[Intel-gfx] limiting modes on 4k monitors on Haswell ULT

[Ville Syrjälä]

On Thu, Oct 02, 2014 at 10:00:50AM +0200, Daniel Vetter wrote:
> On Thu, Oct 02, 2014 at 01:30:48PM +1000, Dave Airlie wrote:
> > Hey guys,
> > 
> > so I have a haswell ULT laptop (lenovo t440s), and got access to a
> > Samsung single panel 4k monitor (no MST).
> > 
> > Now we detect the monitor fine, but unfortunately the ULT hsw can't
> > run it over DP, as it has clock limits in place. However we still
> > offer the 60hz mode and we pick it by default, ensuing black screens.
> > 
> > I've tested the same monitor with a haswell in a t540p and it works
> > fine due to having the higher limits in place.
> 
> We have the IS_HSW_ULX check in intel_dp_max_link_bw, and that's used by
> both compute_config and mode_valid. So it should work, and from a quick
> look I don't see any bugs. But obviously something is busted.
> 
> Can you please printk-augment intel_dp_mode_valid and check what's going
> wrong there?

Or maybe this is about cdclk instead of link clock? I believe the max
cdclk ULT can support is 450MHz. I'm not sure if the default would be
450MHz or 337.5MHz. Also we still don't have support for changing that
during runtime even though I regularly try to trick people into
implementing it. What's the pixel clock of the mode that fails?

-- 
Ville Syrjälä
Intel OTC