[Intel-gfx] i915 SNB: hotplug events when charging causing poor interactivity

[Daniel J Blueman]

Keith, Eric,

When charging Dell E5420 laptops, I see the Sandy Bridge South Display
Engine port C hotplug interrupt fire consistently at around 20Hz. Each
scan on the connectors results in ~100ms hold time for the mode_config
mutex which blocks eg the cursor set ioctl (due to the I2C read
timeouts across the connectors), resulting in terrible GUI
interactivity.

Notably, when only using battery or only using AC, the port C
interrupts don't fire. It feels like the platform is using South
Display Port C for GPIO/I2C; setting the port C pulse duration from
2ms to 100ms doesn't change the behaviour. I'll dump off the GPIO
settings, but what else is good to debug this?

Also, have you come across this kind of pattern before, eg a platform
using these GPIO ports for the something (in this case, feels like the
EC/battery)? Judging by the lack of a quirks, I'd say not. Perhaps
also I should dump off the SDE port C IIR mask register before we
reset it, in case the BIOS intentionally masks out port C hotplug
events.

Thanks,
 Daniel
--
Daniel J Blueman