[RFC] i2c: add deprecation warning for class based instantiation

[Wolfram Sang <wsa@the-dreams.de>]

Class based instantiation can cause huge delays when booting. This
mechanism was used when it was not possible to describe slaves on I2C
busses. We now have other mechanisms, so most embedded I2C will not need
classes and it was explicitly not recommended to use them. Add a
deprecation warning for drivers who want to disable class based in the
near future to gain boot-up time, so users relying on this technique can
switch to something better. They really should.

Signed-off-by: Wolfram Sang <wsa@the-dreams.de>
---
 drivers/i2c/busses/i2c-omap.c |    2 +-
 drivers/i2c/i2c-core.c        |    6 ++++++
 include/linux/i2c.h           |    1 +
 3 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/drivers/i2c/busses/i2c-omap.c b/drivers/i2c/busses/i2c-omap.c
index e02f9e3..f06109f 100644
--- a/drivers/i2c/busses/i2c-omap.c
+++ b/drivers/i2c/busses/i2c-omap.c
@@ -1231,7 +1231,7 @@ omap_i2c_probe(struct platform_device *pdev)
 	adap = &dev->adapter;
 	i2c_set_adapdata(adap, dev);
 	adap->owner = THIS_MODULE;
-	adap->class = I2C_CLASS_HWMON;
+	adap->class = I2C_CLASS_HWMON | I2C_CLASS_DEPRECATED;
 	strlcpy(adap->name, "OMAP I2C adapter", sizeof(adap->name));
 	adap->algo = &omap_i2c_algo;
 	adap->dev.parent = &pdev->dev;
diff --git a/drivers/i2c/i2c-core.c b/drivers/i2c/i2c-core.c
index 48e31ed..47ea1f0 100644
--- a/drivers/i2c/i2c-core.c
+++ b/drivers/i2c/i2c-core.c
@@ -999,6 +999,12 @@ static int i2c_register_adapter(struct i2c_adapter *adap)
 		return -EINVAL;
 	}
 
+	if (adap->class & I2C_CLASS_DEPRECATED)
+		dev_warn(&adap->dev, "This adapter will soon drop class based "
+			"instantiation of slaves\nPlease make sure all its I2C "
+			"devices are instantiated by other means.\nCheck "
+			"'Documentation/i2c/instantiating-devices' for details\n");
+
 	rt_mutex_init(&adap->bus_lock);
 	mutex_init(&adap->userspace_clients_lock);
 	INIT_LIST_HEAD(&adap->userspace_clients);
diff --git a/include/linux/i2c.h b/include/linux/i2c.h
index e988fa9..ffab838 100644
--- a/include/linux/i2c.h
+++ b/include/linux/i2c.h
@@ -473,6 +473,7 @@ void i2c_unlock_adapter(struct i2c_adapter *);
 #define I2C_CLASS_HWMON		(1<<0)	/* lm_sensors, ... */
 #define I2C_CLASS_DDC		(1<<3)	/* DDC bus on graphics adapters */
 #define I2C_CLASS_SPD		(1<<7)	/* Memory modules */
+#define I2C_CLASS_DEPRECATED	(1<<8)	/* Warn users that adapter will stop using classes */
 
 /* Internal numbers to terminate lists */
 #define I2C_CLIENT_END		0xfffeU
-- 
1.7.10.4