linux-kernel - Re: [PATCH] staging: comedi: ni_mio_common: fix AO inttrig backwards compatibility

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Date:	Tue, 16 Aug 2016 23:07:27 -0600
From:	Spencer E Olson <olsonse@...ch.edu>
To:	Ian Abbott <abbotti@....co.uk>
Cc:	Hartley Sweeten <HartleyS@...ionengravers.com>,
	"devel@...verdev.osuosl.org" <devel@...verdev.osuosl.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] staging: comedi: ni_mio_common: fix AO inttrig
 backwards compatibility

Sorry for the very belated reply on this.  I'm assuming that this was
already accepted, but I've been working with this patch for a bit.  This
fixes the problems I raised in any case.

Reviewed-by: Spencer E Olson <olsonse@...ch.edu>

On Wed, 2016-07-20 at 17:07 +0100, Ian Abbott wrote:
> On 20/07/16 16:55, Hartley Sweeten wrote:
> > On Tuesday, July 19, 2016 4:18 AM, Ian Abbott wrote:
> >> Commit ebb657babfa9 ("staging: comedi: ni_mio_common: clarify the
> >> cmd->start_arg validation and use") introduced a backwards compatibility
> >> issue in the use of asynchronous commands on the AO subdevice when
> >> `start_src` is `TRIG_EXT`.  Valid values for `start_src` are `TRIG_INT`
> >> (for internal, software trigger), and `TRIG_EXT` (for external trigger).
> >> When set to `TRIG_EXT`.  In both cases, the driver relies on an
> >> internal, software trigger to set things up (allowing the user
> >> application to write sufficient samples to the data buffer before the
> >> trigger), so it acts as a software "pre-trigger" in the `TRIG_EXT` case.
> >> The software trigger is handled by `ni_ao_inttrig()`.
> >>
> >> Prior to the above change, when `start_src` was `TRIG_INT`, `start_arg`
> >> was required to be 0, and `ni_ao_inttrig()` checked that the software
> >> trigger number was also 0.  After the above change, when `start_src` was
> >> `TRIG_INT`, any value was allowed for `start_arg`, and `ni_ao_inttrig()`
> >> checked that the software trigger number matched this `start_arg` value.
> >> The backwards compatibility issue is that the internal trigger number
> >> now has to match `start_arg` when `start_src` is `TRIG_EXT` when it
> >> previously had to be 0.
> >>
> >> Fix the backwards compatibility issue in `ni_ao_inttrig()` by always
> >> allowing software trigger number 0 when `start_src` is something other
> >> than `TRIG_INT`.
> >>
> >> Thanks to Spencer Olson for reporting the issue.
> >>
> >> Signed-off-by: Ian Abbott <abbotti@....co.uk>
> >> Reported-by: Spencer Olson <olsonse@...ch.edu>
> >> Fixes: ebb657babfa9 ("staging: comedi: ni_mio_common: clarify the
> >>   cmd->start_arg validation and use")
> >> ---
> >>  drivers/staging/comedi/drivers/ni_mio_common.c | 10 +++++++++-
> >>  1 file changed, 9 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/drivers/staging/comedi/drivers/ni_mio_common.c b/drivers/staging/comedi/drivers/ni_mio_common.c
> >> index 8dabb19..9f4036f 100644
> >> --- a/drivers/staging/comedi/drivers/ni_mio_common.c
> >> +++ b/drivers/staging/comedi/drivers/ni_mio_common.c
> >> @@ -2772,7 +2772,15 @@ static int ni_ao_inttrig(struct comedi_device *dev,
> >>  	int i;
> >>  	static const int timeout = 1000;
> >>
> >> -	if (trig_num != cmd->start_arg)
> >> +	/*
> >> +	 * Require trig_num == cmd->start_arg when cmd->start_src == TRIG_INT.
> >> +	 * For backwards compatibility, also allow trig_num == 0 when
> >> +	 * cmd->start_src != TRIG_INT (i.e. when cmd->start_src == TRIG_EXT);
> >> +	 * in that case, the internal trigger is being used as a pre-trigger
> >> +	 * before the external trigger.
> >> +	 */
> >> +	if (!(trig_num == cmd->start_arg ||
> >> +	      (trig_num == 0 && cmd->start_src != TRIG_INT)))
> >>  		return -EINVAL;
> >
> > Ian,
> >
> > I think this test is a bit clearer:
> >
> > +	/*
> > +	 * Require trig_num == cmd->start_arg when cmd->start_src == TRIG_INT.
> > +	 * For backwards compatibility, any trig_num is valid when
> > +	 * cmd->start_src != TRIG_INT (i.e. when cmd->start_src == TRIG_EXT);
> > +	 * in that case, the internal trigger is being used as a pre-trigger
> > +	 * before the external trigger.
> > +	 */
> > +	if (cmd->start_src == TRIG_INT && trig_num != cmd->start_arg)
> > 		return -EINVAL;
> 
> True, but I restricted it to only accept trig_num values that have been 
> valid in the past.
> 
> >
> > But, either way:
> >
> > Reviewed-by: H Hartley Sweeten <hsweeten@...ionengravers.com>
> >
> > Thanks!
> >
> 
> Thanks for the review.
>