Re: i3c: scanning for i2c client devices on the network

[Boris Staletic <bstaletic@axiado.com>]

On Thu, Sep 18, 2025 at 04:15:17PM +0200, Boris Staletic wrote:
> On Tue, Sep 16, 2025 at 01:08:44PM -0400, Frank Li wrote:
> > On Wed, Sep 10, 2025 at 02:01:33PM +0000, Boris Staletic wrote:
> > > Hello,
> > >
> > > We have an I3C bus where the user may ultimately attach an I2C slave with an arbitrary address, with the idea
> > > of using i2c-utils to communicate with the I2C slave from userland.
> > >
> > > With the current kernel, I2C slaves need to be known statically and thus i2cdetect fails to detect any slaves.
> > > All communication attempts get rejected early, because i3c_master_find_i2c_dev_by_addr returns NULL and then i3c_master_i2c_adapter_xfer exits with ENOENT.
> > > I3c_master_find_i2c_dev_by_addr returns NULL because bus->devs.i2c ends up being empty.
> > >
> > > Since the I2C drivers have i2c_detect and i3c_master_controller has its own i2c_adater, I looked into what needs to be done for networks
> > > with I3C masters to use the existing i2c_detect mechanism. That lead me to the following list of changes:
> > >
> > > - i3c_master_controllr's i2c_adapter needs to have its class member set.
> > > - In i2c_detect_address, temp_client->dev needs to be populated and registered before the call to i2c_default_probe.
> > >   - Otherwise the attempt to probe gets rejected early, for the same reason described above (i3c_master_find_i2c_dev_by_addr returning NULL).
> > >
> > > That much was enough to get i2c_detect to work with an I3C master.
> > > To complete the use case I also needed a registered i2c_driver that implements a simple i2c_driver.detect function.
> > > In my test, the detect function simply checked whether i2c_smbus_read_byte_data(client, 0) returns a non-error value.
> > >
> > > Could anyone comment whether this is the right approach for implementing I2C slave detection with an I3C master?
> > 
> > Generally, it is the same as I2C. just send out address, and check if target
> > ACK/NACK this address.
> 
> Are you referring here to i2c_new_scanned_device? If so, that also, in
> my testing, couldn't detect any devices without modificatioins.
> The failure to scan happens because:
> 
> - i2c_new_scanned_device calls i2c_default_probe, which calls
>   i2c_smbus_xfer.
> - that eventually gets to i3c_master_i2c_adapter_xfer, which looks up
>   existing devices by calling i3c_master_find_i2c_dev_by_addr.
> - i3c_master_find_i2c_dev_by_addr returns NULL
> - i3c_master_i2c_adapter_xfer then returns -ENOENT, ven before trying to
>   communicate over the wire.
> 
> I am probably missing something, because, as far as I can tell, neither
> i2c_detect, nor i2c_new_scanned_device can currently be used with an I3C
> master.
> 
> Would you mind elaborating more on the suggested approach?
> 

Replying here to post the diff of what I have tried (and attempted to
explain the opening email). Hopefully it will make it clearer what
is our need and (more so) what my attempted solution does.

-------------------------------------------------------------

This change allows I3C master drivers to trigger I2C slave detection
by i2c slave drivers, leveraging the existing i2c_detect mechanism.
---
 drivers/i2c/busses/Makefile     |  1 +
 drivers/i2c/busses/i2c-axiado.c | 53 +++++++++++++++++++++++++++++++++
 drivers/i2c/i2c-core-base.c     | 12 +++++++-
 drivers/i3c/master.c            |  1 +
 4 files changed, 66 insertions(+), 1 deletion(-)
 create mode 100644 drivers/i2c/busses/i2c-axiado.c

