linux-kernel - [REGRESSION] [PATCH v2] drm/amd/display: fix initial backlight brightness calculation

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Message-ID: <aH2Q_HJvxKbW74vU@hacktheplanet.fi>
Date: Mon, 21 Jul 2025 09:59:40 +0900
From: Lauri Tirkkonen <lauri@...ktheplanet.fi>
To: linux-kernel@...r.kernel.org
Cc: stable@...r.kernel.org, Mario Limonciello <superm1@...nel.org>,
	amd-gfx@...ts.freedesktop.org, Wayne Lin <wayne.lin@....com>,
	Alex Deucher <alexander.deucher@....com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: [REGRESSION] [PATCH v2] drm/amd/display: fix initial backlight
 brightness calculation

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: 8b5f3a229a70 ("drm/amd/display: Fix default DC and AC levels")
Signed-off-by: Lauri Tirkkonen <lauri@...ktheplanet.fi>
Cc: stable@...r.kernel.org
---
 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