staging: comedi: multiq3: tidy up multiq3_encoder_insn_read()

Encoders are not a "normal" subdevice in comedi. For aesthetics, tidy
up this function and add a couple comments to clarify the function and
explain the strange munging of the data.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/staging/comedi/drivers/multiq3.c b/drivers/staging/comedi/drivers/multiq3.c
index b5d2354..fa63637 100644 (file)
--- a/drivers/staging/comedi/drivers/multiq3.c
+++ b/drivers/staging/comedi/drivers/multiq3.c
@@ -185,22 +185,45 @@ static int multiq3_encoder_insn_read(struct comedi_device *dev,
                                     struct comedi_insn *insn,
                                     unsigned int *data)
 {
-       int chan = CR_CHAN(insn->chanspec);
-       int value;
-       int n;
+       unsigned int chan = CR_CHAN(insn->chanspec);
+       unsigned int val;
+       int i;
 
-       for (n = 0; n < insn->n; n++) {
+       for (i = 0; i < insn->n; i++) {
+               /* select encoder channel */
                multiq3_set_ctrl(dev, MULTIQ3_CTRL_EN |
                                      MULTIQ3_CTRL_E_CHAN(chan));
+
+               /* reset the byte pointer */
                outb(MULTIQ3_BP_RESET, dev->iobase + MULTIQ3_ENC_CTRL_REG);
+
+               /* latch the data */
                outb(MULTIQ3_TRSFRCNTR_OL, dev->iobase + MULTIQ3_ENC_CTRL_REG);
-               value = inb(dev->iobase + MULTIQ3_ENC_DATA_REG);
-               value |= (inb(dev->iobase + MULTIQ3_ENC_DATA_REG) << 8);
-               value |= (inb(dev->iobase + MULTIQ3_ENC_DATA_REG) << 16);
-               data[n] = (value + 0x800000) & 0xffffff;
+
+               /* read the 24-bit encoder data (lsb/mid/msb) */
+               val = inb(dev->iobase + MULTIQ3_ENC_DATA_REG);
+               val |= (inb(dev->iobase + MULTIQ3_ENC_DATA_REG) << 8);
+               val |= (inb(dev->iobase + MULTIQ3_ENC_DATA_REG) << 16);
+
+               /*
+                * Munge the data so that the reset value is in the middle
+                * of the maxdata range, i.e.:
+                *
+                * real value   comedi value
+                * 0xffffff     0x7fffff        1 negative count
+                * 0x000000     0x800000        reset value
+                * 0x000001     0x800001        1 positive count
+                *
+                * It's possible for the 24-bit counter to overflow but it
+                * would normally take _quite_ a few turns. A 2000 line
+                * encoder in quadrature results in 8000 counts/rev. So about
+                * 1048 turns in either direction can be measured without
+                * an overflow.
+                */
+               data[i] = (val + ((s->maxdata + 1) >> 1)) & s->maxdata;
        }
 
-       return n;
+       return insn->n;
 }
 
 static void multiq3_encoder_reset(struct comedi_device *dev,