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;
