[Intel-gfx] Solving vlv hpd issues by holding power wells?

[Ville Syrjälä]

On Tue, Apr 05, 2016 at 06:01:54PM -0400, Lyude wrote:
> Hi. Currently I'm working on a bug in the i915 driver where hotplugging seems to
> break if we power on the machine without any connectors attached:
> 
> https://bugzilla.redhat.com/show_bug.cgi?id=1277863
> 
> So the main cause of the issue seems to be that we're not keeping the right
> power wells on for HPD to work. Booting with say, HDMI, seems to result in the
> POWER_DOMAIN_AUDIO domain getting ref'd (even when it's unplugged, which seems
> to be another bug in the driver I've seen on a few systems at this point), which
> stops the entire display domain from being shut off and subsequently keeps hpd
> working. Without anything plugged in nothing turns on any power wells and the
> hpd is non-functional.
> 
> Of course my first thought was to just keep the power wells required for HPD on
> for as long as we have HPD enabled in the driver. I'm not entirely sure of how
> this can be done though, as disabling hotplugging interrupts is done in a
> context where IRQs are disabled and as such it's impossible to grab a reference
> to the power domain we need since that requires us to grab a mutex. I don't see
> any pre/post uninstall hooks for this either, so just having the power well
> turned on and off whenever hpd is toggled doesn't seem very easy.
> 
> On top of this, after talking to a few coworkers I'm not sure if holding a power
> ref as long as hpd interrupts are enabled is the right solution either, since it
> could be costly in terms of power usage. He suggested that I enable polling for
> each connector in the situation where all the power domains are disabled, but
> having hotplugging disabled doesn't sound right to me either.
> 
> So I guess my questions are:
>    1. Should we be falling back to polling when the required power wells for
>       hotplug detection aren't enabled, instead of keeping them enabled to make
>       hotplugging work?
>    2. If polling isn't the right way to go, what is the best way to go about
>       keeping a ref to the required power wells and dropping it whenever
>       hotplugging is disabled?

See https://bugs.freedesktop.org/show_bug.cgi?id=89008 for the
discussion on different options.

-- 
Ville Syrjälä
Intel OTC