Date: Sun, 20 Jul 2025 18:36:54 +0900
From: Lauri Tirkkonen <lauri@hacktheplanet.fi>
To: Mario Limonciello <superm1@kernel.org>
Cc: stable@vger.kernel.org, regressions@lists.linux.dev,
	amd-gfx@lists.freedesktop.org, Wayne Lin <wayne.lin@amd.com>,
	Alex Deucher <alexander.deucher@amd.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Subject: [REGRESSION] [PATCH] drm/amd/display: fix initial backlight
 brightness calculation
Message-ID: <aHy4tohvbwd1HpxI@hacktheplanet.fi>
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 <46de4f2a-8836-42cd-a621-ae3e782bf253@kernel.org>
 <aHru-sP7S2ufH7Im@hacktheplanet.fi>
 <664c5661-0fa8-41db-b55d-7f1f58e40142@kernel.org>
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 <f12cfe85-3597-4cf7-9236-3e00f16c3c38@kernel.org>
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In-Reply-To: <aHy4Ols-BZ3_UgQQ@hacktheplanet.fi>

DIV_ROUND_CLOSEST(x, 100) returns either 0 or 1 if 0<x<=100, so the
division needs to be performed after the multiplication and not the
other way around, to properly scale the value.

Fixes: 6c56c8ec6f97 ("drm/amd/display: Fix default DC and AC levels")
Signed-off-by: Lauri Tirkkonen <lauri@hacktheplanet.fi>
---
 drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
index f58fa5da7fe5..8a5b5dfad1ab 100644
--- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
+++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
@@ -4941,9 +4941,9 @@ amdgpu_dm_register_backlight_device(struct amdgpu_dm_connector *aconnector)
 	caps = &dm->backlight_caps[aconnector->bl_idx];
 	if (get_brightness_range(caps, &min, &max)) {
 		if (power_supply_is_system_supplied() > 0)
-			props.brightness = (max - min) * DIV_ROUND_CLOSEST(caps->ac_level, 100);
+			props.brightness = DIV_ROUND_CLOSEST((max - min) * caps->ac_level, 100);
 		else
-			props.brightness = (max - min) * DIV_ROUND_CLOSEST(caps->dc_level, 100);
+			props.brightness = DIV_ROUND_CLOSEST((max - min) * caps->dc_level, 100);
 		/* min is zero, so max needs to be adjusted */
 		props.max_brightness = max - min;
 		drm_dbg(drm, "Backlight caps: min: %d, max: %d, ac %d, dc %d\n", min, max,
-- 
2.50.1

-- 
Lauri Tirkkonen | lotheac @ IRCnet