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<th>Priority</th>
<td>medium
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<tr>
<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - [Backlight] Very slow response changing backlight on intel video"
href="https://bugs.freedesktop.org/show_bug.cgi?id=72959">72959</a>
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<th>Assignee</th>
<td>chris@chris-wilson.co.uk
</td>
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<th>Summary</th>
<td>[Backlight] Very slow response changing backlight on intel video
</td>
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<th>QA Contact</th>
<td>intel-gfx-bugs@lists.freedesktop.org
</td>
</tr>
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<th>Severity</th>
<td>normal
</td>
</tr>
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<th>Classification</th>
<td>Unclassified
</td>
</tr>
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<th>OS</th>
<td>All
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<th>Reporter</th>
<td>cheetah-freedesktop@fastcat.org
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<th>Hardware</th>
<td>x86-64 (AMD64)
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<th>Status</th>
<td>NEW
</td>
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<th>Version</th>
<td>unspecified
</td>
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<th>Component</th>
<td>Driver/intel
</td>
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<th>Product</th>
<td>xorg
</td>
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<p>
<div>
<pre>References to other reports of this issue:
<a href="https://bugs.launchpad.net/ubuntu/+source/gnome-power-manager/+bug/847001">https://bugs.launchpad.net/ubuntu/+source/gnome-power-manager/+bug/847001</a>
<a href="https://bugs.launchpad.net/ubuntu/+source/linux/+bug/828296">https://bugs.launchpad.net/ubuntu/+source/linux/+bug/828296</a>
<a href="http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=732753">http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=732753</a>
When changing the backlight in the default setup on many Dell and at least some
HP laptops, while the backlight does change, the X environment becomes
unresponsive for several seconds per backlight change (thus changing from
minimum to maximum brighness can cause X to be unresponsive for 30+ seconds).
This seems to be caused by the server preferring the acpi backlight over the
intel backlight.
The second launchpad link has a patch and a workaround that both purport to fix
this. The patch (which is 2 years old, but should still apply after
adjustments for files being moved in the source tree) addresses the issue by
making the intel_backlight sysfs interface be preferred over the acpi
interfaces.
The workaround given (which I can confirm works) is to add
acpi_backlight=vendor to the kernel boot, which effectively disables the acpi
backlight entries and causes the driver to pick (for me) the dell_backlight
entry. From the linked reports, for others the dell backlight entry doesn't
exist, so the intel backlight is picked.
With the server having chosen one of the non-acpi backlight entries, brightness
changes work and are nearly instantaneous.</pre>
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