[Intel-gfx] [PATCH 4/4] drm/i915: Avoid race of intel_crt_detect_hotplug() with HPD interrupt

[Egbert Eich]

Jani Nikula writes:
 > On Wed, 02 Sep 2015, Egbert Eich <eich at suse.com> wrote:
 > > This is exactly the scenatio I'm getting here. I get HPD interrupts at an 
 > > order of 10^4 / sec.
 > 
 > Makes you wonder if either you have faulty hardware or we are
 > configuring the hardware wrong (we overlook some configuration about
 > some voltage/duration threshold maybe, or get irqs from a line that's
 > floating, or something).

It is faulty hardware. But it is not a single machine that broke.
It is an entire series. IMHO due to bad signal routing and poor shilding
there is crosstalk on the SDVO lines signaling the plug status.
Since SDVO uses PCIe lines it is AC coupled, if I recall correctly
from reading the specs long time ago, one status is signalled by a
10MHz signal, the other by 20MHz.

At the time when I implemented this I've seen other reports from systems 
which showed similar problems under certain conditions(*) - although not 
quite as bad, therefore I thought of a general solution to get rid of 
this once and for all. If this had only been one system with this problem, 
I would just have blacklisted it.

(*) It seems that this somewhat depends on the video mode set (supports
the crosstalk theory) but I also had a report where this occurred at
certain charging levels and whether a power supply was connected or
not.

Cheers,
	Egbert.