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

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

Hello,

On Thu, 4 Aug 2016 08:35:24 -0400, Frank Hunleth wrote:

> > 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.  
> 
> Done, but no luck for this particular issue. I have another piece of
> code that differs in performance from Debian by 5% and is more CPU
> bound. I'll try it on that too.

Gaah.

> > Also, are you sure it's really using OpenGL with the Buildroot build?  
> 
> On the Buildroot build, I've tried running the Qt application with
> "-platform eglfs", but that seemed to be the default already. I'm not
> sure how to check deeper than that.

I guess if eglfs support has been built and you're able to use, it
means that it's using the GPU.

> > 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.  
> 
> You're right. On Raspbian, I'm running with X.org. I am naively
> assuming that Qt5 w/ eglfs isn't 10x slower than Qt5 w/ X.org. I
> really hope that's not the case.

I also hope, but using X.org on one side and doing eglfs on the other
side is really a very different configuration. I would also assume that
there should not be a 10x difference between Qt5/X.org vs Qt5/eglfs,
but I have never tested, so I don't know.

Which toolchain are you using ? Are you building everything optimized
for ARMv6 (assuming you're using Rpi1) ?

But a 10x difference really feels like acceleration is not used at all.
You can also check the CPU consumption while running your Qt app to see
what it looks like.

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