[PATCH] staging: comedi: cb_pcidas: reinstate delay removed from trimpot setting

Ian Abbott abbotti at mev.co.uk
Thu Oct 29 14:18:33 UTC 2020

Commit eddd2a4c675c ("staging: comedi: cb_pcidas: refactor
write_calibration_bitstream()") inadvertently removed one of the
`udelay(1)` calls when writing to the calibration register in
`cb_pcidas_calib_write()`.  Reinstate the delay.  It may seem strange
that the delay is placed before the register write, but this function is
called in a loop so the extra delay can make a difference.

This _might_ solve reported issues reading analog inputs on a
PCIe-DAS1602/16 card where the analog input values "were scaled in a
strange way that didn't make sense".  On the same hardware running a
system with a 3.13 kernel, and then a system with a 4.4 kernel, but with
the same application software, the system with the 3.13 kernel was fine,
but the one with the 4.4 kernel exhibited the problem.  Of the 90
changes to the driver between those kernel versions, this change looked
like the most likely culprit.

Fixes: eddd2a4c675c ("staging: comedi: cb_pcidas: refactor write_calibration_bitstream()")
Cc: <stable at vger.kernel.org>
Signed-off-by: Ian Abbott <abbotti at mev.co.uk>
 drivers/staging/comedi/drivers/cb_pcidas.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/staging/comedi/drivers/cb_pcidas.c b/drivers/staging/comedi/drivers/cb_pcidas.c
index 48ec2ee953dc..4f2ac39aa619 100644
--- a/drivers/staging/comedi/drivers/cb_pcidas.c
+++ b/drivers/staging/comedi/drivers/cb_pcidas.c
@@ -529,6 +529,7 @@ static void cb_pcidas_calib_write(struct comedi_device *dev,
 	if (trimpot) {
 		/* select trimpot */
 		calib_bits |= PCIDAS_CALIB_TRIM_SEL;
+		udelay(1);
 		outw(calib_bits, devpriv->pcibar1 + PCIDAS_CALIB_REG);

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