drm: imx: multi-display support questions

[Philipp Zabel]

Hi Gary,

Am Mittwoch, den 27.05.2015, 15:31 +0200 schrieb Gary Bisson:
> > According to the kerneldoc comment for drm_fb_helper_initial_config
> > (which is used by imx-drm via drm_fbdev_cma_init), it should set up a
> > single /dev/fb cloned over all connectors. This works here with LVDS and
> > HDMI.
> 
> Does it require the two displays to have the exact same resolution?
> I'm wondering what is wrong with my setup but with a 1024x768 LVDS and
> a 1920x1080 HDMI display no image is shown on the HDMI (no signal).
> The CRTC settings show that both have the same origin (0,0) so I
> expected the LVDS to display a part of what the HDMI *should* display.

No, but it does require the HDMI and LVDS display to use different clock
sources (unless LVDS serializer clock happens to be the same as the HDMI
pixel clock).
I wonder what we should do about this for devices that have both LVDS
and HDMI output and can only use PLL5 for both. Register a clock
notifier that vetoes changes?

[...]
> > For parallel and LVDS we'd either need to force the parallel panel to be
> > clocked by the IPU internal clock, or move one or the other external
> > clock source off of pll5_video.
> 
> Do you have a preference for one solution over the other?

That depends on the board. Is there an LVDS display that can be driven
by a PLL other than PLL5? Since there is no sane way to change the
LDB_DI parent in a running system, that should be configured in the
device tree.

regards
Philipp