drm/i915/skl: Fix has_ipc on skl and document WaDisableIPC.
authorRodrigo Vivi <rodrigo.vivi@intel.com>
Tue, 3 Oct 2017 06:36:50 +0000 (23:36 -0700)
committerRodrigo Vivi <rodrigo.vivi@intel.com>
Tue, 3 Oct 2017 17:53:48 +0000 (10:53 -0700)
commit4d6ef0da415124ebce578b1326f4e511e35d95f2
treeb06ef070a62341633a7a008a9e3201f9f27b97ae
parente19c1eb885ac4186e64c7e484424124f3145318e
drm/i915/skl: Fix has_ipc on skl and document WaDisableIPC.

According to Spec for SKL+: "Isochronous Priority Control.
If enabled, Display sends demoted requests once the transition
watermark is reached. If transition watermark is not enabled,
Display sends demoted requests when the display buffer is full."

The commit 'e57f1c02155f ("drm/i915/gen9+: Add has_ipc flag in
device info structure")' introduced that as gen9+ but missing many
SKL Skus.

I believe the reason for that is Spec also mentions workarounds for
SKL-ALL: "IPC (Isoch Priority Control) may cause underflows
WA: Do not enable IPC in register ARB_CTL2"

It seems lame to add the feature and forever disable it,
but it will avoid a mistake of enabling it when we are reorganizing
the feature definitions on i915_pci.c later.

It will also allow us to probably extend that workaround for
other platforms.

Cc: Mahesh Kumar <mahesh1.kumar@intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Mahesh Kumar <mahesh1.kumar@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171003063652.17248-1-rodrigo.vivi@intel.com
drivers/gpu/drm/i915/i915_pci.c
drivers/gpu/drm/i915/intel_pm.c