[Intel-gfx] [PATCH] drm/i915/pch: Save/restore PCH_PORT_HOTPLUG across suspend

[Adam Jackson]

On Tue, 2011-07-26 at 14:00 -0700, Jesse Barnes wrote:

> We should be writing this reg.  The only question is whether we should
> be trusting the BIOS values (which may have custom pulse duration
> settings) or just unconditionally enabling hot plug for ports we care
> about with the default 2ms pulse width (per the DP spec).
> 
> If we assume the BIOS programs this reg to a good value (a very big
> assumption) saving and restoring it is safest.  I just wonder if we'll
> find machines where it's broken by default leading to weird DP behavior.

Another option is reading the HPD input enable bits from the platform
value, but setting the pulse width ourselves.  I have a vague fear of
enabling HPD for all ports blindly, I suspect there's some platform out
there that miswires things such that if we did we'd get an infinite irq
storm.

IWBNI the documentation for the pulse width bits in that reg was worded
a bit more clearly.  It's not completely clear whether the "short pulse
duration" part sets a minimum or maximum.  My reading interprets it as
minimum, but then one wonders what the long pulse duration is.

I'll play around with it a bit more.  If - as evidence seems to suggest
- we're really getting two HPD IRQs for each plug, then there's probably
a good reason for it which the code should reflect.

- ajax
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