[ltp] Blanking out when driving QHD 2560x1440 cheap panels with the displayport

[Alex Deucher]

On Thu, Mar 6, 2014 at 1:43 PM, Rubin Abdi <rubin@starset.net> wrote:
> I have an X220, I am running Debian Sid, 3.12.9. I've used this machine
> to drive 27" and larger Dell displays at full resolution via displayport
> to displayport and displayport to dual link DVI without issue.
>
> A few months ago I purchased one of those $400 Apple knock off S-IPS
> displays from Korea through ebay, branded as Crossover at QHD 2560x1440.
> I spent the following month trying to figure out why when driving the
> display through my machine's displayport, it would blank out after about
> a minute or two of constant action on the screen (like playing a video).
> It would go into blank/sleep mode for a second or two, bounce back, do
> this a few times before the display giving up and stating there is
> nothing turn on/plugged into the displayport and shut off. I tried 3
> different displayport cables and assumed the issue wasn't my laptop
> since I was driving a different 30" display at the office over
> displayport just fine.
>
> Talking with the person who sold me the panel, they mailed over a couple
> different boards to swap out inside the display, none of which provided
> any improvement. All that came to an end when I realized a month had
> passed since I purchased the display and the seller stopped returning my
> messages through ebay as their obligation to provide me with support
> ended with my inability to leave them bad feedback.
>
> Since then I learned I could drive the monitor through displayport to
> dual link DVI (with an attached adapter that sucks in power over USB). I
> don't get the show stopping screen blanking issue, but about once every
> 5 minutes of action on the screen (like scrolling a web page), all the
> pixels on the screen vibrate together about 200px back and forth
> horizontally for half a second. I decided that was tolerable and a
> better option than simply trashing the $400 paper weight.
>
> Yesterday I was thinking of purchasing a replacement display of a
> similar design (hoping it wouldn't be as defective) from a retailer over
> Amazon at about the same cost, and using this one sitting on my desk as
> a display for movies to drive over HDMI at a lower 1080p resolution
> which doesn't cause the same symptoms, and place it somewhere else in my
> house.
>
> After price searching a little bit, I decided to try a new test before
> finally calling it quits and ordering this new monitor. I plugged in an
> older Mac Mini I had laying around to the display and started playing a
> movie at full resolution. Surprisingly after 30 minutes of driving the
> thing over displayport I saw no issues. Swapped the cables, no issues
> again. Pulling out an old drive containing a Windows bootable partition
> for gaming, I tested out using my X220 with Windows driving the display
> through displayport, and to my amazement no issues cropped up.
>
> So why is Linux sucking at providing this display clean signal over the
> displayport?
>
> Through the suggestion of this thread...
>
> https://bbs.archlinux.org/viewtopic.php?id=175868
>
> ...I've stuck xorg into UXA mode. That's slightly helped as it takes
> slightly longer before the display craps about, but by a minute or two.
> Other than that thread and attached bug I really haven't seen any other
> references of folks having this problem. And it doesn't seem like events
> happening to the displayport (such as a monitor blanking) don't end up
> in any logs I've found, so debugging this hasn't really happened.
>
> Any thoughts, ideas, or suggestions would be greatly appreciated. Thanks.

I'm not particularly familiar with Intel display hardware, but it
sounds like display underflow to me.  I suspect there is contention
for memory bandwidth and in certain cases the display loses and the
line buffer runs dry.  Probably some fine tuning of the display
watermarks would fix that up.

Alex