[PATCH v3 0/1] i2c: Allow disabling auto detection via devicetree

[PATCH v3 0/1] i2c: Allow disabling auto detection via devicetree

[Vincent Whitchurch]

v3:
- Drop bindings patch since it has been merged in dt-schema:
  https://github.com/devicetree-org/dt-schema/commit/ba55f96c6d8d
- Reword commit message of i2c patch to indicate that the property is
  documented in dt-schema.

v2:
- Change subject prefix of bindings patch
- Reword description of property in bindings patch

When a driver with a ->detect callback (such as lm75) is loaded, the i2c core
performs I2C transactions on the bus to all the addresses listed in that
driver's address_list.  This kind of probing wastes time and as
Documentation/i2c/instantiating-devices.rst says, this method is not
recommended and it is instead advised to list all devices in the devicetree.

However, even if all the devices are listed in the devicetree, there is
currently no way to prevent the core from attempting auto detection short of
patching controller drivers to not pass the I2C_CLASS* bits in adap->class.
The latter is not always possible since generic drivers like i2c-gpio set these
bits.

To avoid this unnecessary probing and reduce boot time, this series adds
support in the I2C core to allow this feature to be disabled.

Cc: linux-i2c@vger.kernel.org
Cc: devicetree@vger.kernel.org
Cc: linux-kernel@vger.kernel.org

Vincent Whitchurch (1):
  i2c: core: support no-detect property

 drivers/i2c/i2c-core-base.c | 8 +++++++-
 include/linux/i2c.h         | 1 +
 2 files changed, 8 insertions(+), 1 deletion(-)

-- 
2.34.1

[PATCH v3 1/1] i2c: core: support no-detect property

[Vincent Whitchurch]

If the devicetree specifies the no-detect property (documented in
dt-schema [0]) for the I2C bus, we know that there are no other devices
on the bus other than the ones listed in the devicetree, so avoid
calling drivers' detect callbacks and wasting time looking for devices
which do not exist.

[0] https://github.com/devicetree-org/dt-schema/commit/ba55f96c6d8d

Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
---
 drivers/i2c/i2c-core-base.c | 8 +++++++-
 include/linux/i2c.h         | 1 +
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/drivers/i2c/i2c-core-base.c b/drivers/i2c/i2c-core-base.c
index d43db2c3876e..d43025b84546 100644
--- a/drivers/i2c/i2c-core-base.c
+++ b/drivers/i2c/i2c-core-base.c
@@ -1341,7 +1341,8 @@ static int i2c_do_add_adapter(struct i2c_driver *driver,
 			      struct i2c_adapter *adap)
 {
 	/* Detect supported devices on that bus, and instantiate them */
-	i2c_detect(adap, driver);
+	if (adap->detect)
+		i2c_detect(adap, driver);
 
 	return 0;
 }
@@ -1432,6 +1433,7 @@ EXPORT_SYMBOL_GPL(i2c_handle_smbus_host_notify);
 
 static int i2c_register_adapter(struct i2c_adapter *adap)
 {
+	struct device_node *np = adap->dev.of_node;
 	int res = -EINVAL;
 
 	/* Can't register until after driver model init */
@@ -1502,6 +1504,10 @@ static int i2c_register_adapter(struct i2c_adapter *adap)
 			 "Failed to create compatibility class link\n");
 #endif
 
+	adap->detect = true;
+	if (np && of_property_read_bool(np, "no-detect"))
+		adap->detect = false;
+
 	/* create pre-declared device nodes */
 	of_i2c_register_devices(adap);
 	i2c_acpi_install_space_handler(adap);
diff --git a/include/linux/i2c.h b/include/linux/i2c.h
index fbda5ada2afc..8fad5fe85685 100644
--- a/include/linux/i2c.h
+++ b/include/linux/i2c.h
@@ -728,6 +728,7 @@ struct i2c_adapter {
 	struct rt_mutex bus_lock;
 	struct rt_mutex mux_lock;
 
+	bool detect;
 	int timeout;			/* in jiffies */
 	int retries;
 	struct device dev;		/* the adapter device */
-- 
2.34.1

Re: [PATCH v3 1/1] i2c: core: support no-detect property

[Wolfram Sang]

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On Fri, Apr 29, 2022 at 03:07:49PM +0200, Vincent Whitchurch wrote:
> If the devicetree specifies the no-detect property (documented in
> dt-schema [0]) for the I2C bus, we know that there are no other devices
> on the bus other than the ones listed in the devicetree, so avoid
> calling drivers' detect callbacks and wasting time looking for devices
> which do not exist.

Setting 'classes' in the adapter struct to 0 (actually to
'I2C_CLASS_DEPRECATED' for a while) is not good enough? Or do you need
class based instantiation on some busses?


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