[Buildroot] Raspberry Pi OpenGL/Qt5 10x slower than Raspbian

[Thomas Petazzoni <thomas.petazzoni@free-electrons.com>]

Hello,

On Wed, 3 Aug 2016 22:39:31 -0400, Frank Hunleth wrote:
> I'm trying to debug a 10x slow down with a simple Qt5 app that draws
> rectangles and a couple lines of text on the screen. I'm comparing the
> performance between a fresh install of Raspbian and the master branch
> of Buildroot (but I also saw this with an older BR release). I've
> narrowed the performance difference down to the rectangle and text
> drawing calls. Nothing else is running. On the Buildroot side, I'm
> using the raspberrypi_defconfig with only the following additions and
> my test app:
> 
> +BR2_PACKAGE_QT5=y
> +BR2_PACKAGE_QT5BASE_LICENSE_APPROVED=y
> +BR2_PACKAGE_QT5BASE_GUI=y
> +BR2_PACKAGE_QT5BASE_WIDGETS=y
> +BR2_PACKAGE_QT5BASE_EGLFS=y
> +BR2_PACKAGE_RPI_USERLAND=y
> 
> I haven't modified any of the config files in the boot partition.
> Changing the config.txt to match the Raspbian version didn't affect
> performance. I also verified that my test app is using eglfs. The
> performance degradation is the same on both a Raspberry Pi Zero and
> Model B.
> 
> This slowdown seems so big that it feels like I'm forgetting some
> important piece of configuration, but I can't figure it out.
> 
> Has anyone else run into this or has any ideas on things to try?

Change -Os by -O2 in the Buildroot configuration, by using
BR2_OPTIMIZE_2. I even believe we should stop using BR2_OPTIMIZE_S by
default, and switch to BR2_OPTIMIZE_2 instead.

Also, are you sure it's really using OpenGL with the Buildroot build?
Under Raspbian, are you also running the eglfs build of Qt, or is it
with X.org? I'm not sure Debian has an eglfs capable build of Qt5.

Thomas
-- 
Thomas Petazzoni, CTO, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com