i2c: add debug message for detected HostNotify alerts

Setting up HostNotify can be tricky. Support debugging by stating
when a HostNotify alert was received independent of the irq being
mapped. Especially useful with the in-kernel i2c testunit. Update
documentation as well.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Andi Shyti <andi.shyti@kernel.org>

Author: Wolfram Sang

Documentation/i2c/slave-testunit-backend.rst

@@ -99,6 +99,11 @@ Example to send a notification after 10ms::
 
   # i2cset -y 0 0x30 0x02 0x42 0x64 0x01 i
 
+If the host controller supports HostNotify, this message with debug level
+should appear (Linux 6.11 and later)::
+
+  Detected HostNotify from address 0x30
+
 0x03 SMBUS_BLOCK_PROC_CALL
 ~~~~~~~~~~~~~~~~~~~~~~~~~~
 

drivers/i2c/i2c-core-base.c

@@ -1468,6 +1468,8 @@ int i2c_handle_smbus_host_notify(struct i2c_adapter *adap, unsigned short addr)
 	if (!adap)
 		return -EINVAL;
 
+	dev_dbg(&adap->dev, "Detected HostNotify from address 0x%02x", addr);
+
 	irq = irq_find_mapping(adap->host_notify_domain, addr);
 	if (irq <= 0)
 		return -ENXIO;