[PATCH v3 5/7] i2c: bcm2835: Add support for Repeated Start Condition

[Noralf Trønnes]

Documentation/i2c/i2c-protocol states that Combined transactions should
separate messages with a Start bit and end the whole transaction with a
Stop bit. This patch adds support for issuing only a Start between
messages instead of a Stop followed by a Start.

This implementation differs from downstream i2c-bcm2708 in 2 respects:
- it uses an interrupt to detect that the transfer is active instead
  of using polling. There is no interrupt for Transfer Active, but by
  not prefilling the FIFO it's possible to use the TXW interrupt.
- when resetting/disabling the controller between transfers it writes
  CLEAR to the control register instead of just zero.
  Using just zero gave many errors. This might be the reason why
  downstream had to disable this feature and make it available with a
  module parameter.

I have run thousands of transfers to a DS1307 (rtc), MMA8451 (accel)
and AT24C32 (eeprom) in parallel without problems.

Signed-off-by: Noralf Trønnes <noralf at tronnes.org>
---
 drivers/i2c/busses/i2c-bcm2835.c | 101 ++++++++++++++++++++++++---------------
 1 file changed, 63 insertions(+), 38 deletions(-)

diff --git a/drivers/i2c/busses/i2c-bcm2835.c b/drivers/i2c/busses/i2c-bcm2835.c
index 99857f8..d42fee2 100644
--- a/drivers/i2c/busses/i2c-bcm2835.c
+++ b/drivers/i2c/busses/i2c-bcm2835.c
@@ -63,6 +63,7 @@ struct bcm2835_i2c_dev {
 	struct i2c_adapter adapter;
 	struct completion completion;
 	struct i2c_msg *curr_msg;
+	int num_msgs;
 	u32 msg_err;
 	u8 *msg_buf;
 	size_t msg_buf_remaining;
@@ -109,6 +110,45 @@ static void bcm2835_drain_rxfifo(struct bcm2835_i2c_dev *i2c_dev)
 	}
 }
 
+/*
+ * Repeated Start Condition (Sr)
+ * The BCM2835 ARM Peripherals datasheet mentions a way to trigger a Sr when it
+ * talks about reading from a slave with 10 bit address. This is achieved by
+ * issuing a write, poll the I2CS.TA flag and wait for it to be set, and then
+ * issue a read.
+ * A comment in https://github.com/raspberrypi/linux/issues/254 shows how the
+ * firmware actually does it using polling and says that it's a workaround for
+ * a problem in the state machine.
+ * It turns out that it is possible to use the TXW interrupt to know when the
+ * transfer is active, provided the FIFO has not been prefilled.
+ */
+
+static void bcm2835_i2c_start_transfer(struct bcm2835_i2c_dev *i2c_dev)
+{
+	u32 c = BCM2835_I2C_C_ST | BCM2835_I2C_C_I2CEN;
+	struct i2c_msg *msg = i2c_dev->curr_msg;
+	bool last_msg = (i2c_dev->num_msgs == 1);
+
+	if (!i2c_dev->num_msgs)
+		return;
+
+	i2c_dev->num_msgs--;
+	i2c_dev->msg_buf = msg->buf;
+	i2c_dev->msg_buf_remaining = msg->len;
+
+	if (msg->flags & I2C_M_RD)
+		c |= BCM2835_I2C_C_READ | BCM2835_I2C_C_INTR;
+	else
+		c |= BCM2835_I2C_C_INTT;
+
+	if (last_msg)
+		c |= BCM2835_I2C_C_INTD;
+
+	bcm2835_i2c_writel(i2c_dev, BCM2835_I2C_A, msg->addr);
+	bcm2835_i2c_writel(i2c_dev, BCM2835_I2C_DLEN, msg->len);
+	bcm2835_i2c_writel(i2c_dev, BCM2835_I2C_C, c);
+}
+
 static irqreturn_t bcm2835_i2c_isr(int this_irq, void *data)
 {
 	struct bcm2835_i2c_dev *i2c_dev = data;
@@ -142,6 +182,12 @@ static irqreturn_t bcm2835_i2c_isr(int this_irq, void *data)
 		}
 
 		bcm2835_fill_txfifo(i2c_dev);
+
+		if (i2c_dev->num_msgs && !i2c_dev->msg_buf_remaining) {
+			i2c_dev->curr_msg++;
+			bcm2835_i2c_start_transfer(i2c_dev);
+		}
+
 		return IRQ_HANDLED;
 	}
 
