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Subject[PATCH 4.4 129/132] staging: comedi: usbduxfast: usbduxfast_ai_cmdtest rounding error
Date
From: Bernd Porr <mail@berndporr.me.uk>

commit 5618332e5b955b4bff06d0b88146b971c8dd7b32 upstream.

The userspace comedilib function 'get_cmd_generic_timed' fills
the cmd structure with an informed guess and then calls the
function 'usbduxfast_ai_cmdtest' in this driver repeatedly while
'usbduxfast_ai_cmdtest' is modifying the cmd struct until it
no longer changes. However, because of rounding errors this never
converged because 'steps = (cmd->convert_arg * 30) / 1000' and then
back to 'cmd->convert_arg = (steps * 1000) / 30' won't be the same
because of rounding errors. 'Steps' should only be converted back to
the 'convert_arg' if 'steps' has actually been modified. In addition
the case of steps being 0 wasn't checked which is also now done.

Signed-off-by: Bernd Porr <mail@berndporr.me.uk>
Cc: <stable@vger.kernel.org> # 4.4+
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Link: https://lore.kernel.org/r/20191118230759.1727-1-mail@berndporr.me.uk
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
drivers/staging/comedi/drivers/usbduxfast.c | 21 ++++++++++++++-------
1 file changed, 14 insertions(+), 7 deletions(-)

--- a/drivers/staging/comedi/drivers/usbduxfast.c
+++ b/drivers/staging/comedi/drivers/usbduxfast.c
@@ -1,5 +1,5 @@
/*
- * Copyright (C) 2004-2014 Bernd Porr, mail@berndporr.me.uk
+ * Copyright (C) 2004-2019 Bernd Porr, mail@berndporr.me.uk
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
@@ -17,7 +17,7 @@
* Description: University of Stirling USB DAQ & INCITE Technology Limited
* Devices: [ITL] USB-DUX-FAST (usbduxfast)
* Author: Bernd Porr <mail@berndporr.me.uk>
- * Updated: 10 Oct 2014
+ * Updated: 16 Nov 2019
* Status: stable
*/

@@ -31,6 +31,7 @@
*
*
* Revision history:
+ * 1.0: Fixed a rounding error in usbduxfast_ai_cmdtest
* 0.9: Dropping the first data packet which seems to be from the last transfer.
* Buffer overflows in the FX2 are handed over to comedi.
* 0.92: Dropping now 4 packets. The quad buffer has to be emptied.
@@ -359,6 +360,7 @@ static int usbduxfast_ai_cmdtest(struct
struct comedi_cmd *cmd)
{
int err = 0;
+ int err2 = 0;
unsigned int steps;
unsigned int arg;

@@ -408,11 +410,16 @@ static int usbduxfast_ai_cmdtest(struct
*/
steps = (cmd->convert_arg * 30) / 1000;
if (cmd->chanlist_len != 1)
- err |= comedi_check_trigger_arg_min(&steps,
- MIN_SAMPLING_PERIOD);
- err |= comedi_check_trigger_arg_max(&steps, MAX_SAMPLING_PERIOD);
- arg = (steps * 1000) / 30;
- err |= comedi_check_trigger_arg_is(&cmd->convert_arg, arg);
+ err2 |= comedi_check_trigger_arg_min(&steps,
+ MIN_SAMPLING_PERIOD);
+ else
+ err2 |= comedi_check_trigger_arg_min(&steps, 1);
+ err2 |= comedi_check_trigger_arg_max(&steps, MAX_SAMPLING_PERIOD);
+ if (err2) {
+ err |= err2;
+ arg = (steps * 1000) / 30;
+ err |= comedi_check_trigger_arg_is(&cmd->convert_arg, arg);
+ }

if (cmd->stop_src == TRIG_COUNT)
err |= comedi_check_trigger_arg_min(&cmd->stop_arg, 1);

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