Re: [igt-dev] [PATCH i-g-t] tests/kms_flip: Fix comparison of unsigned integers

[Petri Latvala <petri.latvala@intel.com>]

On Tue, Mar 05, 2019 at 01:02:58PM +0000, Chris Wilson wrote:
> Quoting Petri Latvala (2019-03-05 12:55:01)
> > On Thu, Feb 07, 2019 at 04:02:12PM +0200, Ville Syrjälä wrote:
> > > On Thu, Feb 07, 2019 at 03:36:52PM +0200, Petri Latvala wrote:
> > > > Difference of unsigned ints is unsigned int, and therefore always
> > > > larger than zero.
> > > > 
> > > > Signed-off-by: Petri Latvala <petri.latvala@intel.com>
> > > > Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
> > > > ---
> > > >  tests/kms_flip.c | 2 +-
> > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > > 
> > > > diff --git a/tests/kms_flip.c b/tests/kms_flip.c
> > > > index 798fc4e8..96f4a2f7 100755
> > > > --- a/tests/kms_flip.c
> > > > +++ b/tests/kms_flip.c
> > > > @@ -502,7 +502,7 @@ static void check_state(const struct test_output *o, const struct event_state *e
> > > >     if (o->flags & TEST_TS_CONT) {
> > > >             /* Ignore seq_step here since vblank waits time out immediately
> > > >              * when we kill the crtc. */
> > > > -           igt_assert_f(es->current_seq - es->last_seq >= 0,
> > > > +           igt_assert_f(es->current_seq >= es->last_seq,
> > > 
> > > That won't handle wraparound.
> > 
> > Are you referring to something other than the checked overflow? Or in
> > other words, are you objecting to this patch?
> 
> /* a is later than b */
> static bool seqno_later(u32 a, u32 b) { return (s32)(a - b) >= 0; }


I see now! My mental gymnastics failed to process the combined
overflows and the type conversion, and how that leads to correct
results but some quick testing made me see the truth.

Although it might be moot anyway if we end up removing this code
altogether.


-- 
Petri Latvala
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