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

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Date: Sat, 16 Mar 2024 11:10:34 -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 16/03/2024 08:59, 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
> Wouldn't be this commit instead?
> https://cgit.freedesktop.org/drm/drm-misc/commit/?id=8b25320887d7feac98875546ea0f521628b745bb
Yes, you're right!

Melissa
>
> Best Regards,
> ~Arthur Grillo
>
>
>>> 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
>>>