drm/stm: ltdc: update planes at next vblank to avoid partial refresh

Plane updates must be synchronized on vblank with the shadow register mechanism
to avoid partial refresh on screen.
Signed-off-by: Yannick Fertré <yannick.fertre@st.com>
Acked-by: Philippe Cornu <philippe.cornu@st.com>
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1554103457-29595-1-git-send-email-yannick.fertre@st.com

drivers/gpu/drm/stm/ltdc.c

@@ -426,8 +426,8 @@ static void ltdc_crtc_atomic_enable(struct drm_crtc *crtc,
 	/* Enable IRQ */
 	reg_set(ldev->regs, LTDC_IER, IER_RRIE | IER_FUIE | IER_TERRIE);
 
-	/* Immediately commit the planes */
-	reg_set(ldev->regs, LTDC_SRCR, SRCR_IMR);
+	/* Commit shadow registers = update planes at next vblank */
+	reg_set(ldev->regs, LTDC_SRCR, SRCR_VBR);
 
 	/* Enable LTDC */
 	reg_set(ldev->regs, LTDC_GCR, GCR_LTDCEN);