Builds from master for RPi3 contain a downstream RPi kernel version 4.4.16. The vc4 drm we are using in this kernel does detect a connected display (actually this is hacked up a bit and doesn't use the gpio based detect on RPi3) and properly reads EDID, and selects a preferred mode (e.g. 60hz 1080p) from those presented. However, setting this mode results in an "out of range" warning on the monitor if this bug is hit.

I verified that this issue occurs on 3 different displays I have handy (which work on other boards like Minnowboard Max/Turbot) including the GeChic 1080p display we have speced for the CES 2017 demos, a Hisense TV, and a Sceptre monitor.

Meanwhile, if I run the upstream stable 4.8 kernel (only patched similarly to the downstream hack to deal with lack of gpio detect of a connected display), these same monitors are all able to sync the select 60hz 1080p mode that's selected by the drm subsystem.

My initial review of the downstream versus upstream driver doesn't show any obvious patches we're missing to address this issue. The upstream driver is quite a bit different now with some major timing related changes. It may be possible to backport this.

A short-term alternative to make the RPi3 build usable with the CES 2017 GeChic display is to hardcode the mode or a specific modeline for the GeChic monitor. Simply adding:

[output]
name=HDMI-A-1
mode=1024x768

to weston.ini was enough for the vc4 drm driver to sync a mode on the GeChic display. It's also possible to specify a complete modeline. Not a great workaround but something usable for a demo. This overrides the default behavior of Weston to use the current mode which will generally be the preferred mode that the vc4 drm driver selects by default (which, with this driver is broken depending on the monitor).