FAILED: patch "[PATCH] drm/i915/backlight: fix CPU mode backlight takeover on LPT" failed to apply to 5.4-stable tree

[<gregkh@linuxfoundation.org>]

The patch below does not apply to the 5.4-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>.

thanks,

greg k-h

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

From bb83d5fb550bb7db75b29e6342417fda2bbb691c Mon Sep 17 00:00:00 2001
From: Jani Nikula <jani.nikula@intel.com>
Date: Fri, 8 Jan 2021 17:28:41 +0200
Subject: [PATCH] drm/i915/backlight: fix CPU mode backlight takeover on LPT
MIME-Version: 1.0
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The pch_get_backlight(), lpt_get_backlight(), and lpt_set_backlight()
functions operate directly on the hardware registers. If inverting the
value is needed, using intel_panel_compute_brightness(), it should only
be done in the interface between hardware registers and
panel->backlight.level.

The CPU mode takeover code added in commit 5b1ec9ac7ab5
("drm/i915/backlight: Fix backlight takeover on LPT, v3.") reads the
hardware register and converts to panel->backlight.level correctly,
however the value written back should remain in the hardware register
"domain".

This hasn't been an issue, because GM45 machines are the only known
users of i915.invert_brightness and the brightness invert quirk, and
without one of them no conversion is made. It's likely nobody's ever hit
the problem.

Fixes: 5b1ec9ac7ab5 ("drm/i915/backlight: Fix backlight takeover on LPT, v3.")
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Lyude Paul <lyude@redhat.com>
Cc: <stable@vger.kernel.org> # v5.1+
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210108152841.6944-1-jani.nikula@intel.com
(cherry picked from commit 0d4ced1c5bfe649196877d90442d4fd618e19153)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>

diff --git a/drivers/gpu/drm/i915/display/intel_panel.c b/drivers/gpu/drm/i915/display/intel_panel.c
index 9f23bac0d792..d64fce1a17cb 100644
--- a/drivers/gpu/drm/i915/display/intel_panel.c
+++ b/drivers/gpu/drm/i915/display/intel_panel.c
@@ -1650,16 +1650,13 @@ static int lpt_setup_backlight(struct intel_connector *connector, enum pipe unus
 		val = pch_get_backlight(connector);
 	else
 		val = lpt_get_backlight(connector);
-	val = intel_panel_compute_brightness(connector, val);
-	panel->backlight.level = clamp(val, panel->backlight.min,
-				       panel->backlight.max);
 
 	if (cpu_mode) {
 		drm_dbg_kms(&dev_priv->drm,
 			    "CPU backlight register was enabled, switching to PCH override\n");
 
 		/* Write converted CPU PWM value to PCH override register */
-		lpt_set_backlight(connector->base.state, panel->backlight.level);
+		lpt_set_backlight(connector->base.state, val);
 		intel_de_write(dev_priv, BLC_PWM_PCH_CTL1,
 			       pch_ctl1 | BLM_PCH_OVERRIDE_ENABLE);
 
@@ -1667,6 +1664,10 @@ static int lpt_setup_backlight(struct intel_connector *connector, enum pipe unus
 			       cpu_ctl2 & ~BLM_PWM_ENABLE);
 	}
 
+	val = intel_panel_compute_brightness(connector, val);
+	panel->backlight.level = clamp(val, panel->backlight.min,
+				       panel->backlight.max);
+
 	return 0;
 }