i2c: s3c2410: Leave the bus disabled unless it is in use

There is a rather odd feature of the exynos i2c controller that if it
is left enabled, it can lock itself up with the clk line held low.
This makes the bus unusable.

Unfortunately, the s3c24xx_i2c_set_master() function does not notice
this, and reports a timeout. From then on the bus cannot be used until
the AP is rebooted.

The problem happens when any sort of interrupt occurs (e.g. due to a
bus transition) when we are not in the middle of a transaction. We
have seen many instances of this when U-Boot leaves the bus apparently
happy, but Linux cannot access it.

The current code is therefore pretty fragile.

This fixes things by leaving the bus disabled unless we are actually
in a transaction. We enable the bus at the start of the transaction and
disable it at the end. That way we won't get interrupts and will not
lock up the bus.

It might be possible to clear pending interrupts on start-up, but this
seems to be a more robust solution. We can't service interrupts when
we are not in a transaction, and anyway would rather not lock up the
bus while we try.

Signed-off-by: Simon Glass <[e-mail]>
Signed-off-by: Naveen Krishna Chatradhi <[e-mail]>
Acked-by: Kyungmin Park <[e-mail]>
Signed-off-by: Wolfram Sang <[e-mail]>

drivers/i2c/busses/i2c-s3c2410.c

···
 	return IRQ_HANDLED;
 }
 
+/*
+ * Disable the bus so that we won't get any interrupts from now on, or try
+ * to drive any lines. This is the default state when we don't have
+ * anything to send/receive.
+ *
+ * If there is an event on the bus, or we have a pre-existing event at
+ * kernel boot time, we may not notice the event and the I2C controller
+ * will lock the bus with the I2C clock line low indefinitely.
+ */
+static inline void s3c24xx_i2c_disable_bus(struct s3c24xx_i2c *i2c)
+{
+	unsigned long tmp;
+
+	/* Stop driving the I2C pins */
+	tmp = readl(i2c->regs + S3C2410_IICSTAT);
+	tmp &= ~S3C2410_IICSTAT_TXRXEN;
+	writel(tmp, i2c->regs + S3C2410_IICSTAT);
+
+	/* We don't expect any interrupts now, and don't want send acks */
+	tmp = readl(i2c->regs + S3C2410_IICCON);
+	tmp &= ~(S3C2410_IICCON_IRQEN | S3C2410_IICCON_IRQPEND |
+		S3C2410_IICCON_ACKEN);
+	writel(tmp, i2c->regs + S3C2410_IICCON);
+}
+
 
 /* s3c24xx_i2c_set_master
  *
···
 
 	s3c24xx_i2c_wait_idle(i2c);
 
+	s3c24xx_i2c_disable_bus(i2c);
+
  out:
+	i2c->state = STATE_IDLE;
+
 	return ret;
 }
 
···
 
 static int s3c24xx_i2c_init(struct s3c24xx_i2c *i2c)
 {
-	unsigned long iicon = S3C2410_IICCON_IRQEN | S3C2410_IICCON_ACKEN;
 	struct s3c2410_platform_i2c *pdata;
 	unsigned int freq;
 
···
 
 	dev_info(i2c->dev, "slave address 0x%02x\n", pdata->slave_addr);
 
-	writel(iicon, i2c->regs + S3C2410_IICCON);
+	writel(0, i2c->regs + S3C2410_IICCON);
+	writel(0, i2c->regs + S3C2410_IICSTAT);
 
 	/* we need to work out the divisors for the clock... */
 
 	if (s3c24xx_i2c_clockrate(i2c, &freq) != 0) {
-		writel(0, i2c->regs + S3C2410_IICCON);
 		dev_err(i2c->dev, "cannot meet bus frequency required\n");
 		return -EINVAL;
 	}
···
 	/* todo - check that the i2c lines aren't being dragged anywhere */
 
 	dev_info(i2c->dev, "bus frequency set to %d KHz\n", freq);
-	dev_dbg(i2c->dev, "S3C2410_IICCON=0x%02lx\n", iicon);
+	dev_dbg(i2c->dev, "S3C2410_IICCON=0x%02x\n",
+		readl(i2c->regs + S3C2410_IICCON));
 
 	return 0;
 }