Re: [PATCH] Revert "drm/i915/backlight: Remove try_vesa_interface"

On 05/06/2026 11:04, Kandpal, Suraj wrote:
>> -----Original Message-----
>> From: Tvrtko Ursulin <[email protected]>
>> Sent: Friday, June 5, 2026 3:23 PM
>> To: Thorsten Leemhuis <[email protected]>; Tvrtko Ursulin
>> <[email protected]>; Dave Airlie <[email protected]>; Simona Vetter
>> <[email protected]>; Kandpal, Suraj <[email protected]>
>> Cc: Nautiyal, Ankit K <[email protected]>; Murthy, Arun R
>> <[email protected]>; [email protected]; intel-
>> [email protected]; Vivi, Rodrigo <[email protected]>; Linux kernel
>> regressions list <[email protected]>; ML dri-devel <dri-
>> [email protected]>; Jani Nikula <[email protected]>
>> Subject: Re: [PATCH] Revert "drm/i915/backlight: Remove try_vesa_interface"
>>
>>
>> Hi Suraj,
>>
>> On 05/06/2026 07:58, Thorsten Leemhuis wrote:
>>> On 6/4/26 15:55, Thorsten Leemhuis wrote:
>>>> On 5/17/26 04:47, Suraj Kandpal wrote:
>>>>> This reverts commit 40d2f5820951dee818d05c14677277048bd85f9f.
>>>>>
>>>>> Removing the try_vesa_interface gate caused a backlight regression
>>>>> on panels whose VBT correctly reports INTEL_BACKLIGHT_DISPLAY_DDI
>>>>> and whose PWM path is the actual backlight control, but whose DPCD
>>>>> optimistically advertises DP_EDP_BACKLIGHT_AUX_ENABLE_CAP /
>> _BRIGHTNESS_AUX_SET_CAP.
>>>>> After the commit such panels silently bind to the VESA AUX backlight
>>>>> funcs; AUX writes complete but the panel ignores them, leaving
>>>>> brightness stuck (no-op backlight). Observed on at least KBL and TGL
>>>>> eDP setups.
>>>>
>>>> Lo! What's the status of this regression fix? It's a -next for two
>>>> weeks now as f30fddb4402313 ("Revert "drm/i915/backlight: Remove
>>>> try_vesa_interface""), but from the outside and checking
>>>> https://gitlab.freedesktop.org/drm/i915/kernel/-/commits/drm-intel-fi
>>>> xes it looks like it's scheduled for merging in the next cycle.
>>>
>>> Resending to Tvrtko, who sent the i915 PR yesterday (which didn't
>>> contain that fix), as well as Dave and Simona.
>>>
>>> FWIW, due to the lack of response to various inquiries I'm considering
>>> to ask Linus to directly pick up the mentioned regression fix to
>>> ensure it makes it into rc7.
>>>
>>> In case anyone wonder what regression I'm talking about:
>>>
>>> * https://gitlab.freedesktop.org/drm/i915/kernel/-/issues/16015 /
>>> https://lore.kernel.org/lkml/CADo9pHjr-
>> zZ9C3%2B026y5%2BXOGPSeRzSJMCHof
>>> [email protected]/
>>> * https://gitlab.freedesktop.org/drm/i915/kernel/-/issues/16043
>>> * https://gitlab.freedesktop.org/drm/i915/kernel/-/work_items/16097 /
>>> https://lore.kernel.org/lkml/d2de7933-e650-4b19-8d88-90d66693dcfc@mess
>>> age-id.googlemail.com/
>>>
>>> Ciao, Thorsten
>>>
>>>> But I think it should be merged this cycle (ideally before -rc7, as
>>>> Linus wants all known regression fixed by -rc6), as it fixes a
>>>> regression that is known since the -rc1 days. I already asked for the
>>>> mainlining plans in gitlab tickets about a week ago (and since then
>>>> affected users spoke up, too), but there was no conclusive answer for
>>>> the plans, which is why I'm trying this way now.
>>
>> Was there a reason the revert was not marked with a Fixes: tag? Or in other
>> words, any particular reason why it should *not* be picked up for drm-intel-
>> fixes?
> 
> There wasn't any particular reason.
> Tbh I didn't think we would require it since this revert was fixing the issue caused by the patch in question being reverted.

Okay then, thank you. I've cherry picked it and sent a new pull request. 
The previous one hasn't been pulled yet so hopefully this is still in time.

