i2c: bcm2835: Fix hang for writing messages larger than 16 bytes

Writing messages larger than the FIFO size results in a hang, rendering
the machine unusable. This is because the RXD status flag is set on the
first interrupt which results in bcm2835_drain_rxfifo() stealing bytes
from the buffer. The controller continues to trigger interrupts waiting
for the missing bytes, but bcm2835_fill_txfifo() has none to give.
In this situation wait_for_completion_timeout() apparently is unable to
stop the madness.

The BCM2835 ARM Peripherals datasheet has this to say about the flags:
  TXD: is set when the FIFO has space for at least one byte of data.
  RXD: is set when the FIFO contains at least one byte of data.
  TXW: is set during a write transfer and the FIFO is less than full.
  RXR: is set during a read transfer and the FIFO is or more full.

Implementing the logic from the downstream i2c-bcm2708 driver solved
the hang problem.

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Martin Sperl <kernel@martin.sperl.org>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>

drivers/i2c/busses/i2c-bcm2835.c

@@ -64,6 +64,7 @@ struct bcm2835_i2c_dev {
 	int irq;
 	struct i2c_adapter adapter;
 	struct completion completion;
+	struct i2c_msg *curr_msg;
 	u32 msg_err;
 	u8 *msg_buf;
 	size_t msg_buf_remaining;
@@ -126,26 +127,30 @@ static irqreturn_t bcm2835_i2c_isr(int this_irq, void *data)
 		return IRQ_HANDLED;
 	}
 
-	if (val & BCM2835_I2C_S_RXD) {
-		bcm2835_drain_rxfifo(i2c_dev);
-		if (!(val & BCM2835_I2C_S_DONE))
-			return IRQ_HANDLED;
-	}
-
 	if (val & BCM2835_I2C_S_DONE) {
-		if (i2c_dev->msg_buf_remaining)
+		if (i2c_dev->curr_msg->flags & I2C_M_RD) {
+			bcm2835_drain_rxfifo(i2c_dev);
+			val = bcm2835_i2c_readl(i2c_dev, BCM2835_I2C_S);
+		}
+
+		if ((val & BCM2835_I2C_S_RXD) || i2c_dev->msg_buf_remaining)
 			i2c_dev->msg_err = BCM2835_I2C_S_LEN;
 		else
 			i2c_dev->msg_err = 0;
 		complete(&i2c_dev->completion);
 		return IRQ_HANDLED;
 	}
 
-	if (val & BCM2835_I2C_S_TXD) {
+	if (val & BCM2835_I2C_S_TXW) {
 		bcm2835_fill_txfifo(i2c_dev);
 		return IRQ_HANDLED;
 	}
 
+	if (val & BCM2835_I2C_S_RXR) {
+		bcm2835_drain_rxfifo(i2c_dev);
+		return IRQ_HANDLED;
+	}
+
 	return IRQ_NONE;
 }
 
@@ -155,6 +160,7 @@ static int bcm2835_i2c_xfer_msg(struct bcm2835_i2c_dev *i2c_dev,
 	u32 c;
 	unsigned long time_left;
 
+	i2c_dev->curr_msg = msg;
 	i2c_dev->msg_buf = msg->buf;
 	i2c_dev->msg_buf_remaining = msg->len;
 	reinit_completion(&i2c_dev->completion);