RX 580 and 5K displays, bandwidth validation failed whith multiple monitors

[Gaël HERMET]

Hi,

I have been facing an issue with my 5K display (iiyama ProLite 
XB2779QQS-S1).

It works fine as long as it is the only active monitor, as soon as I 
activate another monitor the main one (5k) can't display more than 4k.

Debug using "echo 0x4 > /sys/module/drm/parameters/debug" show this :
mai 23 09:01:22 bureau-gael /usr/lib/gdm3/gdm-x-session[3465]: (EE) 
AMDGPU(0): failed to set mode: Invalid argument
mai 23 09:01:22 bureau-gael kernel: [drm:dce112_validate_bandwidth 
[amdgpu]] dce112_validate_bandwidth: Bandwidth validation failed!

I disabled the check by forcing is_display_configuration_supported to 
return true in dce_calcs.c and it works fine.

Anything I can do to correct this bandwidth calculation ?