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

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Date: Wed, 13 Mar 2024 15:47:11 -0300
From: Arthur Grillo <arthurgrillo@...eup.net>
To: Melissa Wen <mwen@...lia.com>
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 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.

Best Regards,
~Arthur Grillo

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