(In reply to Linas Vepstas from comment #33)
> After this, starting X a second time does not generate any video-out, so
> monitors stay blanked. Nothing at all printed in /var/log/syslog. However
> kern.log shows:
> 
> Oct 31 15:41:47 fanny kernel: [ 2761.045902] INFO: task kworker/10:1:135
> blocked for more than 120 seconds.
> Oct 31 15:41:47 fanny kernel: [ 2761.045912]       Not tainted
> 3.18.0-rc2-linas #5
> Oct 31 15:41:47 fanny kernel: [ 2761.045915] "echo 0 >
> /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> Oct 31 15:41:47 fanny kernel: [ 2761.045920] kworker/10:1    D
> ffff88085dd53040     0   135      2 0x00000000
> Oct 31 15:41:47 fanny kernel: [ 2761.045959] Workqueue: events
> output_poll_execute [drm_kms_helper]

This hung task issue has been fixed in v4.16-rc3 with
https://git.kernel.org/linus/d61a5c106351

It is unrelated however to the "failed to idle channel" issue AFAICS.