i2c: imx: Fix race condition in dma read

BugLink: https://bugs.launchpad.net/bugs/1792377

commit bed4ff1e upstream.

This fixes a race condition, where the DMAEN bit ends up being set after
I2C slave has transmitted a byte following the dummy read.  When that
happens, an interrupt is generated instead, and no DMA request is generated
to kickstart the DMA read, and a timeout happens after DMA_TIMEOUT (1 sec).

Fixed by setting the DMAEN bit before the dummy read.
Signed-off-by: Esben Haabendal <eha@deif.com>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>
Cc: stable@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

drivers/i2c/busses/i2c-imx.c

@@ -677,9 +677,6 @@ static int i2c_imx_dma_read(struct imx_i2c_struct *i2c_imx,
 	struct imx_i2c_dma *dma = i2c_imx->dma;
 	struct device *dev = &i2c_imx->adapter.dev;
 
-	temp = imx_i2c_read_reg(i2c_imx, IMX_I2C_I2CR);
-	temp |= I2CR_DMAEN;
-	imx_i2c_write_reg(temp, i2c_imx, IMX_I2C_I2CR);
 
 	dma->chan_using = dma->chan_rx;
 	dma->dma_transfer_dir = DMA_DEV_TO_MEM;
@@ -792,6 +789,7 @@ static int i2c_imx_read(struct imx_i2c_struct *i2c_imx, struct i2c_msg *msgs, bo
 	int i, result;
 	unsigned int temp;
 	int block_data = msgs->flags & I2C_M_RECV_LEN;
+	int use_dma = i2c_imx->dma && msgs->len >= DMA_THRESHOLD && !block_data;
 
 	dev_dbg(&i2c_imx->adapter.dev,
 		"<%s> write slave address: addr=0x%x\n",
@@ -818,12 +816,14 @@ static int i2c_imx_read(struct imx_i2c_struct *i2c_imx, struct i2c_msg *msgs, bo
 	 */
 	if ((msgs->len - 1) || block_data)
 		temp &= ~I2CR_TXAK;
+	if (use_dma)
+		temp |= I2CR_DMAEN;
 	imx_i2c_write_reg(temp, i2c_imx, IMX_I2C_I2CR);
 	imx_i2c_read_reg(i2c_imx, IMX_I2C_I2DR); /* dummy read */
 
 	dev_dbg(&i2c_imx->adapter.dev, "<%s> read data\n", __func__);
 
-	if (i2c_imx->dma && msgs->len >= DMA_THRESHOLD && !block_data)
+	if (use_dma)
 		return i2c_imx_dma_read(i2c_imx, msgs, is_lastmsg);
 
 	/* read data */