RK3568 rk616转HDMI问题

接上串口后，可以看到更多的信息，发现是在系统起来后系统内部执行了个
/usr/bin/modetest程序，此时我的HDMI是没有接显示器的。

NAME=Buildroot
VERSION=2018.02-rc3-00393-g00b5f43b-dirty
ID=buildroot
VERSION_ID=2018.02-rc3
PRETTY_NAME="Buildroot 2018.02-rc3" 
Starting logging: OK
/usr/bin/modetest
[    4.466638] rk618-hdmi rk618-hdmi: failed to get edid
[    4.467542] rockchip-vop2 fe040000.vop: [drm:vop2_crtc_atomic_enable] Update mode to 1280x720p60, type: 17 for vp2
[    4.468378] clk_core_set_rate_nolock: failed to set dclk_vop2 clock to run at 74250000
[    5.816594] rk618-hdmi rk618-hdmi: failed to get edid
[    7.159886] rk618-hdmi rk618-hdmi: failed to get edid
[    8.503241] rk618-hdmi rk618-hdmi: failed to get edid
[    9.843264] rk618-hdmi rk618-hdmi: failed to get edid
[   11.189968] rk618-hdmi rk618-hdmi: failed to get edid
[   12.533222] rk618-hdmi rk618-hdmi: failed to get edid
[   13.876557] rk618-hdmi rk618-hdmi: failed to get edid
Populating /dev using udev: [   13.894374] udevd[145]: starting version 3.2.7
[   13.900454] udevd[145]: specified group 'kvm' unknown
[   13.912600] udevd[146]: starting eudev-3.2.7
done