[PATCH 6/6] video: imx-lcdif: drain write-combine buffer before LCDIF DMA reads pixels

[Johannes Schneider]

From: Thomas Haemmerle <thomas.haemmerle at leica-geosystems.com>

The LCDIF framebuffer is allocated as Normal Non-Cacheable (write-combine)
memory.  After the splash command calls gu_screen_blit(), which copies the
decoded PNG via memcpy() from a cached shadow buffer into the hardware
framebuffer, writes may still reside in the CPU write buffer and not yet
be visible to the LCDIF DMA engine.  On hardware this manifests as
partial or corrupted rendering — the bottom portion of the splash image
renders black or shows garbage.

Implement fb_damage() to execute a DSB SY barrier after each blit,
ensuring all pending write-combine writes are committed to DRAM before
the LCDIF reads the pixel data.

Assisted-by: Claude:claude-sonnet-4-6
Signed-of-by: Thomas Haemmerle <thomas.haemmerle at leica-geosystems.com>
---
 drivers/video/imx-lcdif.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/drivers/video/imx-lcdif.c b/drivers/video/imx-lcdif.c
index ae5976c771..8d9b40be0f 100644
--- a/drivers/video/imx-lcdif.c
+++ b/drivers/video/imx-lcdif.c
@@ -20,6 +20,7 @@
 #include <of_graph.h>
 #include <video/media-bus-format.h>
 #include <video/vpl.h>
+#include <asm/system.h>
 
 /* LCDIF V8 register map */
 #define LCDC_V8_CTRL			0x00
@@ -262,9 +263,21 @@ static void lcdif_fb_disable(struct fb_info *info)
 	clk_disable_unprepare(priv->clk_axi);
 }
 
+static void lcdif_fb_damage(struct fb_info *info, struct fb_rect *rect)
+{
+	/*
+	 * The hardware framebuffer is Normal Non-Cacheable (write-combine).
+	 * Writes from the shadow-buffer memcpy may sit in the CPU write buffer
+	 * and not yet be visible to the LCDIF DMA engine. Execute DSB SY to
+	 * drain the write buffer to DRAM before the LCDIF reads the pixels.
+	 */
+	dsb();
+}
+
 static struct fb_ops lcdif_fb_ops = {
 	.fb_enable  = lcdif_fb_enable,
 	.fb_disable = lcdif_fb_disable,
+	.fb_damage  = lcdif_fb_damage,
 };
 
 static int lcdif_probe(struct device *dev)
-- 
2.43.0