[PATCH 2/4] drm/sun4i: dsi: Change the start delay calculation

[Maxime Ripard]

The current calculation for the video start delay in the current DSI driver
is that it is the total vertical size, minus the backporch and sync length,
plus 1.

However, the Allwinner code has it as the active vertical size, plus the
back porch and the sync length. This doesn't make any difference on the
only panel it has been tested with so far, since in that particular case
the front porch is equal to the sum of the back porch and sync length.

This is not the case for all panels, obviously, so we need to fix it. Since
the Allwinner code has a bunch of extra code to deal with out of bounds
values, so let's add them as well.

Signed-off-by: Maxime Ripard <maxime.ripard at bootlin.com>
---
 drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c b/drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c
index 380fc527a707..e3e4ba90c059 100644
--- a/drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c
+++ b/drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c
@@ -357,7 +357,12 @@ 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;
+	u16 delay = (mode->vsync_end + 1) % mode->vtotal;
+
+	if (!delay)
+		delay = 1;
+
+	return delay;
 }
 
 static void sun6i_dsi_setup_burst(struct sun6i_dsi *dsi,
-- 
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