diff --git a/drivers/i2c/busses/Makefile b/drivers/i2c/busses/Makefile
index 04db855fdfd6..7b51df1433c4 100644
--- a/drivers/i2c/busses/Makefile
+++ b/drivers/i2c/busses/Makefile
@@ -42,6 +42,7 @@ obj-$(CONFIG_I2C_AT91)		+= i2c-at91.o
 i2c-at91-y			:= i2c-at91-core.o i2c-at91-master.o
 i2c-at91-$(CONFIG_I2C_AT91_SLAVE_EXPERIMENTAL)	+= i2c-at91-slave.o
 obj-$(CONFIG_I2C_AU1550)	+= i2c-au1550.o
+obj-y				+= i2c-axiado.o
 obj-$(CONFIG_I2C_AXXIA)		+= i2c-axxia.o
 obj-$(CONFIG_I2C_BCM2835)	+= i2c-bcm2835.o
 obj-$(CONFIG_I2C_BCM_IPROC)	+= i2c-bcm-iproc.o
diff --git a/drivers/i2c/busses/i2c-axiado.c b/drivers/i2c/busses/i2c-axiado.c
new file mode 100644
index 000000000000..d5006e250a3d
--- /dev/null
+++ b/drivers/i2c/busses/i2c-axiado.c
@@ -0,0 +1,53 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * I2C bus driver for the Cadence I2C controller.
+ *
+ * Copyright (C) 2009 - 2014 Xilinx, Inc.
+ */
+
+#include <linux/clk.h>
+#include <linux/delay.h>
+#include <linux/i2c.h>
+#include <linux/interrupt.h>
+#include <linux/io.h>
+#include <linux/iopoll.h>
+#include <linux/module.h>
+#include <linux/platform_device.h>
+#include <linux/of.h>
+#include <linux/pm_runtime.h>
+#include <linux/pinctrl/consumer.h>
+#include <linux/reset.h>
+
+static const unsigned short normal_i2c[] = { 0x48, 0x49, 0x50, 0x51, 0x52, 0x53, I2C_CLIENT_END };
+
+static int ax_i2c_detect(struct i2c_client *client, struct i2c_board_info *info)
+{
+	int byte = i2c_smbus_read_byte_data(client, 0);
+	if(byte < 0)
+		return -ENODEV;
+	strscpy(info->type, "axiado-client", I2C_NAME_SIZE);
+	return 0;
+}
+
+static const struct of_device_id ax_i2c_of_match[] = {
+	{ .compatible = "axiado,ax300",},
+	{ /* end of table */ }
+};
+MODULE_DEVICE_TABLE(of, ax_i2c_of_match);
+
+
+static struct i2c_driver ax_i2c_drv = {
+	.driver = {
+		.name  = "axiado",
+		.of_match_table = ax_i2c_of_match,
+	},
+	.detect = ax_i2c_detect,
+	.address_list = normal_i2c,
+	.class = I2C_CLASS_HWMON
+};
+
+module_i2c_driver(ax_i2c_drv);
+
+MODULE_AUTHOR("Boris Staletic axiado-2532@axiado.com");
+MODULE_DESCRIPTION("Axiado I2C bus driver");
+MODULE_LICENSE("GPL");
diff --git a/drivers/i2c/i2c-core-base.c b/drivers/i2c/i2c-core-base.c
index ecca8c006b02..231c43cf657d 100644
--- a/drivers/i2c/i2c-core-base.c
+++ b/drivers/i2c/i2c-core-base.c
@@ -2467,13 +2467,22 @@ static int i2c_detect_address(struct i2c_client *temp_client,
 		return 0;

 	/* Make sure there is something at this address */
-	if (!i2c_default_probe(adapter, addr))
+	temp_client->dev.bus = &i2c_bus_type;
+	temp_client->dev.type = &i2c_client_type;
+	temp_client->dev.parent = &temp_client->adapter->dev;
+	dev_set_name(&temp_client->dev, "i2c-temp-client");
+	int err3 = device_register(&temp_client->dev);
+	int err2 = i2c_default_probe(adapter, addr);
+	if (!err2) {
+		device_unregister(&temp_client->dev);
 		return 0;
+	}

 	/* Finally call the custom detection function */
 	memset(&info, 0, sizeof(struct i2c_board_info));
 	info.addr = addr;
 	err = driver->detect(temp_client, &info);
+	device_unregister(&temp_client->dev);
 	if (err) {
 		/* -ENODEV is returned if the detection fails. We catch it
 		   here as this isn't an error. */
@@ -2542,6 +2551,7 @@ static int i2c_detect(struct i2c_adapter *adapter, struct i2c_driver *driver)
 		dev_dbg(&adapter->dev,
 			"found normal entry for adapter %d, addr 0x%02x\n",
 			i2c_adapter_id(adapter), address_list[i]);
+		memset(&temp_client->dev, 0, sizeof(temp_client->dev));
 		temp_client->addr = address_list[i];
 		err = i2c_detect_address(temp_client, driver);
 		if (unlikely(err))
diff --git a/drivers/i3c/master.c b/drivers/i3c/master.c
index 2ef898a8fd80..5fabba14de20 100644
--- a/drivers/i3c/master.c
+++ b/drivers/i3c/master.c
@@ -2494,6 +2494,7 @@ static int i3c_master_i2c_adapter_init(struct i3c_master_controller *master)
 	/* FIXME: Should we allow i3c masters to override these values? */
 	adap->timeout = 1000;
 	adap->retries = 3;
+	adap->class = 1;

 	id = of_alias_get_id(master->dev.of_node, "i2c");
 	if (id >= 0) {
> Thanks in advance,
> Boris Staletic
> 
> > 
> > Address 7E, REPEAT START, scan's address. ACK/NACK STOP.
> > 
> > Check ACK/NACK. But you don't know if it is i3c device or i2c device by this
> > way.
> > 
> > send 7E to avoid IBI during you send out address.
> > 
> > Fank
> > 
> > > The most iffy part of this approach is the need for such a dummy driver, just so the I3C master knows that "something exists".
> > > I'd welcome any sort of feedback.
> > >
> > > Thanks,
> > > Boris Staletic
> > > --
> > > linux-i3c mailing list
> > > linux-i3c@lists.infradead.org
> > > http://lists.infradead.org/mailman/listinfo/linux-i3c

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