Backlight control broken on UM325 (OLED) on 5.15 (bisected)

[Alex Deucher]

On Mon, Nov 15, 2021 at 2:35 AM Samuel Čavoj <samuel at cavoj.net> wrote:
>
> Hello,
>
> the backlight control no longer works on my ASUS UM325 (Ryzen 5700U)
> OLED laptop. I have bisected the breakage to commit 7fd13baeb7a3a48.
>
>     commit 7fd13baeb7a3a48cae12c36c52f06bf4e9e7d728 (HEAD, refs/bisect/bad)
>     Author: Alex Deucher <alexander.deucher at amd.com>
>     Date:   Thu Jul 8 16:31:10 2021 -0400
>
>         drm/amdgpu/display: add support for multiple backlights
>
>         On platforms that support multiple backlights, register
>         each one separately.  This lets us manage them independently
>         rather than registering a single backlight and applying the
>         same settings to both.
>
>         v2: fix typo:
>         Reported-by: kernel test robot <lkp at intel.com>
>
>         Reviewed-by: Roman Li <Roman.Li at amd.com>
>         Signed-off-by: Alex Deucher <alexander.deucher at amd.com>
>
> I have encountered another user with the same issue on reddit[0].
>
> The node in /sys/class/backlight exists, writing to it just does
> nothing. I would be glad to help debugging the issue. Thank you very
> much.

That patch adds support for systems with multiple backlights.  Do you
have multiple backlight devices now?  If so, does the other one work?

Can you also try this patch?

diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_acpi.c
b/drivers/gpu/drm/amd/amdgpu/amdgpu_acpi.c
index 4811b0faafd9..67163c9d49e6 100644
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu_acpi.c
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_acpi.c
@@ -854,8 +854,8 @@ int amdgpu_acpi_init(struct amdgpu_device *adev)
                if (amdgpu_device_has_dc_support(adev)) {
 #if defined(CONFIG_DRM_AMD_DC)
                        struct amdgpu_display_manager *dm = &adev->dm;
-                       if (dm->backlight_dev[0])
-                               atif->bd = dm->backlight_dev[0];
+                       if (dm->backlight_dev[1])
+                               atif->bd = dm->backlight_dev[1];
 #endif
                } else {
                        struct drm_encoder *tmp;


Alex

>
> Regards,
> Samuel Čavoj
>
> [0]: https://www.reddit.com/r/AMDLaptops/comments/qst0fm/after_updating_to_linux_515_my_brightness/