i2c: rcar: explain the lockless design

Wolfram Sang · Wolfram Sang · commit 7ce98a5591d2 · 2019-03-12T14:09:33.000+01:00
To make sure people can understand the lockless design of this driver
without the need to dive into git history, add a comment giving an
overview of the situation.

Signed-off-by: Wolfram Sang &lt;wsa+renesas@sang-engineering.com&gt;
Reviewed-by: Simon Horman &lt;horms+renesas@verge.net.au&gt;
Signed-off-by: Wolfram Sang &lt;wsa@the-dreams.de&gt;
diff --git a/drivers/i2c/busses/i2c-rcar.c b/drivers/i2c/busses/i2c-rcar.c
@@ -611,6 +611,15 @@ static bool rcar_i2c_slave_irq(struct rcar_i2c_priv *priv)
 	return true;
 }
 
+/*
+ * This driver has a lock-free design because there are IP cores (at least
+ * R-Car Gen2) which have an inherent race condition in their hardware design.
+ * There, we need to clear RCAR_BUS_MASK_DATA bits as soon as possible after
+ * the interrupt was generated, otherwise an unwanted repeated message gets
+ * generated. It turned out that taking a spinlock at the beginning of the ISR
+ * was already causing repeated messages. Thus, this driver was converted to
+ * the now lockless behaviour. Please keep this in mind when hacking the driver.
+ */
 static irqreturn_t rcar_i2c_irq(int irq, void *ptr)
 {
 	struct rcar_i2c_priv *priv = ptr;
