i3c: scanning for i2c client devices on the network

[Boris Staletic]

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:

Once again, here's a patch of what I have attempted to do, but this has
seen less testing.
I have removed the quotes below the patch for brevity.

-----------------------------------------------------------------------
 drivers/i3c/master.c | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/drivers/i3c/master.c b/drivers/i3c/master.c
index 2ef898a8fd80..de9dbf6c518e 100644
--- a/drivers/i3c/master.c
+++ b/drivers/i3c/master.c
@@ -2849,6 +2849,26 @@ int i3c_master_register(struct i3c_master_controller *master,
 			i2c_scl_rate = I3C_BUS_I2C_FM_SCL_MAX_RATE;
 	}

+	unsigned short addr_list[] = { 0x48, 0x49, 0x50, 0x51, 0x52, 0x53, I2C_CLIENT_END };
+	struct i2c_client *temp_client = kzalloc(sizeof(*temp_client), GFP_KERNEL);
+	for(int i = 0; addr_list[i] != I2C_CLIENT_END; i++) {
+		unsigned short one_address[] = { addr_list[i], I2C_CLIENT_END };
+		struct i2c_board_info info;
+		memset(&info, 0, sizeof(info));
+		info.addr = addr_list[i];
+
+		memset(&temp_client->dev, 0, sizeof(info));
+		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 err = device_register(&temp_client->dev);
+		if(err)
+			continue;
+		i2c_new_scanned_device(&master->i2c, &info, one_address, NULL);
+		i2c_unregister_device(temp_client);
+	}
+
 	ret = i3c_bus_set_mode(i3cbus, mode, i2c_scl_rate);
 	if (ret)
 		goto err_put_dev;