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

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Date:	Wed, 20 Jul 2016 17:07:17 +0100
From:	Ian Abbott <abbotti@....co.uk>
To:	Hartley Sweeten <HartleyS@...ionengravers.com>,
	"devel@...verdev.osuosl.org" <devel@...verdev.osuosl.org>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Spencer Olson <olsonse@...ch.edu>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] staging: comedi: ni_mio_common: fix AO inttrig backwards
 compatibility

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.

-- 
-=( Ian Abbott @ MEV Ltd.    E-mail: <abbotti@....co.uk> )=-
-=(                          Web: http://www.mev.co.uk/  )=-