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

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Message-ID: <yyrvbqpmqplwtqfdsjkhzmx7wrk4h67kn5443bdou7c7uciouy@hac7zfxiff7t>
Date: Tue, 12 Mar 2024 15:27:38 -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 03/06, 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?

Melissa

>  }
>  
>  static inline int drm_fixp2int_ceil(s64 a)
> 
> -- 
> 2.43.0
>