linux-kernel - Re: [PATCH v3] drivers/staging/comedi/drivers/ni

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Date:	Fri, 11 Jul 2014 18:06:36 +0100
From:	Ian Abbott <abbotti@....co.uk>
To:	Dan Carpenter <dan.carpenter@...cle.com>
CC:	<linux-kernel@...r.kernel.org>, <kernel-janitors@...r.kernel.org>,
	<devel@...verdev.osuosl.org>, <gregkh@...uxfoundation.org>,
	<dcb314@...mail.com>
Subject: Re: [PATCH v3] drivers/staging/comedi/drivers/ni_atmio16d.c: remove
 pointless condition

On 2014-07-11 16:34, Dan Carpenter wrote:
> On Fri, Jul 11, 2014 at 03:38:30PM +0100, Ian Abbott wrote:
>> From: Andrey Utkin <andrey.krieger.utkin@...il.com>
>>
>> From: Andrey Utkin <andrey.krieger.utkin@...il.com>
>>
>> The issue was discovered with static analysis and has two instances in
>> this file. The code looks like this
>> if (x < 65536000) {
>> 	...
>> } else if (x < 655360000) {
>> 	...
>> } else if (x <= 0xffffffff /* 6553600000 */) {
>> 	...
>> } else if (x <= 0xffffffff /* 65536000000 */) {
>> 	...
>> }
>>
>> The meaning of this block is to select appropriate clock frequency for
>> interval timer basing on "x", which is amount of time.
>>
>> Notes:
>> 1. That last condition matches previous one - that's the issue.
>> 2. Decimal numbers in comments don't match hex numbers in expressions.
>> But in first case the numbers have same order, while in the second case
>> the hex number is the same, and the decimal one is 10 times bigger.
>> 3. Actually type of "x" is "unsigned int", so its exact upper limit is
>> not obviously known.
>> 4. There's no "else" block.
>>
>> So it makes sense to make an "else" block from last "else if" case. The
>> code inside the block seems correct for such usage.
>>
>> [ Actually, get rid of the final "else if" case and change the
>> next-to-last "else if" case to an "else" as the upper limit of "x" _is_
>> known to be 0xffffffff (UINT_MAX), which is less than 6553600000 -- Ian ]
>>
>> Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=79871
>> Reported-by: David Binderman <dcb314@...mail.com>
>> Signed-off-by: Andrey Utkin <andrey.krieger.utkin@...il.com>
>> Signed-off-by: Ian Abbott <abbotti@....co.uk>
>> ---
>> v2: Removed final "else if" block and changed preceding "else if" block
>> to "else" block as the condition is always true due to limited range of
>> "unsigned int". -- Ian
>> v3: Corrected subject line I messed up in v2. -- Ian
>> ---
>>   drivers/staging/comedi/drivers/ni_atmio16d.c | 10 ++--------
>>   1 file changed, 2 insertions(+), 8 deletions(-)
>>
>> diff --git a/drivers/staging/comedi/drivers/ni_atmio16d.c b/drivers/staging/comedi/drivers/ni_atmio16d.c
>> index 6ad27f5..b1b4744 100644
>> --- a/drivers/staging/comedi/drivers/ni_atmio16d.c
>> +++ b/drivers/staging/comedi/drivers/ni_atmio16d.c
>> @@ -335,12 +335,9 @@ static int atmio16d_ai_cmd(struct comedi_device *dev,
>>   	} else if (cmd->convert_arg < 655360000) {
>>   		base_clock = CLOCK_100_KHZ;
>>   		timer = cmd->convert_arg / 10000;
>> -	} else if (cmd->convert_arg <= 0xffffffff /* 6553600000 */) {
>> +	} else /* cmd->convert_arg < 6553600000 */ {
>
> I think the comment is meant to be /* cmd->convert_arg >= 655360000 */
> There is an extra zero on 6553600000.

That's not what I was intending to convey.  The preceding chain of 
if/else if was checking if cmd->convert_arg < 65536000, else if 
cmd->convert_arg < 655360000, and I was trying to convey that the final 
else part was valid for all remaining values less than 6553600000, which 
in fact is all remaining unsigned int values.  (Obviously, I failed to 
convey this meaning to everyone!)

-- 
-=( Ian Abbott @ MEV Ltd.    E-mail: <abbotti@....co.uk>        )=-
-=( Tel: +44 (0)161 477 1898   FAX: +44 (0)161 718 3587         )=-
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