[PATCH v2 0/2] drm/bridge: lt9211: Add drive-strength-microamp DT property

[PATCH v2 0/2] drm/bridge: lt9211: Add drive-strength-microamp DT property

[Boerge Struempfel]

The LT9211 LVDS TX output driver current is currently hardcoded to
0x8 (~25 uA). Some board layouts require a different drive strength
to meet signal integrity requirements.

This series adds support for the standard 'drive-strength-microamp'
DT property, allowing board DTs to select one of sixteen discrete
current levels between 12 uA and 36 uA. The default preserves the
existing behaviour.

Changes in v2:
- Use u8 instead of u32 for the lookup table array (values fit in u8)
- Warn when drive-strength-microamp exists in DT but is malformed

Link to v1: https://lore.kernel.org/dri-devel/20260512164609.3390700-1-bstruempfel@data-modul.com/

Boerge Struempfel (2):
  dt-bindings: display/bridge: lt9211: Add drive-strength-microamp
    property
  drm/bridge: lt9211: Add drive-strength-microamp DT property

 .../display/bridge/lontium,lt9211.yaml        |  7 +++++
 drivers/gpu/drm/bridge/lontium-lt9211.c       | 31 ++++++++++++++++++-
 2 files changed, 37 insertions(+), 1 deletion(-)

-- 
2.54.0

[PATCH v2 1/2] dt-bindings: display/bridge: lt9211: Add drive-strength-microamp property

[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 <bstruempfel@data-modul.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>

---
 .../devicetree/bindings/display/bridge/lontium,lt9211.yaml | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/Documentation/devicetree/bindings/display/bridge/lontium,lt9211.yaml b/Documentation/devicetree/bindings/display/bridge/lontium,lt9211.yaml
index 9a6e9b25d14a..381b69c761b8 100644
--- a/Documentation/devicetree/bindings/display/bridge/lontium,lt9211.yaml
+++ b/Documentation/devicetree/bindings/display/bridge/lontium,lt9211.yaml
@@ -31,6 +31,13 @@ properties:
   vccio-supply:
     description: Regulator for 1.8V IO power.
 
+  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]
+    default: 25
+
   ports:
     $ref: /schemas/graph.yaml#/properties/ports
 
-- 
2.54.0

[PATCH v2 2/2] drm/bridge: lt9211: Add drive-strength-microamp DT property

[Boerge Struempfel]

The LT9211 LVDS TX output driver current (RG_MLTX_HSDRV_ISEL) was
previously hardcoded to 0x8 (~25 uA), which may not be optimal for all
board layouts.

The hardware supports 16 discrete current levels starting at 12.5 uA
with a step of 1.5625 uA. These are exposed as rounded integer microamp
values in the lookup table.

Add support for the 'drive-strength-microamp' DT property. A lookup
table maps the sixteen supported microamp values (12..36 uA) to the
corresponding register field. Defaults to 25 uA when the property is
absent, preserving the existing behaviour.

Signed-off-by: Boerge Struempfel <bstruempfel@data-modul.com>
---
 drivers/gpu/drm/bridge/lontium-lt9211.c | 31 ++++++++++++++++++++++++-
 1 file changed, 30 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/bridge/lontium-lt9211.c b/drivers/gpu/drm/bridge/lontium-lt9211.c
index 03fc8fd10f20..5edd2d2e3da6 100644
--- a/drivers/gpu/drm/bridge/lontium-lt9211.c
+++ b/drivers/gpu/drm/bridge/lontium-lt9211.c
@@ -40,6 +40,11 @@
 /* DSI lane count - 0 means 4 lanes ; 1, 2, 3 means 1, 2, 3 lanes. */
 #define REG_DSI_LANE_COUNT(n)			((n) & 3)
 
+/* Maps register value (index) to drive-strength-microamp DT property value */
+static const u8 lt9211_hsdrv_microamp[] = {
+	12, 14, 16, 17, 19, 20, 22, 23, 25, 27, 28, 30, 31, 33, 34, 36
+};
+
 struct lt9211 {
 	struct drm_bridge		bridge;
 	struct device			*dev;
@@ -50,6 +55,7 @@ struct lt9211 {
 	struct regulator		*vccio;
 	bool				lvds_dual_link;
 	bool				lvds_dual_link_even_odd_swap;
+	u8				lvds_hsdrv_isel;
 };
 
 static const struct regmap_range lt9211_rw_ranges[] = {
@@ -374,7 +380,8 @@ static int lt9211_configure_tx(struct lt9211 *ctx, bool jeida,
 		/* BIT(7) is LVDS dual-port */
 		{ 0x823b, 0x38 | (ctx->lvds_dual_link ? BIT(7) : 0) },
 		{ 0x823e, 0x92 },
-		{ 0x823f, 0x48 },
+		/* bits 3:0: RG_MLTX_HSDRV_ISEL, LVDS TX driver current */
+		{ 0x823f, 0x40 | ctx->lvds_hsdrv_isel },
 		{ 0x8240, 0x31 },
 		{ 0x8243, 0x80 },
 		{ 0x8244, 0x00 },
@@ -629,7 +636,9 @@ static int lt9211_parse_dt(struct lt9211 *ctx)
 	struct device *dev = ctx->dev;
 	struct drm_panel *panel;
 	int dual_link;
+	u32 microamp;
 	int ret;
+	int i;
 
 	ctx->vccio = devm_regulator_get(dev, "vccio");
 	if (IS_ERR(ctx->vccio))
@@ -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) {
+		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;
+		}
+	} else if (ret != -EINVAL) {
+		dev_warn(dev, "Failed to read drive-strength-microamp: %d, using default\n",
+			 ret);
+	}
+
 	return 0;
 }
 
