Bug#658662: drm/i915: no signal via DisplayPort on Sandy Bridge since Linux 3.2

[Jonathan Nieder <jrnieder@gmail.com>]

Hi,

Peter Colberg wrote:

> To summarize: The external display connected via DisplayPort works
> fine with Linux 3.1, while, with Linux 3.2, it works with lower
> (non-native) resolutions and fails with the maximum (native)
> resolution.

Please report this upstream, following instructions from [1], and let
us know the bug number so we can track it.

The upstream developers may ask you to bisect to find the specific
patch that introduced the problem.  It works like this:

 0. Prerequisites:

	apt-get install git build-essential

 1. Get a copy of the kernel source repository:

	git clone \
	  git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
	cd linux

    Or if you already have a copy, update it:

	cd linux
	git fetch origin

 2. Test v3.2:

	git checkout v3.2
	cp /boot/config-$(uname -r) .config; # current configuration
	make localmodconfig; # optional: minimize configuration
	make deb-pkg; # optionally with -j<n> for parallel build
	dpkg -i ../<name of package>
	reboot
	... test test test ...

    If it doesn't exhibit the problem, declare victory and we can
    try to figure out what Debian-specific patch (unlikely) or
    configuration change (more likely) caused it.  Otherwise:

 3. Test v3.1:

	git checkout v3.1
	make silentoldconfig; # reuse configuration
	make deb-pkg; # maybe with -j4
	dpkg -i ../<name of package>
	reboot
	... test test test ...

    Hopefully it does not exhibit the problem.

 4. Test a version halfway between:

	git bisect start v3.2 v3.1; # checks out a version halfway between to test
	make silentoldconfig; # reuse configuration
	make deb-pkg; # maybe with -j4
	dpkg -i ../<name of package>
	reboot
	... test test test ...

	git bisect bad; # if it exhibits the problem
	git bisect good; # if the external display works fine, after a
	                 # power-cycle and with the right resolution
	git bisect skip; # if some other problem makes it hard to test

 5. Another version to test will be automatically checked out.  So:

	make silentoldconfig
	make deb-pkg; # maybe with -j4
	dpkg -i ../<name of package>
	reboot
	... test ...
	git bisect good/bad/skip

 6. Rinse and repeat until bored.

 7. After sufficiently many iterations, git will indicate the "first
    bad commit" --- hoorah!  If bored before then, you can run "git
    bisect log" to print information about the versions tested in a
    form that will allow someone else to pick up where you left off.
    At any step if the gitk package is installed, you can run "git
    bisect visualize" to watch the regression range narrowing.

Hope that helps,
Jonathan

[1] http://intellinuxgraphics.org/how_to_report_bug.html