[PATCH weston 1/8] shared: Add timespec_normalize helper

[Pekka Paalanen]

On Tue, 12 Dec 2017 12:36:39 +0200
Alexandros Frantzis <alexandros.frantzis at collabora.com> wrote:

> On Tue, Dec 12, 2017 at 11:50:49AM +0200, Pekka Paalanen wrote:
> > On Mon,  4 Dec 2017 15:34:01 +0200
> > Alexandros Frantzis <alexandros.frantzis at collabora.com> wrote:
> >   
> > > Add a helper function to normalize struct timespec values so that the
> > > nanoseconds part is less than 1 second and has the same sign as the
> > > seconds part (if the seconds part is not 0).
> > > 
> > > Normalization is required to ensure we can safely convert timespec
> > > values to wayland protocol data, i.e, to tv_sec_hi, tv_sec_lo,
> > > tv_sec_nsec triplets, and will be used in upcoming commits.
> > > 
> > > Signed-off-by: Alexandros Frantzis <alexandros.frantzis at collabora.com>
> > > ---
> > >  shared/timespec-util.h | 28 ++++++++++++++++++++++
> > >  tests/timespec-test.c  | 65 ++++++++++++++++++++++++++++++++++++++++++++++++++
> > >  2 files changed, 93 insertions(+)
> > > 
> > > diff --git a/shared/timespec-util.h b/shared/timespec-util.h
> > > index f9736c27..a10edf5b 100644
> > > --- a/shared/timespec-util.h
> > > +++ b/shared/timespec-util.h
> > > @@ -33,6 +33,34 @@
> > >  
> > >  #define NSEC_PER_SEC 1000000000
> > >  
> > > +/* Normalize a timespec
> > > + *
> > > + * \param r[out] normalized timespec
> > > + * \param a[in] timespec to normalize
> > > + *
> > > + * Normalize a timespec so that tv_nsec is less than 1 second
> > > + * and has the same sign as tv_sec (if tv_sec is non-zero).  
> > 
> > Hi,  
> 
> Hi Pekka,
> 
> thanks for the review.
> 
> > why does it need to have the same sign?
> > E.g. timespec_sub() ensures nsec is non-negative, as do the add
> > functions.  
> 
> The goal was to make timespec_normalize more generally useful and
> independent of any current behavior. I didn't want to assume that the
> provided timespec would be produced necessarily by using the
> aforementioned functions that already provide the same-sign guarantee.

They don't provide the same-sign guarantee. They ensure nsec is
non-negative, regardless of the sign of sec.

The question here is, what is the normalized form?

We have not had use for negative time values in the protocol, and the
new proposal does not either, so protocol specs do not offer any
precendent. Computationally they could appear in programs, so the
definition of "normalized" for these helper functions will be purely
libweston-internal.

I'd go with non-negative instead of same-sign, because the existing
code is already like that. I'd like to hear about any better
justification one way or another, since I have little else. I believe
non-negative lead to simpler code.


Thanks,
pq
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