[PATCH RFC 0/2] regmap: support firmware-described non-sequential register reads

[PATCH RFC 0/2] regmap: support firmware-described non-sequential register reads

[Richard Leitner]

Hi all,

this series adds support for firmware/device-tree described constraints
on sequential register reads.

The motivating use case are camera modules from Vision Components.
These modules may integrate image sensors that normally support
incrementing multi-register reads, while the MCU (which sits basically
"between" the I2C host and the camera sensors) breaks them. In such
cases, single-register reads work, but sequential/bulk register reads do
not.

This limitation is therefore not a property of the sensor silicon
itself, but of the hardware module/integration in which the device is
used. As a result, it cannot be identified from the deivce driver and
handling it in each affected device driver does not scale well.

To address this, the series introduces a common firmware property
(no-sequential-read) describing hardware integrations where reading
multiple consecutive registers in a single operation is not reliable.
regmap then consumes that property and maps it to its existing
use_single_read configuration. This keeps the hardware constraint
described in firmware/device-tree while applying it in the layer that
already owns the register read transfer policy.

The initial discussion that motivated this approach can be found here:
https://lore.kernel.org/lkml/20260721151614.GN50424@killaraus.ideasonboard.com/

Feedback is especially welcome on the property naming, the placement of
the common binding schema, and if regmap is the best place to implment this.

Thanks,
Richard

Link: https://lore.kernel.org/lkml/20260721151614.GN50424@killaraus.ideasonboard.com/
Signed-off-by: Richard Leitner <richard.leitner@linux.dev>
---
Richard Leitner (2):
      dt-bindings: regmap: add common schema for no-sequential-read
      regmap: implement no-sequential-read firmware property

 .../devicetree/bindings/regmap/common.yaml         | 26 ++++++++++++++++++++++
 drivers/base/regmap/regmap.c                       | 14 +++++++++++-
 2 files changed, 39 insertions(+), 1 deletion(-)
---
base-commit: 248951ddc14de84de3910f9b13f51491a8cd91df
change-id: 20260722-regmap-single-read-a2e95070cd66

Best regards,
--  
Richard Leitner <richard.leitner@linux.dev>

[PATCH RFC 1/2] dt-bindings: regmap: add common schema for no-sequential-read

[Richard Leitner]

Introduce a common regmap dt-binding schema with an optional boolean
property no-sequential-read to describe hardware integrations where
reading multiple consecutive registers in a single operation is not
reliable and registers must be read individually.

A typical case are imaging sensors used on vendor-provided camera
modules whose bus integration does not support reads that advance across
consecutive register addresses. In such cases, single-register reads
work, but bulk reads do not. This restriction is a property of the
concrete hardware integration, not of the chip itself. It therefore
cannot be identified from the device driver and duplicating firmware
parsing in each affected driver does not scale.

Signed-off-by: Richard Leitner <richard.leitner@linux.dev>
---
 .../devicetree/bindings/regmap/common.yaml         | 26 ++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/Documentation/devicetree/bindings/regmap/common.yaml b/Documentation/devicetree/bindings/regmap/common.yaml
new file mode 100644
index 0000000000000..1d713bc3217da
--- /dev/null
+++ b/Documentation/devicetree/bindings/regmap/common.yaml
@@ -0,0 +1,26 @@
+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/regmap/common.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Common register access constraints
+
+maintainers:
+  - TBD
+
+description:
+  Common properties describing register access constraints.
+
+properties:
+  no-sequential-read:
+    type: boolean
+    description:
+      Indicates that this device instance does not support sequential
+      register reads.
+
+      Reads may start at a given register address, but reading multiple
+      consecutive registers in a single operation is not reliable.
+      Software must instead read registers individually.
+
+additionalProperties: true

-- 
2.53.0

[PATCH RFC 2/2] regmap: implement no-sequential-read firmware property

[Richard Leitner]

Some devices are integrated in hardware modules where incrementing
multi-register reads are not reliable, even though the underlying chip
normally supports them.

regmap already owns a configuration that selects between bulk reads and
single-register reads. Letting regmap consume this firmware property
makes use of this configuration and therefore keeps the override in the
actual layer of implementation.

If the property is absent, existing behaviour is unchanged.

Signed-off-by: Richard Leitner <richard.leitner@linux.dev>
---
 drivers/base/regmap/regmap.c | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/drivers/base/regmap/regmap.c b/drivers/base/regmap/regmap.c
index e6e022b026375..ed08db764cdcb 100644
--- a/drivers/base/regmap/regmap.c
+++ b/drivers/base/regmap/regmap.c
@@ -674,6 +674,18 @@ enum regmap_endian regmap_get_val_endian(struct device *dev,
 }
 EXPORT_SYMBOL_GPL(regmap_get_val_endian);
 
+static bool regmap_get_use_single_read(struct device *dev,
+				       const struct regmap_bus *bus,
+				       const struct regmap_config *config)
+{
+	struct fwnode_handle *fwnode = dev ? dev_fwnode(dev) : NULL;
+
+	if (fwnode_property_read_bool(fwnode, "no-sequential-read"))
+		return true;
+
+	return config->use_single_read || !(config->read || (bus && bus->read));
+}
+
 struct regmap *__regmap_init(struct device *dev,
 			     const struct regmap_bus *bus,
 			     void *bus_context,
@@ -786,7 +798,7 @@ struct regmap *__regmap_init(struct device *dev,
 		map->reg_stride_order = ilog2(map->reg_stride);
 	else
 		map->reg_stride_order = -1;
-	map->use_single_read = config->use_single_read || !(config->read || (bus && bus->read));
+	map->use_single_read = regmap_get_use_single_read(dev, bus, config);
 	map->use_single_write = config->use_single_write || !(config->write || (bus && bus->write));
 	map->can_multi_write = config->can_multi_write && (config->write || (bus && bus->write));
 	if (bus) {

-- 
2.53.0