[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

[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.

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

[Claude Code Review Bot]

Overall Series Review

Subject: drm/bridge: lt9211: Add drive-strength-microamp DT property
Author: Boerge Struempfel <bstruempfel@data-modul.com>
Patches: 8
Reviewed: 2026-05-16T11:56:03.253260

---

This is a clean, straightforward 2-patch series that adds a configurable LVDS TX drive strength to the LT9211 bridge driver via the standard `drive-strength-microamp` DT property. The series is well-structured: patch 1 adds the DT binding, patch 2 adds the driver implementation. The default value (25 uA, register index 8) preserves backward compatibility with the previously hardcoded `0x48`. The register-level change from `0x48` to `0x40 | ctx->lvds_hsdrv_isel` is correct — the default index 8 produces `0x40 | 0x08 = 0x48`, matching the original. The lookup table values are internally consistent with the described formula (12.5 uA base + 1.5625 uA steps, rounded to integer).

No functional issues found. A couple of minor style observations below.

**Verdict: Series looks good.**

---
Generated by Claude Code Patch Reviewer

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

[Claude Code Review Bot]

Patch Review

This patch adds the `drive-strength-microamp` property to the LT9211 binding YAML with an enum of 16 discrete values and a default of 25.

**Looks good.** The property is correctly placed in the `properties:` block (needed since the schema has `additionalProperties: false`). The property name `drive-strength-microamp` is a standard DT property name. The enum values match the 16 hardware levels. The default of 25 preserves existing behavior. Already has Conor's Acked-by.

No issues.

---
Generated by Claude Code Patch Reviewer

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

[Claude Code Review Bot]

Patch Review

The driver patch adds:
- A `static const u8` lookup table mapping register index to microamp value
- A `u8 lvds_hsdrv_isel` field in the `lt9211` struct
- DT parsing in `lt9211_parse_dt()`
- The register write change in `lt9211_configure_tx()`

**Register change is correct:**
```c
-		{ 0x823f, 0x48 },
+		/* bits 3:0: RG_MLTX_HSDRV_ISEL, LVDS TX driver current */
+		{ 0x823f, 0x40 | ctx->lvds_hsdrv_isel },
```
Original `0x48` = `0100_1000` — bit 6 set, bits 3:0 = 8. New code: `0x40` (bit 6) OR'd with the 4-bit index (0–15), giving range `0x40`–`0x4F`. Default index 8 → `0x48`. Matches.

**DT parsing logic is sound:**
```c
+	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);
+	}
```

The three-way handling is correct: success → lookup, `-EINVAL` → property absent (silent default), other error → warn and continue with default. The linear scan of 16 entries is fine for a one-time probe path.

**Minor observations (non-blocking):**

1. The existing driver uses `of_property_read_u32()` via `dev->of_node`, which is consistent with the driver's existing OF usage (`of_graph_get_port_by_id`, etc.). Some reviewers may suggest `device_property_read_u32(dev, ...)` as the more generic API, but given this driver is inherently OF-only, either is acceptable.

2. The `dev_err` path for an invalid microamp value would only trigger if the DT binding schema validation is bypassed (since the binding constrains values via enum). Returning `-EINVAL` here is still the right defensive choice.

No functional issues.

---
Generated by Claude Code Patch Reviewer

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

[Claude Code Review Bot]

Overall Series Review

Subject: drm/bridge: lt9211: Add drive-strength-microamp DT property
Author: Boerge Struempfel <bstruempfel@data-modul.com>
Patches: 7
Reviewed: 2026-05-16T13:11:28.036918

---

This is a clean, well-structured 2-patch series adding a configurable LVDS TX drive-strength property to the LT9211 bridge driver. The binding and driver changes are split correctly across the two patches. The default value (`0x8` = 25 uA) preserves backward compatibility. The code is correct and consistent with existing patterns in the driver.

One concern worth flagging: the values 12-36 in `drive-strength-microamp` seem implausibly low for LVDS output drive current (standard LVDS specifies ~3.5 mA nominal differential current). These may represent an internal bias/reference current rather than the actual output current, or there may be a units error (milliamps vs. microamps). This should be verified against the LT9211 datasheet.

Overall: **Looks good with minor comments.**

---

---
Generated by Claude Code Patch Reviewer