@@ -166,30 +212,25 @@ static irqreturn_t bcm2835_i2c_isr(int this_irq, void *data)
 	return IRQ_HANDLED;
 }
 
-static int bcm2835_i2c_xfer_msg(struct bcm2835_i2c_dev *i2c_dev,
-				struct i2c_msg *msg)
+static int bcm2835_i2c_xfer(struct i2c_adapter *adap, struct i2c_msg msgs[],
+			    int num)
 {
-	u32 c;
+	struct bcm2835_i2c_dev *i2c_dev = i2c_get_adapdata(adap);
 	unsigned long time_left;
+	int i;
 
-	i2c_dev->curr_msg = msg;
-	i2c_dev->msg_buf = msg->buf;
-	i2c_dev->msg_buf_remaining = msg->len;
-	reinit_completion(&i2c_dev->completion);
-
-	bcm2835_i2c_writel(i2c_dev, BCM2835_I2C_C, BCM2835_I2C_C_CLEAR);
+	for (i = 0; i < (num - 1); i++)
+		if (msgs[i].flags & I2C_M_RD) {
+			dev_warn_once(i2c_dev->dev,
+				      "only one read message supported, has to be last\n");
+			return -EOPNOTSUPP;
+		}
 
-	if (msg->flags & I2C_M_RD) {
-		c = BCM2835_I2C_C_READ | BCM2835_I2C_C_INTR;
-	} else {
-		c = BCM2835_I2C_C_INTT;
-		bcm2835_fill_txfifo(i2c_dev);
-	}
-	c |= BCM2835_I2C_C_ST | BCM2835_I2C_C_INTD | BCM2835_I2C_C_I2CEN;
+	i2c_dev->curr_msg = msgs;
+	i2c_dev->num_msgs = num;
+	reinit_completion(&i2c_dev->completion);
 
-	bcm2835_i2c_writel(i2c_dev, BCM2835_I2C_A, msg->addr);
-	bcm2835_i2c_writel(i2c_dev, BCM2835_I2C_DLEN, msg->len);
-	bcm2835_i2c_writel(i2c_dev, BCM2835_I2C_C, c);
+	bcm2835_i2c_start_transfer(i2c_dev);
 
 	time_left = wait_for_completion_timeout(&i2c_dev->completion,
 						BCM2835_I2C_TIMEOUT);
@@ -200,31 +241,15 @@ static int bcm2835_i2c_xfer_msg(struct bcm2835_i2c_dev *i2c_dev,
 		return -ETIMEDOUT;
 	}
 
-	if (likely(!i2c_dev->msg_err))
-		return 0;
+	if (!i2c_dev->msg_err)
+		return num;
 
 	dev_dbg(i2c_dev->dev, "i2c transfer failed: %x\n", i2c_dev->msg_err);
 
 	if (i2c_dev->msg_err & BCM2835_I2C_S_ERR)
 		return -EREMOTEIO;
-	else
-		return -EIO;
-}
-
-static int bcm2835_i2c_xfer(struct i2c_adapter *adap, struct i2c_msg msgs[],
-			    int num)
-{
-	struct bcm2835_i2c_dev *i2c_dev = i2c_get_adapdata(adap);
-	int i;
-	int ret = 0;
-
-	for (i = 0; i < num; i++) {
-		ret = bcm2835_i2c_xfer_msg(i2c_dev, &msgs[i]);
-		if (ret)
-			break;
-	}
 
-	return ret ?: i;
+	return -EIO;
 }
 
 static u32 bcm2835_i2c_func(struct i2c_adapter *adap)
-- 
2.8.2