[PATCH v8 7/8] drm/sun4i: sun6i_mipi_dsi: Refactor vertical video start delay

[Jagan Teki <jagan@xxxxxxxxxxxxxxxxxxxx>]

Vertical video start delay is computed by excluding vertical porch
value from total vertical timings, but the current driver excluding
vertical porch along with vertical sync values from total vertical
timings resulting wrong start delay.

This patch trying to update the video start delay by subtracting
vertical porch from vertical total, on the other hand it added 1
extra start_delay line for TCON based on the Allwinner BSP reference.

BSP code form BPI-M64-bsp is computing video start delay as
(from linux-sunxi/
drivers/video/sunxi/disp2/disp/de/lowlevel_sun50iw1/de_dsi.c)

u32 vfp = panel->lcd_vt - panel->lcd_y - panel->lcd_vbp;
=> (panel->lcd_vt) - panel->lcd_y - (panel->lcd_vbp)
=> (timmings->ver_front_porch + panel->lcd_vbp + panel->lcd_y)
   - panel->lcd_y - (panel->lcd_vbp)
=> timmings->ver_front_porch + panel->lcd_vbp + panel->lcd_y
  			     - panel->lcd_y - panel->lcd_vbp
=> timmings->ver_front_porch

So, update the start delay computation accordingly.

Signed-off-by: Jagan Teki <jagan@xxxxxxxxxxxxxxxxxxxx>
---
 drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c | 19 ++++++++++++++++++-
 1 file changed, 18 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c b/drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c
index a08dfdcbe9e8..31cf9c58e98d 100644
--- a/drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c
+++ b/drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c
@@ -358,7 +358,24 @@ static void sun6i_dsi_inst_init(struct sun6i_dsi *dsi,
 static u16 sun6i_dsi_get_video_start_delay(struct sun6i_dsi *dsi,
 					   struct drm_display_mode *mode)
 {
-	return mode->vtotal - (mode->vsync_end - mode->vdisplay) + 1;
+	u32 vfp, start_delay;
+
+	/* vertical front porch */
+	vfp = mode->vsync_start - mode->vdisplay;
+
+	/* start_delay = vertical total - vertical front porch */
+	start_delay = mode->vtotal - vfp;
+
+	/* add extra 1 delay line for TCON, as per Allwinner BSP */
+	start_delay = 1;
+
+	if (start_delay > mode->vtotal)
+		start_delay -= mode->vtotal;
+
+	if (!start_delay)
+		start_delay = 1;
+
+	return start_delay;
 }
 
 static void sun6i_dsi_setup_burst(struct sun6i_dsi *dsi,
-- 
2.18.0.321.gffc6fa0e3

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