GNOME's Mutter Picks Up Another Optimization For Helping DisplayLink-Type Hardware
Written by Michael Larabel in GNOME on 12 July 2019 at 07:02 AM EDT. 2 Comments
Collabora's Pekka Paalanen landed another optimization this week into GNOME's Mutter for further enhancing the performance of using DisplayLink hardware and similar secondary GPUs under this Linux desktop.

Over the past few cycles we've seen a lot of improvements made for bettering the performance of DisplayLink USB graphics connected displays under the GNOME desktop environment. While the experience has already improved a lot, for GNOME 3.34 due out in September will be more optimizations.

This latest optimization is using the primary GPU of the system for copying to secondary outputs. This is a fallback to the hardware accelerated copies from the primary GPU memory and an alternative to the slow CPU-based copy path. This new copy path copies using the primary GPU writing into the memory of the secondary GPU.

Pekka explained, "This attempts a somewhat notorious operation: using a GPU to render into a DRM dumb buffer. Normally that is illegal, but in this case we already have a working fallback just in case it doesn't work. The DRM dumb buffer is allocated on the secondary GPU device to ensure it will work for KMS on the secondary's outputs. The primary GPU copy is only attempted if the secondary GPU copy path has already been deemed unavailable. Therefore this MR cannot regress any "PRIME" use cases that are already using the secondary GPU to do the copy. The primary GPU copy path can only be used where previously the CPU copy path was used, and there, when it works, it should be a major performance win."

This new copy path yielded lower CPU utilization for Mutter and also much lower wall times per call. More details via this now-honored MR.
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