[PATCH] misc: suppress build warning

[PATCH] misc: suppress build warning

[Lad, Prabhakar]

this patch fixes following build warning:

drivers/misc/ioc4.c: In function ‘ioc4_probe’:
drivers/misc/ioc4.c:194:16: warning: ‘start’ may be used uninitialized in this function [-Wmaybe-uninitialized]
  period = (end - start) /
                ^
drivers/misc/ioc4.c:148:11: note: ‘start’ was declared here
  uint64_t start, end, period;

Signed-off-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
---
 Note:- I have compile tested only, and this was noticed
 on linux-next

 drivers/misc/ioc4.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/misc/ioc4.c b/drivers/misc/ioc4.c
index 3336ddc..c1f1e6c 100644
--- a/drivers/misc/ioc4.c
+++ b/drivers/misc/ioc4.c
@@ -145,7 +145,7 @@ ioc4_clock_calibrate(struct ioc4_driver_data *idd)
 	union ioc4_int_out int_out;
 	union ioc4_gpcr gpcr;
 	unsigned int state, last_state = 1;
-	uint64_t start, end, period;
+	uint64_t start = 0, end, period;
 	unsigned int count = 0;
 
 	/* Enable output */
-- 
1.9.1

Re: [PATCH] misc: suppress build warning

[Arnd Bergmann]

On Thursday 04 December 2014 14:38:30 Lad, Prabhakar wrote:
> this patch fixes following build warning:
> 
> drivers/misc/ioc4.c: In function ‘ioc4_probe’:
> drivers/misc/ioc4.c:194:16: warning: ‘start’ may be used uninitialized in this function [-Wmaybe-uninitialized]
>   period = (end - start) /
>                 ^
> drivers/misc/ioc4.c:148:11: note: ‘start’ was declared here
>   uint64_t start, end, period;
> 
> Signed-off-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>

Please explain why the compiler thinks there is a bug, why you 
are sure that there isn't, and why you picked '0' as the
initialization value.

	Arnd

Re: [PATCH] misc: suppress build warning

[Prabhakar Lad]

On Thu, Dec 4, 2014 at 2:59 PM, Arnd Bergmann <arnd@arndb.de> wrote:
> On Thursday 04 December 2014 14:38:30 Lad, Prabhakar wrote:
>> this patch fixes following build warning:
>>
>> drivers/misc/ioc4.c: In function ‘ioc4_probe’:
>> drivers/misc/ioc4.c:194:16: warning: ‘start’ may be used uninitialized in this function [-Wmaybe-uninitialized]
>>   period = (end - start) /
>>                 ^
>> drivers/misc/ioc4.c:148:11: note: ‘start’ was declared here
>>   uint64_t start, end, period;
>>
>> Signed-off-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
>
> Please explain why the compiler thinks there is a bug, why you
> are sure that there isn't, and why you picked '0' as the
> initialization value.
>
Its a false positive, to suppress the warning '0' was picked.

Thanks,
--Prabhakar Lad

Re: [PATCH] misc: suppress build warning

[Greg Kroah-Hartman]

On Thu, Dec 04, 2014 at 03:13:00PM +0000, Prabhakar Lad wrote:
> On Thu, Dec 4, 2014 at 2:59 PM, Arnd Bergmann <arnd@arndb.de> wrote:
> > On Thursday 04 December 2014 14:38:30 Lad, Prabhakar wrote:
> >> this patch fixes following build warning:
> >>
> >> drivers/misc/ioc4.c: In function ‘ioc4_probe’:
> >> drivers/misc/ioc4.c:194:16: warning: ‘start’ may be used uninitialized in this function [-Wmaybe-uninitialized]
> >>   period = (end - start) /
> >>                 ^
> >> drivers/misc/ioc4.c:148:11: note: ‘start’ was declared here
> >>   uint64_t start, end, period;
> >>
> >> Signed-off-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
> >
> > Please explain why the compiler thinks there is a bug, why you
> > are sure that there isn't, and why you picked '0' as the
> > initialization value.
> >
> Its a false positive, to suppress the warning '0' was picked.

Are you _sure_ it's a false positive?  That odd do/while loop looks like
it might just not ever initialize the start variable, are you sure the
logic there is correct?

thanks,

greg k-h

Re: [PATCH] misc: suppress build warning

[Andrew Morton]

On Thu, 4 Dec 2014 08:30:32 -0800 Greg Kroah-Hartman <gregkh@linuxfoundation.org> wrote:

> On Thu, Dec 04, 2014 at 03:13:00PM +0000, Prabhakar Lad wrote:
> > On Thu, Dec 4, 2014 at 2:59 PM, Arnd Bergmann <arnd@arndb.de> wrote:
> > > On Thursday 04 December 2014 14:38:30 Lad, Prabhakar wrote:
> > >> this patch fixes following build warning:
> > >>
> > >> drivers/misc/ioc4.c: In function ___ioc4_probe___:
> > >> drivers/misc/ioc4.c:194:16: warning: ___start___ may be used uninitialized in this function [-Wmaybe-uninitialized]
> > >>   period = (end - start) /
> > >>                 ^
> > >> drivers/misc/ioc4.c:148:11: note: ___start___ was declared here
> > >>   uint64_t start, end, period;
> > >>
> > >> Signed-off-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
> > >
> > > Please explain why the compiler thinks there is a bug, why you
> > > are sure that there isn't, and why you picked '0' as the
> > > initialization value.
> > >
> > Its a false positive, to suppress the warning '0' was picked.
> 
> Are you _sure_ it's a false positive?  That odd do/while loop looks like
> it might just not ever initialize the start variable, are you sure the
> logic there is correct?
> 

As long as IOC4_CALIBRATE_END is greater than IOC4_CALIBRATE_DISCARD (it is),
`start' is written to.

It would be nice to simplify the code, but I'm not sure how.

And I really dislike this initialize-it-to-zero-to-stop-the-warning
thing which we do all over the place.  The reader doesn't know *why*
it's initialized to zero and the initialization can conceal bugs if we
get a code path which should have written to it but forgot to.  And it
adds unneeded code to vlinux.

I much prefer unintialized_var() which fixes the documentation issue
and doesn't add code.  But Linus and Ingo had a dummy-spit over it.