linux-kernel - Re: [PATCH 1/7] drm: Fix drm_fixp2int

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <nzce3m2okiqdd6iqj6ynymus64vjcpdep3jwqgs4uw3rvkvqkz@tz4i34w7b6es>
Date: Thu, 14 Mar 2024 09:59:39 -0300
From: Melissa Wen <mwen@...lia.com>
To: Arthur Grillo <arthurgrillo@...eup.net>
Cc: Rodrigo Siqueira <rodrigosiqueiramelo@...il.com>, 
	Melissa Wen <melissa.srw@...il.com>, Maíra Canal <mairacanal@...eup.net>, 
	Haneen Mohammed <hamohammed.sa@...il.com>, Daniel Vetter <daniel@...ll.ch>, 
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, Maxime Ripard <mripard@...nel.org>, 
	Thomas Zimmermann <tzimmermann@...e.de>, David Airlie <airlied@...il.com>, 
	Jonathan Corbet <corbet@....net>, pekka.paalanen@...oniitty.fi, 
	Louis Chauvet <louis.chauvet@...tlin.com>, dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org, 
	jeremie.dautheribes@...tlin.com, miquel.raynal@...tlin.com, thomas.petazzoni@...tlin.com, 
	seanpaul@...gle.com, marcheu@...gle.com, nicolejadeyee@...gle.com, 
	Pekka Paalanen <pekka.paalanen@...labora.com>
Subject: Re: [PATCH 1/7] drm: Fix drm_fixp2int_round() making it add 0.5

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@...labora.com>
> >> Signed-off-by: Arthur Grillo <arthurgrillo@...eup.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
> >>