[meta-freescale] Framebuffer problem with Qt on Kontron SMARC sAMX6i

[Nikita Yushchenko]

> Hello,
> 
> I have been able to get HDMI output working on Yocto Daisy with the Kontron
> patch for SMARC sAMX6i. Now I am trying to run an embedded Qt application
> using eglfs. 
> 
> The application starts up and picture is displayed on HDMI, however, I get
> random blank display when e.g., moving the mouse cursor around. On the
> console, I get an error message
> 
> mxc_sdc_fb mxc_sdc_fb.19: timeout when waiting for flip irq
> 
> multiple times.

I have seen this while working on sAMX6i BSP, both 3.10 based
(daisy/dizzy) and 3.14 based (fido).

Not sure message is directly related - at least, it does not show always
when issue arises.

Per my understanding, this is caused by not enough bus throughput at
some point. When something else is accessing framebuffer memory, and/or
IPU is doing something else, something on HDMI data path is not able to
read or process data in time, this causes HDMI output signal
interruption, this causes temporary black screen.  Problem can become
even worse on massive screen update (i.e. video playback) when screen
goes completely black.

I don't know what in particular is reason of this. Maybe some clock is
running at slower rate than it should?
I did not modify anything related to IPU or HDMI in sAMX6i kernel, this
part is unmodified linux-imx. I can guess that problem should be visible
sometimes on other imx6 boards, although I've never seen any reports of
that.

Possible workarounds are:
- make sure that bandwidth is not being wasted for other screen
(echo 1 > /sys/class/grapthis/fb2/blank)
- use lower framebuffer bpp
- use lower screen resolution

Nikita