[PATCH] i2c-cadence: fix repeated start in message sequence

[Giuseppe Cantavenera]

Wrong i2c transactions have been observed in reads made up of
a write followed by a read supposed to be interleaved by a
repeated start.
cdns_i2c_mrecv() and cdns_i2c_msend() were clearing the
HOLD bit in the control register before issuing the last
transaction of a series, which somehow reflects the
Zynq-7000 Reference Manual but is incorrect for most slaves.
Seems like most of the times the clearing of the HOLD bit
takes some istants to be effective:
the controller sends a restart on the bus.
Sometimes the clearing does take effect and the last call
to cdns_i2c_mrecv() or cdns_i2c_msend() results in:
STOP + START instead of simply a RESTART (often mandatory).

Modify the command sequence:
1- enable interrupts
2- issue the write/read operation
3- clear the HOLD bit if needed

Signed-off-by: Giuseppe Cantavenera <giuseppe.cantavenera at azcom.it>
Tested-by: Giuseppe Cantavenera <giuseppe.cantavenera at azcom.it>
Cc: stable at vger.kernel.org
Cc: michal.simek at xilinx.com
Cc: soren.brinkmann at xilinx.com
---
 drivers/i2c/busses/i2c-cadence.c | 25 ++++++++++++++++---------
 1 file changed, 16 insertions(+), 9 deletions(-)

diff --git a/drivers/i2c/busses/i2c-cadence.c b/drivers/i2c/busses/i2c-cadence.c
index 7234e32..8a36df2 100644
--- a/drivers/i2c/busses/i2c-cadence.c
+++ b/drivers/i2c/busses/i2c-cadence.c
@@ -428,15 +428,21 @@ static void cdns_i2c_mrecv(struct cdns_i2c *id)
 		cdns_i2c_writereg(id->recv_count, CDNS_I2C_XFER_SIZE_OFFSET);
 	}
 
-	/* Clear the bus hold flag if bytes to receive is less than FIFO size */
+	cdns_i2c_writereg(CDNS_I2C_ENABLED_INTR_MASK, CDNS_I2C_IER_OFFSET);
+	/* Set the slave address in address register - triggers operation */
+	cdns_i2c_writereg(id->p_msg->addr & CDNS_I2C_ADDR_MASK,
+						CDNS_I2C_ADDR_OFFSET);
+
+	/*
+	 * Clear the bus hold flag if bytes to receive is less than FIFO size.
+	 * This needs to be after the read is triggered, otherwise any read
+	 * made up of a write plus a read interleaved by a repeated start
+	 * may fail.
+	 */
 	if (!id->bus_hold_flag &&
 		((id->p_msg->flags & I2C_M_RECV_LEN) != I2C_M_RECV_LEN) &&
 		(id->recv_count <= CDNS_I2C_FIFO_DEPTH))
 			cdns_i2c_clear_bus_hold(id);
-	/* Set the slave address in address register - triggers operation */
-	cdns_i2c_writereg(id->p_msg->addr & CDNS_I2C_ADDR_MASK,
-						CDNS_I2C_ADDR_OFFSET);
-	cdns_i2c_writereg(CDNS_I2C_ENABLED_INTR_MASK, CDNS_I2C_IER_OFFSET);
 }
 
 /**
@@ -489,17 +495,18 @@ static void cdns_i2c_msend(struct cdns_i2c *id)
 		id->send_count--;
 	}
 
+
+	cdns_i2c_writereg(CDNS_I2C_ENABLED_INTR_MASK, CDNS_I2C_IER_OFFSET);
+	/* Set the slave address in address register - triggers operation. */
+	cdns_i2c_writereg(id->p_msg->addr & CDNS_I2C_ADDR_MASK,
+						CDNS_I2C_ADDR_OFFSET);
 	/*
 	 * Clear the bus hold flag if there is no more data
 	 * and if it is the last message.
 	 */
 	if (!id->bus_hold_flag && !id->send_count)
 		cdns_i2c_clear_bus_hold(id);
-	/* Set the slave address in address register - triggers operation. */
-	cdns_i2c_writereg(id->p_msg->addr & CDNS_I2C_ADDR_MASK,
-						CDNS_I2C_ADDR_OFFSET);
 
-	cdns_i2c_writereg(CDNS_I2C_ENABLED_INTR_MASK, CDNS_I2C_IER_OFFSET);
 }
 
 /**
-- 
1.9.1