Subject: FAILED: patch "[PATCH] drm/amd/display: Scale custom brightness curve from full" failed to apply to 6.18-stable tree
To: zhitaevakh@gmail.com,alexander.deucher@amd.com,mario.limonciello@amd.com,superm1@kernel.org
Cc: <stable@vger.kernel.org>
From: <gregkh@linuxfoundation.org>
Date: Wed, 09 Sep 2026 13:53:00 +0200
Message-ID: <2026090900-lip-ribbon-1726@gregkh>
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The patch below does not apply to the 6.18-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.18.y
git checkout FETCH_HEAD
git cherry-pick -x 6fd83a1c2cdea48c396f600795217fbdfb8124f6
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '2026090900-lip-ribbon-1726@gregkh' --subject-prefix 'PATCH 6.18.y' 'HEAD^..'

Possible dependencies:


thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 6fd83a1c2cdea48c396f600795217fbdfb8124f6 Mon Sep 17 00:00:00 2001
From: Akhmed Zhitaev <zhitaevakh@gmail.com>
Date: Thu, 13 Aug 2026 22:09:59 +0500
Subject: [PATCH] drm/amd/display: Scale custom brightness curve from full
 range

Custom brightness curves use an 8-bit input signal.  After exporting the
full PWM range to userspace, the curve normalizer still divides requests
by the physical PWM span.  On panels with a nonzero minimum PWM level,
this can produce a curve input greater than 255 and send an invalid
backlight level to DC.

Scale the userspace [0..max] range to the curve's [0..255] range
instead.  This retains the full advertised range and keeps the reverse
readback conversion unchanged.

Fixes: 8dbd72cb7900 ("drm/amd/display: Export full brightness range to userspace")
Cc: stable@vger.kernel.org
Signed-off-by: Akhmed Zhitaev <zhitaevakh@gmail.com>
Reviewed-by: Mario Limonciello (AMD) <superm1@kernel.org>
(Move to amdgpu_dm_backlight.c)
Link: https://patch.msgid.link/20260813170959.22073-1-zhitaevakh@gmail.com
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

diff --git a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_backlight.c b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_backlight.c
index b66ca60e697d..e61bbc310f33 100644
--- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_backlight.c
+++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_backlight.c
@@ -106,10 +106,10 @@ int get_brightness_range(const struct amdgpu_dm_backlight_caps *caps,
 }
 EXPORT_IF_KUNIT(get_brightness_range);
 
-/* Rescale from [min..max] to [0..AMDGPU_MAX_BL_LEVEL] */
-static inline u32 scale_input_to_fw(int min, int max, u64 input)
+/* Rescale userspace [0..max] to the firmware curve's [0..255]. */
+static inline u32 scale_input_to_fw(int max, u64 input)
 {
-	return DIV_ROUND_CLOSEST_ULL(input * AMDGPU_MAX_BL_LEVEL, max - min);
+	return DIV_ROUND_CLOSEST_ULL(input * AMDGPU_MAX_BL_LEVEL, max);
 }
 
 /* Rescale from [0..AMDGPU_MAX_BL_LEVEL] to [min..max] */
@@ -123,7 +123,7 @@ void convert_custom_brightness(const struct amdgpu_dm_backlight_caps *caps,
 			       unsigned int min, unsigned int max,
 			       uint32_t *user_brightness)
 {
-	u32 brightness = scale_input_to_fw(min, max, *user_brightness);
+	u32 brightness = scale_input_to_fw(max, *user_brightness);
 	u8 lower_signal, upper_signal, upper_lum, lower_lum, lum;
 	int left, right;