PREEMPT_RT vs i915

[Mike Galbraith]

On Thu, 2025-07-10 at 18:50 +0300, Ville Syrjälä wrote:
> On Thu, Jul 10, 2025 at 06:52:58AM +0200, Mike Galbraith wrote:
> > > 
> > > Until someone actually does the work to confirm the thing is working
> > > reliably there's no point in posting anything.
> > 
> > What does that entail?
> 
> Basic testing would be something like this:
> - enable CONFIG_DRM_I915_DEBUG_VBLANK_EVADE
> - set i915.enable_dsb=0 to make sure everything takes the
>   mmio path

I can give that a go while tossing rocks to check for moaning..

> - stress the heck out of it and make sure the histogram
>   doesn't look significantly worse than on !RT
>   (kms_atomic_transition --extended might take care of the display
>   side here, but it should probably be accompanied with some
>   horrendous system loads which is a less well defined part)
> - ideally do that on a potato
>   (some VLV/CHV (Atom) thing would probably be a good candidate)
> - repeat with lockdep enabled to make everything even harder

..forwarding any performance goop emitted, but lockdep runs out of lock
counting fingers and turns itself off in short order with RT builds.

	-Mike