[Bug 72961] [hdmi to dual-link dvi regression] Patch to hdmi_portclock_limit disables resolutions that worked with the 3.13.6 kernel

[bugzilla-daemon at bugzilla.kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=72961

--- Comment #5 from Nicholas Vinson <nvinson at comcast.net> ---
(In reply to Ville Syrjala from comment #4)
> Created attachment 130731 [details]
> [PATCH] drm/i915: Allow user modes to exceed DVI 165MHz limit
> 
> Based on what the cable looks like, I think it's just a plain old passive
> single link DVI <-> HDMI cable. So it looks like your monitor accepts out of
> spec frequencies on the DVI connector. Probably it's just a HDMI input w/ a

I can confirm that the monitor will accept out of spec frequencies because I
have attached a HDMI <-> single-link DVI cable to it and got 2560x1440 as well.

As for the cable I'm currently using, I am not completely sure.  As you can
tell from Amazon's website it claims to be able to handle resolutions beyond
what a single-link DVI cable can (1440p and 4Kx2K for example).  I guess the
only way I could ever really know would be to take the cable apart and see if
it just wires or if it has circuitry in it.

> DVI connector. But the monitor apparently doesn't report back the HDMI VSDB
> EDID block so the source can't identify it as a HDMI monitor, which means
> the driver can't know that the monitor will accept HDMI signals.
> 
> In this case we just filter out the high resolution modes, so you still get
> a picture on the screen, but if we were to revert the patch then people with
> monitors which don't accept out of spec DVI signals would get no picture at
> all. So we get to pick the option where everyone gets at least some kind of
> picture.
> 
> But the patch does seem to be a bit too strict. We should probably still
> allow you to manually add the mode w/ xrandr. In that case you're basically
> telling the driver that you know what you're doing. This patch attempts to
> fix that. Let me know if it works.

I added the patch to the kernel sources and rebooted.  The maximum resolution I
can get with the console is 1920x1200.  Inside X, I was able to add the
2560x1440 mode back using xrandr and achieve that resolution.  I have verified
that I can add a configuration file to /etc/X11/xorg.conf.d to get X to start
with the 2560x1440 resolution.  Personally, I am satisfied with this solution. 
Thank you.

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