UPSTREAM: 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.

TEST=boot casta, run google play

Cc: Mahesh Kumar <>
Cc: Maarten Lankhorst <>
Cc: Chris Wilson <>
Signed-off-by: Rodrigo Vivi <>
Reviewed-by: Mahesh Kumar <>
(cherry picked from commit 4d6ef0da415124ebce578b1326f4e511e35d95f2)
Signed-off-by: Chiou, Cooper <>

Change-Id: Icc2f873345c765ea62844cf9b54582ebbeff76be
Tested-by: Cooper Chiou <>
Tested-by: Cooper Chiou <>
Commit-Ready: Cooper Chiou <>
Legacy-Commit-Queue: Commit Bot <>
Reviewed-by: St├ęphane Marchesin <>
Reviewed-by: Sean Paul <>
(cherry picked from commit bb613a7875c1f440a65cbc73fc6effa78e9c5357)
Reviewed-by: Young H Kim <>
Reviewed-by: Justin TerAvest <>
Tested-by: Young H Kim <>
Tested-by: Justin TerAvest <>
Commit-Queue: Justin TerAvest <>
2 files changed