[3.16.y-ckt stable] Patch "drm/i915: fix backlight after resume on 855gm" has been added to staging queue

[Luis Henriques]

This is a note to let you know that I have just added a patch titled

    drm/i915: fix backlight after resume on 855gm

to the linux-3.16.y-queue branch of the 3.16.y-ckt extended stable tree 
which can be found at:

    http://kernel.ubuntu.com/git/ubuntu/linux.git/log/?h=linux-3.16.y-queue

This patch is scheduled to be released in version 3.16.7-ckt15.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.16.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

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>From 0f74d97f49e15a3b083a307a05351eb6ee5a5667 Mon Sep 17 00:00:00 2001
From: Jani Nikula <jani.nikula at intel.com>
Date: Fri, 26 Jun 2015 14:18:56 +0300
Subject: drm/i915: fix backlight after resume on 855gm
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

commit 2059ac3b1304cb6a82f9d90762dea9f556831627 upstream.

Some 855gm models (at least ThinkPad X40) regressed because of

commit b0cd324faed23d10d66ba6ade66579c681feef6f
Author: Jani Nikula <jani.nikula at intel.com>
Date:   Wed Nov 12 16:25:43 2014 +0200

    drm/i915: don't save/restore backlight hist ctl registers

which tried to make our driver more robust by not blindly saving and
restoring registers, but it failed to take into account

commit 0eb96d6ed38430b72897adde58f5477a6b71757a
Author: Jesse Barnes <jbarnes at virtuousgeek.org>
Date:   Wed Oct 14 12:33:41 2009 -0700

    drm/i915: save/restore BLC histogram control reg across suspend/resume

Fix the regression by enabling hist ctl on gen2.

v2: Improved the comment.

v3: Improved the comment, again.

Reported-and-tested-by: Philipp Gesang <phg at phi-gamma.net>
References: http://mid.gmane.org/20150623222648.GD12335@acheron
Fixes: b0cd324faed2 ("drm/i915: don't save/restore backlight hist ctl registers")
Cc: Ville Syrjälä <ville.syrjala at linux.intel.com>
Acked-by: Chris Wilson <chris at chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula at intel.com>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 drivers/gpu/drm/i915/i915_reg.h    | 1 +
 drivers/gpu/drm/i915/intel_panel.c | 8 ++++++++
 2 files changed, 9 insertions(+)

diff --git a/drivers/gpu/drm/i915/i915_reg.h b/drivers/gpu/drm/i915/i915_reg.h
index dda970cb01c1..fa0ec5aed9ed 100644
--- a/drivers/gpu/drm/i915/i915_reg.h
+++ b/drivers/gpu/drm/i915/i915_reg.h
@@ -2898,6 +2898,7 @@ enum punit_power_well {
 #define   BLM_POLARITY_PNV			(1 << 0) /* pnv only */

 #define BLC_HIST_CTL	(dev_priv->info.display_mmio_offset + 0x61260)
+#define  BLM_HISTOGRAM_ENABLE			(1 << 31)

 /* New registers for PCH-split platforms. Safe where new bits show up, the
  * register layout machtes with gen4 BLC_PWM_CTL[12]. */
diff --git a/drivers/gpu/drm/i915/intel_panel.c b/drivers/gpu/drm/i915/intel_panel.c
index 59b294c5fe41..a25816cb3df1 100644
--- a/drivers/gpu/drm/i915/intel_panel.c
+++ b/drivers/gpu/drm/i915/intel_panel.c
@@ -774,6 +774,14 @@ static void i9xx_enable_backlight(struct intel_connector *connector)

 	/* XXX: combine this into above write? */
 	intel_panel_actually_set_backlight(connector, panel->backlight.level);
+
+	/*
+	 * Needed to enable backlight on some 855gm models. BLC_HIST_CTL is
+	 * 855gm only, but checking for gen2 is safe, as 855gm is the only gen2
+	 * that has backlight.
+	 */
+	if (IS_GEN2(dev))
+		I915_WRITE(BLC_HIST_CTL, BLM_HISTOGRAM_ENABLE);
 }

 static void i965_enable_backlight(struct intel_connector *connector)