[Intel-gfx] [PATCH 3/4] drm/i915: Disable displays at the user's request

[Chris Wilson]

Quoting Daniel Vetter (2018-10-19 09:22:15)
> On Mon, Oct 15, 2018 at 12:17:41PM +0100, Chris Wilson wrote:
> > If the user passes i915.disable_display=1 we want to disable all the
> > displays and associated HW like the powerwells on their behalf. Instead
> > of short circuiting the HW probe, let it run and setup all the
> > bookkeeping for the known HW. Afterwards, instead of taking over the
> > BIOS fb and installing the fbcon, we shutdown all the outputs and
> > teardown the bookkeeping, leaving us with no attached outputs or crtcs,
> > and all the HW powered down.
> > 
> > Open: wq flushes should be required but seem to deadlock the modprobe
> > under CI.
> > 
> > Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> > Cc: Imre Deak <imre.deak at intel.com>
> > Cc: Ville Syrjälä <ville.syrjala at linux.intel.com>
> 
> i915.disable_display was for those server chips where doing all the init
> resulted in a dead machine. So not sure we want this.

For those server chips, we don't use i915.disable_display but detect when
the fuses are lies and directly set num_pipes == 0. If we had such a
machine in CI, you would already have seen a lot of the fun with KMS being
allowed without any backing hw. Hence why Ville suggested we disable KMS
for machines without pipes to avoid having to add a lot of defense
around the driver.

> What's the issue with power wells still being on and all that? On real hw
> without display they won't exist, and I don't understand why we'd care for
> testing.

For testing. We do use .disable_display and expect rpm to still work, and
to not get random display related failures interfering in displayless
tests.

Quite clearly we haven't been testing the displayless setups at all.
-Chris