Dual LVDS output (available on the SN65DSI84) requires HSYNC_PULSE_WIDTH
and HORIZONTAL_BACK_PORCH to be divided by two with respect to the values
used for single LVDS output.

While not clearly stated in the datasheet, this is needed according to the
DSI Tuner [0] output. It also makes sense intuitively because in dual LVDS
output two pixels at a time are output and so the output clock is half of
the pixel clock.

Some dual-LVDS panels refuse to show any picture without this fix.

Divide by two HORIZONTAL_FRONT_PORCH too, even though this register is used
only for test pattern generation which is not currently implemented by this
driver.

[0] https://www.ti.com/tool/DSI-TUNER

Fixes: ceb515ba29ba ("drm/bridge: ti-sn65dsi83: Add TI SN65DSI83 and SN65DSI84 driver")
Cc: stable@vger.kernel.org
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
---
 drivers/gpu/drm/bridge/ti-sn65dsi83.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/bridge/ti-sn65dsi83.c b/drivers/gpu/drm/bridge/ti-sn65dsi83.c
index d2a81175d279..17a885244e1e 100644
--- a/drivers/gpu/drm/bridge/ti-sn65dsi83.c
+++ b/drivers/gpu/drm/bridge/ti-sn65dsi83.c
@@ -517,6 +517,7 @@ static void sn65dsi83_atomic_pre_enable(struct drm_bridge *bridge,
 					struct drm_atomic_state *state)
 {
 	struct sn65dsi83 *ctx = bridge_to_sn65dsi83(bridge);
+	const unsigned int dual_factor = ctx->lvds_dual_link ? 2 : 1;
 	const struct drm_bridge_state *bridge_state;
 	const struct drm_crtc_state *crtc_state;
 	const struct drm_display_mode *mode;
@@ -653,18 +654,18 @@ static void sn65dsi83_atomic_pre_enable(struct drm_bridge *bridge,
 	/* 32 + 1 pixel clock to ensure proper operation */
 	le16val = cpu_to_le16(32 + 1);
 	regmap_bulk_write(ctx->regmap, REG_VID_CHA_SYNC_DELAY_LOW, &le16val, 2);
-	le16val = cpu_to_le16(mode->hsync_end - mode->hsync_start);
+	le16val = cpu_to_le16((mode->hsync_end - mode->hsync_start) / dual_factor);
 	regmap_bulk_write(ctx->regmap, REG_VID_CHA_HSYNC_PULSE_WIDTH_LOW,
 			  &le16val, 2);
 	le16val = cpu_to_le16(mode->vsync_end - mode->vsync_start);
 	regmap_bulk_write(ctx->regmap, REG_VID_CHA_VSYNC_PULSE_WIDTH_LOW,
 			  &le16val, 2);
 	regmap_write(ctx->regmap, REG_VID_CHA_HORIZONTAL_BACK_PORCH,
-		     mode->htotal - mode->hsync_end);
+		     (mode->htotal - mode->hsync_end) / dual_factor);
 	regmap_write(ctx->regmap, REG_VID_CHA_VERTICAL_BACK_PORCH,
 		     mode->vtotal - mode->vsync_end);
 	regmap_write(ctx->regmap, REG_VID_CHA_HORIZONTAL_FRONT_PORCH,
-		     mode->hsync_start - mode->hdisplay);
+		     (mode->hsync_start - mode->hdisplay) / dual_factor);
 	regmap_write(ctx->regmap, REG_VID_CHA_VERTICAL_FRONT_PORCH,
 		     mode->vsync_start - mode->vdisplay);
 	regmap_write(ctx->regmap, REG_VID_CHA_TEST_PATTERN, 0x00);

-- 
2.53.0

On 2/26/26 5:16 PM, Luca Ceresoli wrote:
> Dual LVDS output (available on the SN65DSI84) requires HSYNC_PULSE_WIDTH
> and HORIZONTAL_BACK_PORCH to be divided by two with respect to the values
> used for single LVDS output.
> 
> While not clearly stated in the datasheet, this is needed according to the
> DSI Tuner [0] output. It also makes sense intuitively because in dual LVDS
> output two pixels at a time are output and so the output clock is half of
> the pixel clock.
> 
> Some dual-LVDS panels refuse to show any picture without this fix.
> 
> Divide by two HORIZONTAL_FRONT_PORCH too, even though this register is used
> only for test pattern generation which is not currently implemented by this
> driver.

Reviewed-by: Marek Vasut <marek.vasut@mailbox.org>

Thanks !