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

[Pekka Paalanen]

On Thu, 14 Mar 2024 09:59:39 -0300
Melissa Wen <mwen at igalia.com> wrote:

> 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.

Those are constants, the compiler should compute it at build time in
both styles.

I like the DRM_FIXED_ONE / 2 style, it's semantically obvious. Bit
shift is less obvious.


Thanks,
pq
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