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Date: Thu, 14 Mar 2024 15:29:12 +0200
From: Pekka Paalanen <pekka.paalanen@collabora.com>
To: Melissa Wen <mwen@igalia.com>
Cc: Arthur Grillo <arthurgrillo@riseup.net>, Rodrigo Siqueira
 <rodrigosiqueiramelo@gmail.com>, Melissa Wen <melissa.srw@gmail.com>,
 =?UTF-8?B?TWHDrXJh?= Canal <mairacanal@riseup.net>, Haneen Mohammed
 <hamohammed.sa@gmail.com>, Daniel Vetter <daniel@ffwll.ch>, Maarten
 Lankhorst <maarten.lankhorst@linux.intel.com>, Maxime Ripard
 <mripard@kernel.org>, Thomas Zimmermann <tzimmermann@suse.de>, David Airlie
 <airlied@gmail.com>, Jonathan Corbet <corbet@lwn.net>, Louis Chauvet
 <louis.chauvet@bootlin.com>, dri-devel@lists.freedesktop.org,
 linux-kernel@vger.kernel.org, jeremie.dautheribes@bootlin.com,
 miquel.raynal@bootlin.com, thomas.petazzoni@bootlin.com,
 seanpaul@google.com, marcheu@google.com, nicolejadeyee@google.com
Subject: Re: [PATCH 1/7] drm: Fix drm_fixp2int_round() making it add 0.5
Message-ID: <20240314152912.7925242f.pekka.paalanen@collabora.com>
In-Reply-To: <nzce3m2okiqdd6iqj6ynymus64vjcpdep3jwqgs4uw3rvkvqkz@tz4i34w7b6es>
References: <20240306-louis-vkms-conv-v1-0-5bfe7d129fdd@riseup.net>
	<20240306-louis-vkms-conv-v1-1-5bfe7d129fdd@riseup.net>
	<yyrvbqpmqplwtqfdsjkhzmx7wrk4h67kn5443bdou7c7uciouy@hac7zfxiff7t>
	<16f8867c-147a-4149-ba96-ae70f8eaf903@riseup.net>
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On Thu, 14 Mar 2024 09:59:39 -0300
Melissa Wen <mwen@igalia.com> wrote:

> On 03/13, Arthur Grillo wrote:
> >=20
> >=20
> > On 12/03/24 15:27, Melissa Wen wrote: =20
> > > On 03/06, Arthur Grillo wrote: =20
> > >> As well noted by Pekka[1], the rounding of drm_fixp2int_round is wro=
ng.
> > >> 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.paala=
nen@collabora.com/
> > >> =20
> > > Hi Arthur,
> > >=20
> > > thanks for addressing this issue.
> > >=20
> > > 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=3Dab87f558dcfb2562c3497e89600dec798a446665 =20
> > >> Suggested-by: Pekka Paalanen <pekka.paalanen@collabora.com>
> > >> Signed-off-by: Arthur Grillo <arthurgrillo@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)
> > >> =20
> > >>  static inline int drm_fixp2int_round(s64 a)
> > >>  {
> > >> -	return drm_fixp2int(a + (1 << (DRM_FIXED_POINT_HALF - 1))); =20
> > > Also, this is the only usage of DRM_FIXED_POINT_HALF. Can you also
> > > remove it as it won't be used anymore?
> > >  =20
> > >> +	return drm_fixp2int(a + DRM_FIXED_ONE / 2); =20
> > > Would this division be equivalent to just shifting 1ULL by 31 instead=
 of
> > > 32 as done in DRM_FIXED_ONE? =20
> >=20
> > Yes, but I think the division makes it easier to understand what is
> > going on. =20
>=20
> 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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