[Intel-gfx] [PATCH] drm/i915: Check live status before reading edid

[Chris Wilson]

On Tue, Sep 15, 2015 at 09:44:20AM +0530, Sonika Jindal wrote:
> The Bspec is very clear that Live status must be checked about before
> trying to read EDID over DDC channel. This patch makes sure that HDMI
> EDID is read only when live status is up.
> 
> The live status doesn't seem to perform very consistent across various
> platforms when tested with different monitors. The reason behind that is
> some monitors are late to provide right voltage to set live_status up.
> So, after getting the interrupt, for a small duration, live status reg
> fluctuates, and then settles down showing the correct staus.
> 
> This is explained here in, in a rough way:
> HPD line  ________________
> 			 |\ T1 = Monitor Hotplug causing IRQ
> 			 | \______________________________________
> 			 | |
>                          | |
> 			 | |   T2 = Live status is stable
> 			 | |  _____________________________________
> 			 | | /|
> Live status _____________|_|/ |
> 			 | |  |
> 			 | |  |
> 			 | |  |
> 			T0 T1  T2
> 
> (Between T1 and T2 Live status fluctuates or can be even low, depending on
>  the monitor)
> 
> After several experiments, we have concluded that a max delay
> of 30ms is enough to allow the live status to settle down with
> most of the monitors. This total delay of 30ms has been split into
> a resolution of 3 retries of 10ms each, for the better cases.
> 
> This delay is kept at 30ms, keeping in consideration that, HDCP compliance
> expect the HPD handler to respond a plug out in 100ms, by disabling port.

This is a regression-fest. Revert with stable@?
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre