staging: comedi: ni_pcidio: Grab MITE spinlock while preparing DMA.

When setting up the DMA for 'read' streaming acquisition command, grab
the MITE channel spinlock before preparing and arming the DMA.

Change inspired by ni_ai_setup_MITE_dma() in ni_mio_common.c.
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/drivers/ni_pcidio.c

@@ -436,6 +436,7 @@ static int ni_pcidio_request_di_mite_channel(struct comedi_device *dev)
 		comedi_error(dev, "failed to reserve mite dma channel.");
 		return -EBUSY;
 	}
+	devpriv->di_mite_chan->dir = COMEDI_INPUT;
 	writeb(primary_DMAChannel_bits(devpriv->di_mite_chan->channel) |
 	       secondary_DMAChannel_bits(devpriv->di_mite_chan->channel),
 	       devpriv->mite->daq_io_addr + DMA_Line_Control_Group1);
@@ -1005,20 +1006,24 @@ static int ni_pcidio_cmd(struct comedi_device *dev, struct comedi_subdevice *s)
 static int setup_mite_dma(struct comedi_device *dev, struct comedi_subdevice *s)
 {
 	int retval;
+	unsigned long flags;
 
 	retval = ni_pcidio_request_di_mite_channel(dev);
 	if (retval)
 		return retval;
 
-	devpriv->di_mite_chan->dir = COMEDI_INPUT;
-
 	/* write alloc the entire buffer */
 	comedi_buf_write_alloc(s->async, s->async->prealloc_bufsz);
 
-	mite_prep_dma(devpriv->di_mite_chan, 32, 32);
+	spin_lock_irqsave(&devpriv->mite_channel_lock, flags);
+	if (devpriv->di_mite_chan) {
+		mite_prep_dma(devpriv->di_mite_chan, 32, 32);
+		mite_dma_arm(devpriv->di_mite_chan);
+	} else
+		retval = -EIO;
+	spin_unlock_irqrestore(&devpriv->mite_channel_lock, flags);
 
-	mite_dma_arm(devpriv->di_mite_chan);
-	return 0;
+	return retval;
 }
 
 static int ni_pcidio_inttrig(struct comedi_device *dev,