staging: comedi: dt2814: Clear stale AI data before operation

When performing a Comedi read instruction or setting up an asynchronous
command on the AI subdevice, clear any stale data on the A/D registers
by waiting for the Status register's BUSY bit to clear (if set) and then
if the FINISH or ERR bit is set, reading the A/D Data register twice to
clear the stale data.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Link: https://lore.kernel.org/r/20210301165757.243065-2-abbotti@mev.co.uk
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/staging/comedi/drivers/dt2814.c b/drivers/staging/comedi/drivers/dt2814.c
index bd329d7b489320161f02fb38f3d42049aa06f0b0..7e73aa094eea23d8ccf63173b300416cea396f54 100644 (file)
--- a/drivers/staging/comedi/drivers/dt2814.c
+++ b/drivers/staging/comedi/drivers/dt2814.c
@@ -52,6 +52,43 @@ struct dt2814_private {
 #define DT2814_TIMEOUT 10
 #define DT2814_MAX_SPEED 100000        /* Arbitrary 10 khz limit */
 
+static int dt2814_ai_notbusy(struct comedi_device *dev,
+                            struct comedi_subdevice *s,
+                            struct comedi_insn *insn,
+                            unsigned long context)
+{
+       unsigned int status;
+
+       status = inb(dev->iobase + DT2814_CSR);
+       if (context)
+               *(unsigned int *)context = status;
+       if (status & DT2814_BUSY)
+               return -EBUSY;
+       return 0;
+}
+
+static int dt2814_ai_clear(struct comedi_device *dev)
+{
+       unsigned int status = 0;
+       int ret;
+
+       /* Wait until not busy and get status register value. */
+       ret = comedi_timeout(dev, NULL, NULL, dt2814_ai_notbusy,
+                            (unsigned long)&status);
+       if (ret)
+               return ret;
+
+       if (status & (DT2814_FINISH | DT2814_ERR)) {
+               /*
+                * There unread data, or the error flag is set.
+                * Read the data register twice to clear the condition.
+                */
+               inb(dev->iobase + DT2814_DATA);
+               inb(dev->iobase + DT2814_DATA);
+       }
+       return 0;
+}
+
 static int dt2814_ai_eoc(struct comedi_device *dev,
                         struct comedi_subdevice *s,
                         struct comedi_insn *insn,
@@ -73,6 +110,7 @@ static int dt2814_ai_insn_read(struct comedi_device *dev,
        int chan;
        int ret;
 
+       dt2814_ai_clear(dev);   /* clear stale data or error */
        for (n = 0; n < insn->n; n++) {
                chan = CR_CHAN(insn->chanspec);
 
@@ -174,6 +212,7 @@ static int dt2814_ai_cmd(struct comedi_device *dev, struct comedi_subdevice *s)
        int chan;
        int trigvar;
 
+       dt2814_ai_clear(dev);   /* clear stale data or error */
        trigvar = dt2814_ns_to_timer(&cmd->scan_begin_arg, cmd->flags);
 
        chan = CR_CHAN(cmd->chanlist[0]);