drm/bridge: lt9211: Add drive-strength-microamp DT property

Add the 'drive-strength-microamp' property to allow board DTs to configure the LT9211 LVDS TX output driver current. Sixteen discrete levels are supported, ranging from 12 uA to 36 uA. Defaults to 25 uA.

Signed-off-by Boerge Struempfel <[email protected]>

Acked-by Conor Dooley <[email protected]>

Quoted 6 lines · Boerge Struempfel

Add the 'drive-strength-microamp' property to allow board DTs to configure the LT9211 LVDS TX output driver current. Sixteen discrete levels are supported, ranging from 12 uA to 36 uA. Defaults to 25 uA.

Signed-off-by Boerge Struempfel <[email protected]>

Acked-by Conor Dooley <[email protected]>

AB goes before SoB line.

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] The values for drive-strength-microamp are scaled incorrectly by a factor of 100, violating the standard device tree unit suffix. --

commit ab265d46e886455894986f313701d72a9407a58e
Author: Boerge Struempfel <[email protected]>

dt-bindings: display/bridge: lt9211: Add drive-strength-microamp property

This commit adds the drive-strength-microamp property to configure the LT9211 LVDS TX output driver current. It specifies sixteen discrete levels ranging from 12 to 36.

Quoted 3 lines

Documentation/devicetree/bindings/display/bridge/lontium,lt9211.yaml +0 -0

[ ... ]

Documentation/devicetree/bindings/display/bridge/lontium,lt9211.yaml properties: · line 38

Quoted 5 lines

Documentation/devicetree/bindings/display/bridge/lontium,lt9211.yaml +5 -0

+  drive-strength-microamp:
+    description:
+      LVDS TX output driver current. Sixteen discrete levels are supported,
+      corresponding to the following nominal values in microamps.
+    enum: [12, 14, 16, 17, 19, 20, 22, 23, 25, 27, 28, 30, 31, 33, 34, 36]

Are these values scaled incorrectly for a property ending in -microamp?

Properties with the -microamp suffix typically require values in true microamperes. A standard LVDS driver requires differential currents in the milliamp range, typically 1.25 mA to 3.59 mA, to generate a standard 350 mV swing over a 100-ohm termination.

Values from 12 to 36 microamps appear to be physically insufficient for LVDS signaling.

If the actual intended currents are 1.25 mA to 3.59 mA, shouldn't the enum values be 1250, 1406, 1562, up to 3594 to accurately reflect true microamperes?

Signature · 1 line

Sashiko AI review   https://sashiko.dev/#/patchset/[email protected]?part=1

drivers/gpu/drm/bridge/lontium-lt9211.c +12 -0

+	if (!ret) {
+		for (i = 0; i < ARRAY_SIZE(lt9211_hsdrv_microamp); i++) {
+			if (lt9211_hsdrv_microamp[i] == microamp) {
+				ctx->lvds_hsdrv_isel = i;
+				break;
+			}
+		}
+		if (i == ARRAY_SIZE(lt9211_hsdrv_microamp)) {
+			dev_err(dev, "Invalid drive-strength-microamp value %u\n",
+				microamp);
+			return -EINVAL;
+		}

+
return 0;
}

On 5/13/26 3:31 PM, Boerge Struempfel wrote:

[...]

drivers/gpu/drm/bridge/lontium-lt9211.c static int lt9211_parse_dt(struct lt9211 *ctx) · line 680

Quoted 7 lines

drivers/gpu/drm/bridge/lontium-lt9211.c +3 -0

@@ -666,6 +675,26 @@ static int lt9211_parse_dt(struct lt9211 *ctx)
 
  	ctx->panel_bridge = panel_bridge;
 
+	ctx->lvds_hsdrv_isel = 8; /* default: 25 uA */
+	ret = of_property_read_u32(dev->of_node, "drive-strength-microamp",
+				   &microamp);

if (ret && ret != -EINVAL) {
   ... error handling ...
   return ret;
}

Then you won't need the if (!ret) conditional and can reduce indent.

I also wonder whether it wouldn't be better to simply bail with dev_err_probe() if the property is invalid, let the user know they should fix their DTs before it is too late.

Quoted 13 lines

drivers/gpu/drm/bridge/lontium-lt9211.c +12 -0

+	if (!ret) {
+		for (i = 0; i < ARRAY_SIZE(lt9211_hsdrv_microamp); i++) {
+			if (lt9211_hsdrv_microamp[i] == microamp) {
+				ctx->lvds_hsdrv_isel = i;
+				break;
+			}
+		}
+		if (i == ARRAY_SIZE(lt9211_hsdrv_microamp)) {
+			dev_err(dev, "Invalid drive-strength-microamp value %u\n",
+				microamp);
+			return -EINVAL;
+		}

[...]

Quoted 3 lines

+
return 0;
}

Quoted 17 lines · Marek Vasut

On 5/13/26 3:31 PM, Boerge Struempfel wrote:

[...]

drivers/gpu/drm/bridge/lontium-lt9211.c static int lt9211_parse_dt(struct lt9211 *ctx) · line 680

Quoted 7 lines

drivers/gpu/drm/bridge/lontium-lt9211.c +3 -0

@@ -666,6 +675,26 @@ static int lt9211_parse_dt(struct lt9211 *ctx)
 
   	ctx->panel_bridge = panel_bridge;
 
+	ctx->lvds_hsdrv_isel = 8; /* default: 25 uA */
+	ret = of_property_read_u32(dev->of_node, "drive-strength-microamp",
+				   &microamp);

if (ret && ret != -EINVAL) {
    ... error handling ...
    return ret;
}

Quoted 1 line

Then you won't need the if (!ret) conditional and can reduce indent.

Thank you for this suggestion. We will than need to do the lookup even in case of no value being supplied, but that would make the code cleaner. Therefore I am changing this in v3.