-- 
2.54.0

Re: [PATCH v2 1/2] dt-bindings: display/bridge: lt9211: Add drive-strength-microamp property

[Marek Vasut]

On 5/13/26 3:31 PM, Boerge Struempfel wrote:
> 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 <bstruempfel@data-modul.com>
> Acked-by: Conor Dooley <conor.dooley@microchip.com>
AB goes before SoB line.

Re: [PATCH v2 2/2] drm/bridge: lt9211: Add drive-strength-microamp DT property

[Marek Vasut]

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

[...]

> @@ -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.

> +	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;
>   }

Re: [PATCH v2 1/2] dt-bindings: display/bridge: lt9211: Add drive-strength-microamp property

[sashiko-bot]

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 <bstruempfel@data-modul.com>

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.

> diff --git a/Documentation/devicetree/bindings/display/bridge/lontium,lt9211.yaml b/Documentation/devicetree/bindings/display/bridge/lontium,lt9211.yaml
> --- a/Documentation/devicetree/bindings/display/bridge/lontium,lt9211.yaml
> +++ b/Documentation/devicetree/bindings/display/bridge/lontium,lt9211.yaml
[ ... ]
> +  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?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260513133127.376458-1-bstruempfel@data-modul.com?part=1

Re: [PATCH v2 1/2] dt-bindings: display/bridge: lt9211: Add drive-strength-microamp property

[Börge Strümpfel]

On Thu, May 14, 2026 at 05:26:03AM +0000, sashiko-bot@kernel.org wrote:
> 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.
> --
> 

Thank you for your review

> commit ab265d46e886455894986f313701d72a9407a58e
> Author: Boerge Struempfel <bstruempfel@data-modul.com>
> 
> 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.
> 
> > diff --git a/Documentation/devicetree/bindings/display/bridge/lontium,lt9211.yaml b/Documentation/devicetree/bindings/display/bridge/lontium,lt9211.yaml
> > --- a/Documentation/devicetree/bindings/display/bridge/lontium,lt9211.yaml
> > +++ b/Documentation/devicetree/bindings/display/bridge/lontium,lt9211.yaml
> [ ... ]
> > +  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?
> 

I double-checked the partial datasheet I was provided by Lontium. It
states for RG_MLTX_HSDRV_ISEL: "Port A/B mipi/lvds tx hstx current
select, 4'b0000 = 12.5 µA, 4'b1111 = 35.9735 µA, step = 1.5625 µA".
Those are the values I took as a reference for this commit. However, I
did not measure them myself and this might be a mistake in the datasheet
or there might be some amplification happening after, which is not
described in the part of the datasheet I got.

Maybe someone else has more insight into this?

In the meantime, I will try to follow up with our hardware team and try
to contact Lontium to clarify where this discrepancy between the
datasheet values and the expected LVDS output current range comes from.

> -- 
> Sashiko AI review   https://sashiko.dev/#/patchset/20260513133127.376458-1-bstruempfel@data-modul.com?part=1

-- 
Best regards,
Börge Strümpfel

DATA MODUL AG
Landsberger Str. 322
80687 München

Re: [PATCH v2 1/2] dt-bindings: display/bridge: lt9211: Add drive-strength-microamp property

[Börge Strümpfel]

On Wed, May 13, 2026 at 03:41:41PM +0200, Marek Vasut wrote:
> On 5/13/26 3:31 PM, Boerge Struempfel wrote:
> > 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 <bstruempfel@data-modul.com>
> > Acked-by: Conor Dooley <conor.dooley@microchip.com>
> AB goes before SoB line.
> 
Thank you Marek. I will fix this in v3.

-- 
Best regards,
Börge Strümpfel

DATA MODUL AG
Landsberger Str. 322
80687 München

Re: [PATCH v2 2/2] drm/bridge: lt9211: Add drive-strength-microamp DT property

[Börge Strümpfel]

On Wed, May 13, 2026 at 03:45:43PM +0200, Marek Vasut wrote:
> On 5/13/26 3:31 PM, Boerge Struempfel wrote:
> 
> [...]
> 
> > @@ -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.

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.
> 
> 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.
Ok. I'll change that in v3.
> 
> > +	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;
> >   }
> 

-- 
Best regards,
Börge Strümpfel

DATA MODUL AG
Landsberger Str. 322
80687 München

Re: [PATCH v2 2/2] drm/bridge: lt9211: Add drive-strength-microamp DT property

[Marek Vasut]

On 5/14/26 1:20 PM, Börge Strümpfel wrote:
> On Wed, May 13, 2026 at 03:45:43PM +0200, Marek Vasut wrote:
>> On 5/13/26 3:31 PM, Boerge Struempfel wrote:
>>
>> [...]
>>
>>> @@ -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.
> 
> 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.
of_property_read_u32() returns -ENODATA if property does not have a 
value -> the error handling above would detect that.