Debugging kernel issue (Kernel modeset to be particular)
 

Hi all,

I was trying to enable kernel modesetting in Ubuntu 14.04 but the screen blanked out so I couldn't even see the log messages. Hence my question(s)

1. How can I debug issues like this if the screen is off? I have heard about enabling kernel debug mode but I'm not sure if that's gonna work if the screen is off.
2. Suppose that I have debug mode enabled which writes the error message to my HDD. Then would I have to move that HDD somewhere to check for that error message?


P.S : I have no expertise in debugging kernel issues so I might be talking out of my ass for now. Bear with me :-)

I would have thought KMS would be on by default in any recent Ubuntu kernel.

You'd better describe in detail what you did and what happened - in detail. When does the screen "blank out" ?.
Do you get a grub boot option screen ?. If not, did you ever get one ?.

Data like that will help us get started.

As an aside, about the only debugging you will be able to do without the appropriate kernel options already compiled in is using a serial console to get the messages. But you still need to be able to update the boot options to get even that ... :rolleyes:

Indeed. However, I compiled a special kernel with some patches. Now the screen works iff I specify nomodeset in the boot parameters or if I boot in recovery mode which also specifies nomodeset.

I compiled a custom Linux kernel. I managed to get to the boot prompt where I selected my new kernel. Everything went well except the screen went blank halfway through. I managed to ssh into the system to get some useful info from dmesg. I found two messages that hopefully will give a more helpful hint on what's wrong here:

[drm:i915_gem_init_stolen [i915]] *ERROR* conflict detected with stolen region: [0x7ce00000 - 0x7ee00000]
[drm:intel_dp_complete_link_train [i915]] *ERROR* failed to train DP, aborting

I then googled the two messages to see what's going on. I'm still working on the first message.

According to https://bugs.freedesktop.org/show_bug.cgi?id=73694, it suggests that the latter does not give a conclusive hint on what's going wrong.

Moderator Response
 

Moved: This thread is more suitable in <Linux-Kernel> and has been moved accordingly to help your thread/question get the exposure it deserves.