[PATCH v3 2/2] drm/bridge: fsl-ldb: Allow the termination resistor to be enabled

[Fabio Estevam <festevam@gmail.com>]

From: Fabio Estevam <festevam@nabladev.com>

The LVDS Control Register (LVDS_CTRL) register has an HS_EN bit that allows
the 100 Ohm termination resistor in the chip to be enabled.

Add support to setting the HS_EN bit when the optional property
"nxp,enable-termination-resistor" is present.

The motivation for introducing this property was a custom i.MX8MP board
that was showing visual artifacts. After enabling the 100 Ohm termination
resistor the LVDS signal quality improved causing the artifacts to
disappear.

Signed-off-by: Fabio Estevam <festevam@nabladev.com>
---
Changes since v2:
- Rename variable to 'use_termination_resistor'. (Liu Ying)
- Remove clearing the LVDS_CTRL_HS_EN bit. (Liu Ying)
- Use dev->of_node. (Liu Ying)

 drivers/gpu/drm/bridge/fsl-ldb.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/gpu/drm/bridge/fsl-ldb.c b/drivers/gpu/drm/bridge/fsl-ldb.c
index 5c3cf37200bc..7b71cde173e0 100644
--- a/drivers/gpu/drm/bridge/fsl-ldb.c
+++ b/drivers/gpu/drm/bridge/fsl-ldb.c
@@ -92,6 +92,7 @@ struct fsl_ldb {
 	const struct fsl_ldb_devdata *devdata;
 	bool ch0_enabled;
 	bool ch1_enabled;
+	bool use_termination_resistor;
 };
 
 static bool fsl_ldb_is_dual(const struct fsl_ldb *fsl_ldb)
@@ -212,6 +213,9 @@ static void fsl_ldb_atomic_enable(struct drm_bridge *bridge,
 	/* Program LVDS_CTRL */
 	reg = LVDS_CTRL_CC_ADJ(2) | LVDS_CTRL_PRE_EMPH_EN |
 	      LVDS_CTRL_PRE_EMPH_ADJ(3) | LVDS_CTRL_VBG_EN;
+
+	if (fsl_ldb->use_termination_resistor)
+		reg |= LVDS_CTRL_HS_EN;
 	regmap_write(fsl_ldb->regmap, fsl_ldb->devdata->lvds_ctrl, reg);
 
 	/* Wait for VBG to stabilize. */
@@ -340,6 +344,9 @@ static int fsl_ldb_probe(struct platform_device *pdev)
 	if (IS_ERR(panel))
 		return PTR_ERR(panel);
 
+	if (of_property_present(dev->of_node, "nxp,enable-termination-resistor"))
+		fsl_ldb->use_termination_resistor = true;
+
 	fsl_ldb->panel_bridge = devm_drm_panel_bridge_add(dev, panel);
 	if (IS_ERR(fsl_ldb->panel_bridge))
 		return PTR_ERR(fsl_ldb->panel_bridge);
-- 
2.34.1