This is a note to let you know that I've just added the patch titled

staging: comedi: usbduxfast: usbduxfast_ai_cmdtest rounding error

to my staging git tree which can be found at git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/staging.git in the staging-testing branch.

The patch will show up in the next release of the linux-next tree (usually sometime within the next 24 hours during the week.)

The patch will be merged to the staging-next branch sometime soon, after it passes testing, and the merge window is open.

If you have any questions about this process, please let me know.

From 222f7381cf6f1eab2ea3a534cbea0815a2b53326 Mon Sep 17 00:00:00 2001

From: Bernd Porr mail@berndporr.me.uk Date: Mon, 18 Nov 2019 23:07:59 +0000 Subject: staging: comedi: usbduxfast: usbduxfast_ai_cmdtest rounding error

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(-)

diff --git a/drivers/staging/comedi/drivers/usbduxfast.c b/drivers/staging/comedi/drivers/usbduxfast.c index 04bc488385e6..4af012968cb6 100644 --- a/drivers/staging/comedi/drivers/usbduxfast.c +++ b/drivers/staging/comedi/drivers/usbduxfast.c @@ -1,6 +1,6 @@ // SPDX-License-Identifier: GPL-2.0+ /* - * Copyright (C) 2004-2014 Bernd Porr, mail@berndporr.me.uk + * Copyright (C) 2004-2019 Bernd Porr, mail@berndporr.me.uk */

/* @@ -8,7 +8,7 @@ * 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 */

@@ -22,6 +22,7 @@ * * * 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. @@ -350,6 +351,7 @@ static int usbduxfast_ai_cmdtest(struct comedi_device *dev, struct comedi_cmd *cmd) { int err = 0; + int err2 = 0; unsigned int steps; unsigned int arg;

@@ -399,11 +401,16 @@ static int usbduxfast_ai_cmdtest(struct comedi_device *dev, */ 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);