Re: limiting modes on 4k monitors on Haswell ULT

[Damien Lespiau <damien.lespiau@intel.com>]

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.

Just to be a bit more specific:

ULT != ULX, the two SKUs have different limits. The test in
intel_dp_max_link_bw() is about eDP/DP max link rate: HBR2(ULT) Vs
HBR(ULX).

I'd go with Ville, that sounds like CDCLK is the limiting factor, and
we're missing these checks. From a cursory glance, it doesn't seem we
can do 4k on ULT.

-- 
Damien