[Intel-gfx] Dual monitor stuttering bug

[Chris Wilson]

Quoting Renato Ferreira (2017-12-06 23:50:26)
> Hello,
> 
> I have an issue though i'm not really sure where it belongs to, i915,
> mesa, drm or xorg, so i'm really sorry if it doesn't belong here.
> 
> The issue is as follows:
> 
> I em experiencing a very noticeable stuttering (mainly on scroll) in any
> app *only* when an external monitor is enabled, and the stutter happens
> *only* on the external monitor (normally, see below), connected through
> HDMI.

Ok, try switching the RandR Primary (e.g. xrandr --output HDMI --primary).
If you are using comptom and it is doing a fullscreen pageflip, then the
update rate will be combined response of both of the monitors. That
doesn't explain why the problem is mostly visible on one, as they both
should feel stuttery.

Pretty much you want to measure the input-output latency of your
actions, and then trace that to find the cause. Off hand, I don't have a
better suggestion than an evening spent reading debug logs
(xf86-video-intel --enable-debug=full will give you everything you need
to know, and so much that you don't).
-Chris