[Bug 99247] glReadPixels on Intel N3700 (Braswell) slower than Raspberry Pi

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=99247

            Bug ID: 99247
           Summary: glReadPixels on Intel N3700 (Braswell) slower than
                    Raspberry Pi
           Product: Mesa
           Version: git
          Hardware: x86-64 (AMD64)
                OS: Linux (All)
            Status: NEW
          Severity: normal
          Priority: medium
         Component: Drivers/DRI/i965
          Assignee: intel-3d-bugs at lists.freedesktop.org
          Reporter: pierre at digitalstar.net
        QA Contact: intel-3d-bugs at lists.freedesktop.org

I wrote an application recently that was ported to both Raspberry PI and Intel
Braswell NUC5PPYH NUC (N3700 cpu). The NUC's glReadPixels performance was worse
than the RPI (10 fps vs 15fps overall app performance where glReadPixels is the
bottleneck). On my Intel HD4000 I can get 60fps under Linux.

I tried to get PBO's working on the NUC (they aren't supported under GLES) for
faster pixel transfer and got no performance difference.

I then installed Windows 10 on the NUC and my performance went up to 36fps
(~29fps without PBOs). This application is driving a large scale public light
installation so I'm very sad to have to resort to Windows 10 as I find Linux
more reliable for this kind of work.

Are PBOs not working on Braswell? Why is there such a huge difference between
the open and closed drivers for a GPU -> CPU pixel transfer?

Performance on the NUC under Linux seemed fine otherwise (although I saw some
glitches in the Ubuntu compositor under latest git Mesa), I got decent shader
performance.

I tried a number of Mesa versions, from the stable Ubuntu 16.04 version to
latest git via a PPA. No performance difference was noted across these
versions.

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