i2c: atr: add passthrough flag

Some I2C ATRs can have other I2C ATRs as children. The I2C messages of
the child ATRs need to be forwarded as-is if the parent I2C ATR can
only do static mapping.

In the case of GMSL, the deserializer I2C ATR actually doesn't have I2C
address remapping hardware capabilities, but it is able to select which
GMSL link to talk to, allowing it to change the address of the
serializer.

The child ATRs need to have their alias pools defined in such a way to
prevent overlapping addresses between them, but there's no way around
this without orchestration between multiple ATR instances.

To allow for this use-case, add a flag that allows unmapped addresses
to be passed through, since they are already remapped by the child ATRs.

There's no case where an address that has not been remapped by the child
ATR will hit the parent ATR.

Signed-off-by: Cosmin Tanislav <demonsingur@gmail.com>
Reviewed-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Reviewed-by: Romain Gantois <romain.gantois@bootlin.com>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>

Author: Demon000

drivers/i2c/i2c-atr.c

@@ -394,6 +394,9 @@ static int i2c_atr_map_msgs(struct i2c_atr_chan *chan, struct i2c_msg *msgs,
 		c2a = i2c_atr_get_mapping_by_addr(chan, msgs[i].addr);
 
 		if (!c2a) {
+			if (atr->flags & I2C_ATR_F_PASSTHROUGH)
+				continue;
+
 			dev_err(atr->dev, "client 0x%02x not mapped!\n",
 				msgs[i].addr);
 
@@ -486,13 +489,13 @@ static int i2c_atr_smbus_xfer(struct i2c_adapter *adap, u16 addr,
 
 	c2a = i2c_atr_get_mapping_by_addr(chan, addr);
 
-	if (!c2a) {
+	if (!c2a && !(atr->flags & I2C_ATR_F_PASSTHROUGH)) {
 		dev_err(atr->dev, "client 0x%02x not mapped!\n", addr);
 		mutex_unlock(&chan->alias_pairs_lock);
 		return -ENXIO;
 	}
 
-	alias = c2a->alias;
+	alias = c2a ? c2a->alias : addr;
 
 	mutex_unlock(&chan->alias_pairs_lock);
 

include/linux/i2c-atr.h

@@ -26,9 +26,11 @@ struct i2c_atr;
  *                    devices being added or removed from a child bus.
  *                    The ATR pool will have to be big enough to accomodate all
  *                    devices expected to be added to the child buses.
+ * @I2C_ATR_F_PASSTHROUGH: Allow unmapped incoming addresses to pass through
  */
 enum i2c_atr_flags {
 	I2C_ATR_F_STATIC = BIT(0),
+	I2C_ATR_F_PASSTHROUGH = BIT(1),
 };
 
 /**