I recently stumbled across an explanation of why you clear the dma flip-flop...

diff --git a/comedi/drivers/das1800.c b/comedi/drivers/das1800.c
index 3da830d3568003b6407814e0034f937e82366170..99313bd243e4078d8e2b8cc54a151c637e3ac171 100644 (file)
--- a/comedi/drivers/das1800.c
+++ b/comedi/drivers/das1800.c
@@ -657,12 +657,10 @@ static int das1800_attach(comedi_device *dev, comedi_devconfig *it)
                                return -ENOMEM;
                        flags = claim_dma_lock();
                        disable_dma(devpriv->dma0);
-                       clear_dma_ff(devpriv->dma0);
                        set_dma_mode(devpriv->dma0, DMA_MODE_READ);
                        if(dma1)
                        {
                                disable_dma(devpriv->dma1);
-                               clear_dma_ff(devpriv->dma1);
                                set_dma_mode(devpriv->dma1, DMA_MODE_READ);
                        }
                        release_dma_lock(flags);
@@ -930,6 +928,9 @@ static void das1800_handle_dma(comedi_device *dev, comedi_subdevice *s)
 
        flags = claim_dma_lock();
        disable_dma(devpriv->dma_current);
+       /* clear flip-flop to make sure 2-byte registers for
+        * count and address get set correctly */
+       clear_dma_ff(devpriv->dma_current);
 
        // figure out how many points to read
        maxPoints = devpriv->dma_buf_size  / sizeof(short);
@@ -1397,6 +1398,9 @@ void setup_dma(comedi_device *dev, comedi_cmd cmd)
 
        lock_flags = claim_dma_lock();
        disable_dma(devpriv->dma0);
+       /* clear flip-flop to make sure 2-byte registers for
+        * count and address get set correctly */
+       clear_dma_ff(devpriv->dma0);
        set_dma_addr(devpriv->dma0, (unsigned int) devpriv->dma_buf0);
        // set appropriate size of transfer
        if(devpriv->count * sizeof(short) >= devpriv->dma_buf_size || devpriv->forever)
@@ -1407,6 +1411,9 @@ void setup_dma(comedi_device *dev, comedi_cmd cmd)
        if(dual_dma && (devpriv->count * sizeof(short) > devpriv->dma_buf_size || devpriv->forever))
        {
                disable_dma(devpriv->dma1);
+               /* clear flip-flop to make sure 2-byte registers for
+                * count and address get set correctly */
+               clear_dma_ff(devpriv->dma1);
                set_dma_addr(devpriv->dma1, (unsigned int) devpriv->dma_buf1);
                // set appropriate size of transfer
                if(devpriv->count * sizeof(short) >= 2 * devpriv->dma_buf_size || devpriv->forever)
@@ -1452,7 +1459,8 @@ static int das1800_ai_do_cmd(comedi_device *dev, comedi_subdevice *s)
 {
        int ret;
        int control_a, control_c;
-       comedi_cmd cmd = s->async->cmd;
+       comedi_async *async = s->async;
+       comedi_cmd cmd = async->cmd;
 
        if(!dev->irq)
        {
@@ -1508,6 +1516,7 @@ static int das1800_ai_do_cmd(comedi_device *dev, comedi_subdevice *s)
                return ret;
        }
        setup_dma(dev, cmd);
+       async->events = 0;
        outb(control_c, dev->iobase + DAS1800_CONTROL_C);
        // set conversion rate and length for burst mode
        if(control_c & BMDE)