Regards,

Tvrtko

>>>> Ciao, Thorsten
>>>>
>>>>
>>>>> Signed-off-by: Suraj Kandpal <[email protected]>
>>>>> ---
>>>>>    .../drm/i915/display/intel_dp_aux_backlight.c | 19 ++++++++++++-------
>>>>>    1 file changed, 12 insertions(+), 7 deletions(-)
>>>>>
>>>>> 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..7a6c07f6aaeb 100644
>>>>> --- a/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
>>>>> +++ b/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
>>>>> @@ -691,10 +691,9 @@ int intel_dp_aux_init_backlight_funcs(struct
>> intel_connector *connector)
>>>>>    	struct intel_dp *intel_dp = intel_attached_dp(connector);
>>>>>    	struct drm_device *dev = connector->base.dev;
>>>>>    	struct intel_panel *panel = &connector->panel;
>>>>> -	bool try_intel_interface = false;
>>>>> +	bool try_intel_interface = false, try_vesa_interface = false;
>>>>>
>>>>> -	/*
>>>>> -	 * Check the VBT and user's module parameters to figure out which
>>>>> +	/* Check the VBT and user's module parameters to figure out which
>>>>>    	 * interfaces to probe
>>>>>    	 */
>>>>>    	switch (display->params.enable_dpcd_backlight) { @@ -703,6 +702,7
>>>>> @@ int intel_dp_aux_init_backlight_funcs(struct intel_connector
>> *connector)
>>>>>    	case INTEL_DP_AUX_BACKLIGHT_AUTO:
>>>>>    		switch (panel->vbt.backlight.type) {
>>>>>    		case INTEL_BACKLIGHT_VESA_EDP_AUX_INTERFACE:
>>>>> +			try_vesa_interface = true;
>>>>>    			break;
>>>>>    		case INTEL_BACKLIGHT_DISPLAY_DDI:
>>>>>    			try_intel_interface = true;
>>>>> @@ -715,12 +715,20 @@ int intel_dp_aux_init_backlight_funcs(struct
>> intel_connector *connector)
>>>>>    		if (panel->vbt.backlight.type !=
>> INTEL_BACKLIGHT_VESA_EDP_AUX_INTERFACE)
>>>>>    			try_intel_interface = true;
>>>>>
>>>>> +		try_vesa_interface = true;
>>>>> +		break;
>>>>> +	case INTEL_DP_AUX_BACKLIGHT_FORCE_VESA:
>>>>> +		try_vesa_interface = true;
>>>>>    		break;
>>>>>    	case INTEL_DP_AUX_BACKLIGHT_FORCE_INTEL:
>>>>>    		try_intel_interface = true;
>>>>>    		break;
>>>>>    	}
>>>>>
>>>>> +	/* For eDP 1.5 and above we are supposed to use VESA interface for
>> brightness control */
>>>>> +	if (intel_dp->edp_dpcd[0] >= DP_EDP_15)
>>>>> +		try_vesa_interface = true;
>>>>> +
>>>>>    	/*
>>>>>    	 * Since Intel has their own backlight control interface, the majority of
>> machines out there
>>>>>    	 * using DPCD backlight controls with Intel GPUs will be using
>>>>> this interface as opposed to @@ -733,9 +741,6 @@ int
>> intel_dp_aux_init_backlight_funcs(struct intel_connector *connector)
>>>>>    	 * panel with Intel's OUI - which is also required for us to be able to
>> detect Intel's
>>>>>    	 * backlight interface at all. This means that the only sensible way for us
>> to detect both
>>>>>    	 * interfaces is to probe for Intel's first, and VESA's second.
>>>>> -	 *
>>>>> -	 * Also there is a chance some VBTs may advertise false Intel backlight
>> support even if the
>>>>> -	 * TCON DPCD says otherwise. This means we keep VESA interface as
>> fallback in that case.
>>>>>    	 */
>>>>>    	if (try_intel_interface && intel_dp->edp_dpcd[0] <= DP_EDP_14b &&
>>>>>    	    intel_dp_aux_supports_hdr_backlight(connector)) { @@ -745,7
>>>>> +750,7 @@ int intel_dp_aux_init_backlight_funcs(struct intel_connector
>> *connector)
>>>>>    		return 0;
>>>>>    	}
>>>>>
>>>>> -	if (intel_dp_aux_supports_vesa_backlight(connector)) {
>>>>> +	if (try_vesa_interface &&
>>>>> +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;
>>>>
>>>
>