linux-kernel - [PATCH v2] drm: Fix drm_fixp2int

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Message-Id: <20240316-drm_fixed-v2-1-c1bc2665b5ed@riseup.net>
Date: Sat, 16 Mar 2024 13:25:20 -0300
From: Arthur Grillo <arthurgrillo@...eup.net>
To: Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, 
 Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>, 
 David Airlie <airlied@...il.com>, Daniel Vetter <daniel@...ll.ch>, 
 Maíra Canal <mcanal@...lia.com>, 
 Melissa Wen <mwen@...lia.com>, 
 Pekka Paalanen <pekka.paalanen@...labora.com>, 
 Harry Wentland <harry.wentland@....com>
Cc: dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org, 
 Arthur Grillo <arthurgrillo@...eup.net>
Subject: [PATCH v2] drm: Fix drm_fixp2int_round() making it add 0.5

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/

Fixes: 8b25320887d7 ("drm: Add fixed-point helper to get rounded integer values")
Suggested-by: Pekka Paalanen <pekka.paalanen@...labora.com>
Reviewed-by: Harry Wentland <harry.wentland@....com>
Signed-off-by: Arthur Grillo <arthurgrillo@...eup.net>
---
Changes in v2:
- Add Fixes tag (Melissa Wen)
- Remove DRM_FIXED_POINT_HALF (Melissa Wen)
- Link to v1: https://lore.kernel.org/all/20240306-louis-vkms-conv-v1-1-5bfe7d129fdd@riseup.net/
---
 include/drm/drm_fixed.h | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/include/drm/drm_fixed.h b/include/drm/drm_fixed.h
index 0c9f917a4d4b..81572d32db0c 100644
--- a/include/drm/drm_fixed.h
+++ b/include/drm/drm_fixed.h
@@ -71,7 +71,6 @@ static inline u32 dfixed_div(fixed20_12 A, fixed20_12 B)
 }
 
 #define DRM_FIXED_POINT		32
-#define DRM_FIXED_POINT_HALF	16
 #define DRM_FIXED_ONE		(1ULL << DRM_FIXED_POINT)
 #define DRM_FIXED_DECIMAL_MASK	(DRM_FIXED_ONE - 1)
 #define DRM_FIXED_DIGITS_MASK	(~DRM_FIXED_DECIMAL_MASK)
@@ -90,7 +89,7 @@ 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)));
+	return drm_fixp2int(a + DRM_FIXED_ONE / 2);
 }
 
 static inline int drm_fixp2int_ceil(s64 a)

---
base-commit: f89632a9e5fa6c4787c14458cd42a9ef42025434
change-id: 20240315-drm_fixed-c680ba078ecb

Best regards,
-- 
Arthur Grillo <arthurgrillo@...eup.net>