Linux 4.7 Adding DP++ Dongle Detection For The Intel DRM Driver

[phoronix]

Phoronix: Linux 4.7 Adding DP++ Dongle Detection For The Intel DRM Driver

Following the DRM feature pull for Linux 4.7 sent at the beginning of the week, David Airlie has now sent in a batch of "DRM fixes" for Linux 4.7 that does include some new functionality too...

http://www.phoronix.com/scan.php?page=news_item&px=Linux-4.7-Intel-DP-Plus-Plus

[Tomin]

So is this somehow different from what my XPS13 does when I plug in HDMI adapter? It displays picture just fine so maybe the adapter is smarter than these DP++ adapters. It cost me something like 20 €, I think.

[syrjala]

"DP++ is supported by most hardware out there but the Intel driver up to now hasn't detected DP++ adapters, thus making these DP-to-HDMI/DVI dongles not work. "

That's not really correct. As long as you limit the TMDS clock below 165 MHz these dongles do work without any special support in the driver. The problems start when you exceed 165 MHz as the dongle degrades the signal quality. Some dongles are rated higher than 165 MHz, some are not. So in order to guarantee a sufficiently high quality signal, the driver must detect the dongle and limit the TMDS clock below the dongle's rating. Adding the 12bpc deep color support to i915 caused a regression for many users as 12bpc requires 50% higher TMDS clock than 8bpc, and eg. for 1080p60 that pushed the clock way past the safe 165 MHz limit (8bpc = 148.5 MHz, 12bpc = 222.75 MHz).

[StefanBruens]

"DP++ is supported by most hardware out there but the Intel driver up to now hasn't detected DP++ adapters, thus making these DP-to-HDMI/DVI dongles not work. "

That's not really correct. ...

Thanks for the clarification. Waiting for the day Michael does any fact checks.