i915: framebuffer compression causing occasional screen flickering

i915: framebuffer compression causing occasional screen flickering

[Petr Kulhavy]

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Hi,

I would like to post a bug report for the Intel i915 driver, not sure if 
this is the right place to do it.

The framebuffer compression feature is sometimes causing screen 
flickering on my Asus UX501 laptop (i7-4720HQ with the HD4600 graphics). 
Konsole out
I'm trying to reduce my battery usage and the kernel parameter 
"i915.enable_fbc=1" saves about 2W actually!

I'm running Kubuntu 15.04 with mainstream kernel 4.1.0 and the latest 
Intel drivers from 01.org version 1.2.0.
However I was experiencing the same problem also with previous versions.

The flickering itself is as follows. In most cases the picture (X with 
KDE desktop) is OK. However with certain image patterns the screen 
starts to flicker multiple times per second, sometimes it affects only 
part of the screen, and it looks like the picture switching fast between 
normal and a bit scrambled. The screen is still readable.

An example of an image pattern triggering the flickering  Firefox with 
youtube at the end of the video when you see the tiles with other 
similar videos to watch. If I navigate to a different page the 
flickering disappears. It is really picture dependent.

Not sure what it exactly is but with the enable_fbc=0 the problem 
reliably disappears. So I believe it is related to the framebuffer 
compression.

It would be really great if you could make the FBC working well also on 
this HW. It saves a lot of battery.

Please let me know if you need some more information.

Thanks a lot
Petr



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