Flickering on built-in display (Intel Haswell graphics) since kernel v4.1 [bisected and workaround attached]

Is the patch in upstream Linux?

No, I made it for myself (using another patch (https://bugs.freedesktop.org/show_bug.cgi?id=91393#c31) as a reference for adding i915 module parameter). Not sure if upstream would accept it. This patch doesn't fix root issue but provides ability to disable DRRS power-saving feature which causes it (I didn't find any other way to do that). By the way, Intel® Graphics Command Center on Windows also has an option to enable/disable "Dynamic Refresh Rate Switching" (https://www.intel.com/content/www/us/en/support/articles/000057188/graphics.html).

Do you know what's the offending commit?

Trying to find the reason for this issue I tested kernels from https://kernel.ubuntu.com/~kernel-ppa/mainline/ on Ubuntu 14.04

v4.0.9-wily - flickering didn't happen (but that's because DRRS wasn't activated in the 4.0.y series)
v4.1-rc3-unstable - problem started here.

So it's quite possible that on a particular machine this issue is present since v4.1-rc1

I found that this is DRRS by looking at i915 commits in the 4.1.y tree.
https://github.com/gregkh/linux/commits/linux-4.1.y/drivers/gpu/drm/i915/
intel_pc.c and intel_display.c have some DRRS-related changes

I can try to revert some commits from there.
Some interesting commits:
* drm/i915: Add support for DRRS in intel_dp_set_m_n
(https://github.com/gregkh/linux/commit/fe3cd48d6b616efc76b6a4003e82e933618c788a#diff-83261d1489be7f0a6d1b8306ff0fddbeb13ceb74768ea99274b009dce13ffd2e)
* drm/i915/bdw: Add support for DRRS to switch RR
(https://github.com/gregkh/linux/commit/a4c30b1d108c1ef60667ed357af4aeabc3d05eca#diff-83261d1489be7f0a6d1b8306ff0fddbeb13ceb74768ea99274b009dce13ffd2e)

It'll be useful if the offending commit can be found...

I still believe that this bug is present since initial implementation of DRRS (kernels 4.0 and 4.1).
For now I have to disable DRRS by default with a patch. Updated version in attachment.

Hello there,

I am having a problem that sounds quite similar to yours. https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1940285

Could you tell me what exactly I must type on terminal to check how my laptop's refresh rate changes (if any), just to check if it behaves like yours.

Thank you.

@alepasgar

1) Add options to your GRUB
sudo nano /etc/default/grub
At the line with "GRUB_CMDLINE_LINUX_DEFAULT" add "drm.debug=0x1e log_buf_len=4M"
Press Ctrl+x , confirm changes with Y
2) sudo update-grub
3) Restart
4) sudo dmesg | grep "Refresh Rate"

 I changed that line so that it says:

GRUB_CMDLINE_LINUX_DEFAULT="quiet splash drm.debug=0x1e log_buf_len=4M"

and follower the rest of the steps.

when typing:
sudo dmesg | grep "Refresh Rate"
on the terminal, nothing really happens.

I am not sure I am doing that last step well, since I checked the previous ones.

I guess you have some different issue. In my case HP Folio 1040 G1 had this that problem since kernel 4.1 and it was never fixed. But still you can analyze dmesg for anything suspicious (maybe try it with different kernels)
Here's a command to output information into dmesg.log in your home folder.
sudo dmesg > ~/dmesg.log