[REGRESSION] bisected: 7.1rc1 breaks backlight control on Thinkpad T480

[REGRESSION] bisected: 7.1rc1 breaks backlight control on Thinkpad T480

[Stefan Seyfried]

Hi all,

I found that 7.1rc1 broke the backlight control on my Thinkpad T480.
Bisected it to

> 40d2f5820951dee818d05c14677277048bd85f9f is the first bad commit
> commit 40d2f5820951dee818d05c14677277048bd85f9f
> Author: Suraj Kandpal <suraj.kandpal@intel.com>
> Date:   Tue Feb 24 08:43:22 2026 +0530
> 
>     drm/i915/backlight: Remove try_vesa_interface
>     
>     Some panels need VESA DPCD AUX backlight but VBT says otherwise.
>     This is why we try with Intel backlight interface over VESA backlight
>     interface. This causes a blankout on such panels without any fallback
>     mechanism.
>     Remove try_vesa_interface and use VESA AUX backlight interface as a
>     fallback mechanism.
>     While at in sneak in a small comment cleanup too.
>     
>     Closes: https://gitlab.freedesktop.org/drm/i915/kernel/-/issues/15679
>     Signed-off-by: Suraj Kandpal <suraj.kandpal@intel.com>
>     Reviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
>     Link: https://patch.msgid.link/20260224031322.2568874-1-suraj.kandpal@intel.com
> 
>  drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c | 19 +++++++------------
>  1 file changed, 7 insertions(+), 12 deletions(-)

Please revert :-)

Best regards,
-- 
Stefan Seyfried

"For a successful technology, reality must take precedence over
  public relations, for nature cannot be fooled." -- Richard Feynman

Re: [REGRESSION] bisected: 7.1rc1 breaks backlight control on Thinkpad T480

[Thorsten Leemhuis]

On 5/15/26 13:47, Stefan Seyfried wrote:
> 
> I found that 7.1rc1 broke the backlight control on my Thinkpad T480.
> Bisected it to
>
>> 40d2f5820951dee818d05c14677277048bd85f9f is the first bad commit
>> commit 40d2f5820951dee818d05c14677277048bd85f9f
>> Author: Suraj Kandpal <suraj.kandpal@intel.com>
>> Date:   Tue Feb 24 08:43:22 2026 +0530
>>
>>     drm/i915/backlight: Remove try_vesa_interface

FWIW: Here are two bug reports that contain proposed fixes for what
might or might not be the issue your face:

https://gitlab.freedesktop.org/drm/i915/kernel/-/work_items/16043
https://gitlab.freedesktop.org/drm/i915/kernel/-/work_items/16015

Ciao,  Thorsten

>>         Some panels need VESA DPCD AUX backlight but VBT says otherwise.
>>     This is why we try with Intel backlight interface over VESA backlight
>>     interface. This causes a blankout on such panels without any fallback
>>     mechanism.
>>     Remove try_vesa_interface and use VESA AUX backlight interface as a
>>     fallback mechanism.
>>     While at in sneak in a small comment cleanup too.
>>         Closes: https://gitlab.freedesktop.org/drm/i915/kernel/-/
>> issues/15679
>>     Signed-off-by: Suraj Kandpal <suraj.kandpal@intel.com>
>>     Reviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
>>     Link: https://patch.msgid.link/20260224031322.2568874-1-
>> suraj.kandpal@intel.com
>>
>>  drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c | 19 ++++++
>> +------------
>>  1 file changed, 7 insertions(+), 12 deletions(-)
> 
> Please revert :-)
> 
> Best regards,

Re: [REGRESSION] bisected: 7.1rc1 breaks backlight control on Thinkpad T480

[Stefan Seyfried]

Am 15.05.26 um 14:13 schrieb Thorsten Leemhuis:
> On 5/15/26 13:47, Stefan Seyfried wrote:
>>
>> I found that 7.1rc1 broke the backlight control on my Thinkpad T480.
>> Bisected it to
>>
>>> 40d2f5820951dee818d05c14677277048bd85f9f is the first bad commit
>>> commit 40d2f5820951dee818d05c14677277048bd85f9f
>>> Author: Suraj Kandpal <suraj.kandpal@intel.com>
>>> Date:   Tue Feb 24 08:43:22 2026 +0530
>>>
>>>      drm/i915/backlight: Remove try_vesa_interface
> 
> FWIW: Here are two bug reports that contain proposed fixes for what
> might or might not be the issue your face:
> 
> https://gitlab.freedesktop.org/drm/i915/kernel/-/work_items/16043
> https://gitlab.freedesktop.org/drm/i915/kernel/-/work_items/16015

Thanks, will look into testing the patches floating around there.

For now, this

  diff --git a/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c 
b/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
  index a8d56ebf06a2..c4ac46559669 100644
  --- a/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
  +++ b/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
  @@ -745,7 +745,7 @@ int intel_dp_aux_init_backlight_funcs(struct 
intel_connector *connector)
                  return 0;
          }

  -       if (intel_dp_aux_supports_vesa_backlight(connector)) {
  +       if (0 && intel_dp_aux_supports_vesa_backlight(connector)) {
                  drm_dbg_kms(dev, "[CONNECTOR:%d:%s] Using VESA eDP 
backlight controls\n",
                              connector->base.base.id, 
connector->base.name);
                  panel->backlight.funcs = &intel_dp_vesa_bl_funcs;

is enough to get the backlight control on the T480 going again.

I know nothing about that code (obviously), but maybe 
"intel_dp_aux_supports_vesa_backlight()" is wrongly determining VESA 
backlight support on my machine?

-- 
Stefan Seyfried

"For a successful technology, reality must take precedence over
  public relations, for nature cannot be fooled." -- Richard Feynman