Executive summary: speed of 2D engine doubles if LCD FIFO settings are tuned. Also: no more busywaits with latest DRM kernel.
Hi all, While debugging something different late last night, I noticed some flags in one of Glamo's registers which looked interesting: FIFO settings for the LCD engine. This reminded me of an observation by Lars a few weeks ago that the LCD engine seems to conflict with the 2D engine on memory accesses, leading to slower performance of accelerated 2D operations when the screen is switched on. So I turned the FIFO up to "8 stages" (from 1) to see what happened. The result was much faster 2D operations - literally twice the speed! At "8 stages", the price of this was that the display became jittery and unstable. However, the same speed improvement is seen at the "4 stages" setting. I've also seen some occasional artifacts with this setting, so I'm using 2 stages at the moment, where the speed is still right up there. I'll be testing some more and seeing if we can tune the memory access even more. Because we don't make the maximum use possible of the 2D engine, this doesn't immediately translate into a huge increase in the UI speed. But the differences are very obvious with x11perf and some of my test programs. In addition, this week I've been working on kernel code to eliminate the busywaits which are currently used when the CPU needs to wait for the GPU. I did this by implementing fence objects and a wait queue mechanism where processes are sent to sleep until woken up by an interrupt when Glamo is finished. That means that we can now do accelerated operations at flat out speed with little or no CPU usage. In practice it's not so simple, of course, because there's an overhead associated with sending the process to sleep then waking it up (and X seems to like accelerating even very small operations), but combined with the FIFO improvements described above this seems to make a noticeable improvement in the overall GUI speed. Anyone brave/stupid enough to test this will need the latest drm-tracking kernel and xf86-video-glamo with KMS enabled. For people who don't currently use KMS, here's patch against current andy-tracking which enabled the "magic FIFO" - but the busywait removal cannot easily be implemented without DRM: http://www.bitwiz.org.uk/glamo-lcd-fifo.patch Tom -- Thomas White <t...@bitwiz.org.uk> _______________________________________________ devel mailing list devel@lists.openmoko.org https://lists.openmoko.org/mailman/listinfo/devel
