[PATCH 1/7] drm: Fix drm_fixp2int_round() making it add 0.5

[Melissa Wen]

On 03/13, Arthur Grillo wrote:
> 
> 
> On 12/03/24 15:27, Melissa Wen wrote:
> > On 03/06, Arthur Grillo wrote:
> >> As well noted by Pekka[1], the rounding of drm_fixp2int_round is wrong.
> >> To round a number, you need to add 0.5 to the number and floor that,
> >> drm_fixp2int_round() is adding 0.0000076. Make it add 0.5.
> >>
> >> [1]: https://lore.kernel.org/all/20240301135327.22efe0dd.pekka.paalanen@collabora.com/
> >>
> > Hi Arthur,
> > 
> > thanks for addressing this issue.
> > 
> > Please, add a fix tag to the commit that you are fixing, so we can
> > easily backport. Might be this commit:
> > https://cgit.freedesktop.org/drm/drm-misc/commit/drivers/gpu/drm/vkms?id=ab87f558dcfb2562c3497e89600dec798a446665
> >> Suggested-by: Pekka Paalanen <pekka.paalanen at collabora.com>
> >> Signed-off-by: Arthur Grillo <arthurgrillo at riseup.net>
> >> ---
> >>  include/drm/drm_fixed.h | 2 +-
> >>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/include/drm/drm_fixed.h b/include/drm/drm_fixed.h
> >> index 0c9f917a4d4b..de3a79909ac9 100644
> >> --- a/include/drm/drm_fixed.h
> >> +++ b/include/drm/drm_fixed.h
> >> @@ -90,7 +90,7 @@ static inline int drm_fixp2int(s64 a)
> >>  
> >>  static inline int drm_fixp2int_round(s64 a)
> >>  {
> >> -	return drm_fixp2int(a + (1 << (DRM_FIXED_POINT_HALF - 1)));
> > Also, this is the only usage of DRM_FIXED_POINT_HALF. Can you also
> > remove it as it won't be used anymore?
> > 
> >> +	return drm_fixp2int(a + DRM_FIXED_ONE / 2);
> > Would this division be equivalent to just shifting 1ULL by 31 instead of
> > 32 as done in DRM_FIXED_ONE?
> 
> Yes, but I think the division makes it easier to understand what is
> going on.

Right. I was thinking about slightly better performance, but I don't
have any data. We can go with this since you consider more readable,
anyway.

Can you send another version addressing the other comments? Then I can
cherry-pick and already apply the fix.

Thanks,

Melissa

> 
> Best Regards,
> ~Arthur Grillo
> 
> > 
> > Melissa
> > 
> >>  }
> >>  
> >>  static inline int drm_fixp2int_ceil(s64 a)
> >>
> >> -- 
> >> 2.43.0
> >>