[PATCH 5.15.y] drm/bridge: ti-sn65dsi83: halve horizontal syncs for dual LVDS output

[Sasha Levin <sashal@kernel.org>]

From: Luca Ceresoli <luca.ceresoli@bootlin.com>

[ Upstream commit d0d727746944096a6681dc6adb5f123fc5aa018d ]

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
Reviewed-by: Marek Vasut <marek.vasut@mailbox.org>
Link: https://patch.msgid.link/20260226-ti-sn65dsi83-dual-lvds-fixes-and-test-pattern-v1-2-2e15f5a9a6a0@bootlin.com
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
[ adapted variable declaration placement ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 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 f96c0a89854b8..8fa43096bf32a 100644
--- a/drivers/gpu/drm/bridge/ti-sn65dsi83.c
+++ b/drivers/gpu/drm/bridge/ti-sn65dsi83.c
@@ -370,6 +370,7 @@ static void sn65dsi83_atomic_enable(struct drm_bridge *bridge,
 				    struct drm_bridge_state *old_bridge_state)
 {
 	struct sn65dsi83 *ctx = bridge_to_sn65dsi83(bridge);
+	const unsigned int dual_factor = ctx->lvds_dual_link ? 2 : 1;
 	struct drm_atomic_state *state = old_bridge_state->base.state;
 	const struct drm_bridge_state *bridge_state;
 	const struct drm_crtc_state *crtc_state;
@@ -489,18 +490,18 @@ static void sn65dsi83_atomic_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);
-- 
2.51.0