Display errors with a R9 290x and a resolution of 3840x2160

Submitted by TarDingens

Assigned to Default DRI bug account

Description

Using the radeon driver with an AMD R9 290x and a 4k monitor causes display corruption when set to the native resolution.

The problem was introduced with this change:
drm/radeon: change vblank_times calculation method to reduce computational error.
https://patchwork.kernel.org/patch/9403897/

The issue is very easy to see when the mclk speed is forced to low, but it occurs in normal usage too. Before the patch mclk switching was disabled at 4k and the bug was not there.

The Windows driver has no such problems and does fine, even with mclk switching.