i2c: rcar: avoid race when unregistering slave

Due to the lockless design of the driver, it is theoretically possible
to access a NULL pointer, if a slave interrupt was running while we were
unregistering the slave. To make this rock solid, disable the interrupt
for a short time while we are clearing the interrupt_enable register.
This patch is purely based on code inspection. The OOPS is super-hard to
trigger because clearing SAR (the address) makes interrupts even more
unlikely to happen as well. While here, reinit SCR to SDBS because this
bit should always be set according to documentation. There is no effect,
though, because the interface is disabled.

Fixes: 7b814d85 ("i2c: rcar: avoid race when unregistering slave client")
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Wolfram Sang <wsa@kernel.org>

drivers/i2c/busses/i2c-rcar.c

@@ -866,12 +866,14 @@ static int rcar_unreg_slave(struct i2c_client *slave)
 
 	WARN_ON(!priv->slave);
 
-	/* disable irqs and ensure none is running before clearing ptr */
+	/* ensure no irq is running before clearing ptr */
+	disable_irq(priv->irq);
 	rcar_i2c_write(priv, ICSIER, 0);
-	rcar_i2c_write(priv, ICSCR, 0);
+	rcar_i2c_write(priv, ICSSR, 0);
+	enable_irq(priv->irq);
+	rcar_i2c_write(priv, ICSCR, SDBS);
 	rcar_i2c_write(priv, ICSAR, 0); /* Gen2: must be 0 if not using slave */
 
-	synchronize_irq(priv->irq);
 	priv->slave = NULL;
 
 	pm_runtime_put(rcar_i2c_priv_to_dev(priv));