurFrameDoneInterruptEnable);
-	}
 	ELCDIF_RgbModeStart(config->base);
 
 	return 0;