[Bug 77436] KDE Konsole very slow scrolling with TearFree and QT graphics system native

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=77436

--- Comment #158 from Chris Wilson <chris at chris-wilson.co.uk> ---
Yours @ 1024x768:
Testing a8r8g8b8 with Src: ref=0.273359, out=0.420982
Testing x8r8g8b8 with Src: ref=0.273589, out=0.421422
Testing a8r8g8b8 with Over: ref=0.775864, out=0.441080
Testing x8r8g8b8 with Over: ref=0.274387, out=0.416628

Mine @ 1024x768:
Testing a8r8g8b8 with Src: ref=0.870605, out=0.593283
Testing x8r8g8b8 with Src: ref=0.858216, out=0.589730
Testing a8r8g8b8 with Over: ref=1.947748, out=0.767362
Testing x8r8g8b8 with Over: ref=0.857667, out=0.586849

Looking at the reference, which uses pure software rasterisation, your
cpu/memory is a little over 2x faster (indeed that is most likely due to the
dual channel advantage you have). But it seems that the GPU is completely
memory bandwidth limited. Comparing Over with argb, which is the only operation
there that does perform a read from dst and blend with src before writing, we
do expect to see a significant impact from the extra read.

Can you please 'cat /sys/kernel/debug/dri/0/i915_ring_freq_table' and 'cat
/proc/cpuinfo'

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