Especially in cases where pre-os does not enable display, cdclk might
not be in sane state. During sanitization initialize cdclk with maximum
value till we get dynamic cdclk support.

v2: Check if BIOS programmed correctly rather than always calling init
    - Do validation of programmed cdctl and what it is expected
    - Only do slk_init_cdclk if validation failed else reuse BIOS
      programmed value

v3: Move the validation logic in a separate sanitize function (Ville)

v4: No need to check LCPLL after sanitize and use max_cdclk_freq instead
    of hardcoded value (Ville)

Cc: Imre Deak <imre.deak@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Shobhit Kumar <shobhit.kumar@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1445344992-14658-1-git-send-email-shobhit.kumar@intel.comReviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

Break out common code from gen8_gt_irq_handler and put it in to
an always inlined function. gcc optimises out the shift at compile
time. (Thomas Daniel/Daniel Vetter/Chris Wilson)

Issue: VIZ-4277
Signed-off-by: Nick Hoath <nicholas.hoath@intel.com>
Cc: Thomas Daniel <thomas.daniel@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1445333036-22164-3-git-send-email-nicholas.hoath@intel.comSigned-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

Renamed tmp variable to the more descriptive iir. (Daniel Vetter/
Thomas Daniel)

Issue: VIZ-4277
Signed-off-by: Nick Hoath <nicholas.hoath@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: David Gordon <david.s.gordon@intel.com>
Cc: Thomas Daniel <thomas.daniel@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1445333036-22164-2-git-send-email-nicholas.hoath@intel.comSigned-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

Just pull the info out of the state structures rather than staging
it in an additional set of structures.  To make this more
straightforward, we change the signature of several internal WM
functions to take the crtc state as a parameter.

v2:
 - Don't forget to skip cursor planes on a loop in the DDB allocation
   function to match original behavior.  (Ander)
 - Change a use of intel_crtc->active to cstate->active.  They should
   be identical, but it's better to be consistent.  (Ander)
 - Rework more function signatures to pass states rather than crtc for
   consistency. (Ander)

v3:
  - Add missing "+ 1" to skl_wm_plane_id()'s 'overlay' case. (Maarten)
  - Packed formats should pass '0' to drm_format_plane_cpp(), not 1.
    (Maarten)
  - Drop unwanted WARN_ON() for disabled planes when calculating data
    rate for SKL.  (Maarten)

v4:
 - Don't include cursor plane in total relative data rate calculation;
   we've already handled the cursor allocation earlier.
 - Fix 'bytes_per_pixel' calculation braindamage.  Somehow I hardcoded
   the NV12 format as a parameter rather than the actual
   fb->pixel_format, and even then still managed to get the format plane
   wrong.  (Ville)
 - Use plane->state->fb rather than plane->fb in
   skl_allocate_pipe_ddb(); the plane->fb pointer isn't updated until
   after we've done our watermark recalculation, so it has stale
   values.  (Bob Paauwe)
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by(v3): Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Paauwe, Bob J <bob.j.paauwe@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
References: http://lists.freedesktop.org/archives/intel-gfx/2015-September/077060.html
References: http://lists.freedesktop.org/archives/intel-gfx/2015-October/077721.html
Smoke-tested-by(v4): Paulo Zanoni <paulo.r.zanoni@intel.com> (SKL)
Link: http://patchwork.freedesktop.org/patch/61968/

Signed-off-by: David Henningsson <david.henningsson@canonical.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1444987464-8657-1-git-send-email-david.henningsson@canonical.comSigned-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

Since the beginning we have conflated the size of the global GTT with
that of the per-process context sizes. In recent times (gen8+), those
are no longer the same where the global GTT is limited to 2/4GiB but the
per-process GTT may be anything up to 256TiB. Userspace knows nothing of
this discrepancy and outside of one or two hacks, uses the getaperture
ioctl to determine the maximum size it can use. Let's leave that as
reporting the global GTT and use the context reporting method to
describe the per-process value (which naturally fallsback to reporting
the aliasing or global on older platforms, so userspace can always use
this method where available).

Testcase: igt/gem_userptr_blits/minor-normal-sync
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=90065Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

If we have llc coherency, we can write directly into the ringbuffer
using ordinary cached writes rather than forcing WC access.

v2: An important consequence is that we can forgo the mappable request
for WB ringbuffers, allowing for many more simultaneous contexts.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

Use 48b addresses if hw supports it (i915.enable_ppgtt=3).
Update the sanitize_enable_ppgtt for 48 bit PPGTT mode.

Note, aliasing PPGTT remains 32b only.

v2: s/full_64b/full_48b/. (Akash)
v3: Add sanitize_enable_ppgtt changes until here. (Akash)
v4: Update param description (Chris)

Cc: Akash Goel <akash.goel@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Michel Thierry <michel.thierry@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

The atomic helpers set planes_changed on a crtc_state if there is
any plane_state bound to that crtc. If there's none and there is
no pipe update required the crtc has nothing to update, so vblank
evasion can be skipped.
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

The crtc->active guards are no longer needed now that all state
updates are outside the commit.

Changes since v1:
- Only check crtc->state->active before calling commit_planes_on_crtc.
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

In the next commit commit_plane will no longer check if the crtc is active.
To prevent issues with legacy page flips the check should be performed inside
update_primary_planes.
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

This should allow not running plane commit when the crtc is off.
While the atomic helpers update those, crtc->x/y is only updated
during modesets, and primary plane is updated after this function
returns.

Unfortunately non-atomic watermarks and fbc still depend on this
state inside i915, so it has to be kept in sync.

Changes since v1:
- Add comment that the legacy state is updated for fbc.
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

Now that we agreed on not preserving framebuffers pinning is finally
allowed to fail because of signals. Use this to make pinning
and acquire the mutex in an interruptible way too.

Unpinning is still uninterruptible, because it happens as a cleanup
of old state, or undoing pins after one of the pins failed.

The intel_pin_and_fence_fb_obj in page_flip will also wait interruptibly,
and can be aborted now.
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Ander Conselvan de Oliveira <conselvan2@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

atomic->disabled_planes is a hack that had to exist because
prepare_fb was only called when a new fb was set. This messed
up fb tracking in some circumstances like aborts from
interruptible waits. As a result interruptible waiting in
prepare_plane_fb was forbidden, but other errors could still
cause frontbuffer tracking to be messed up.

Now that prepare_fb is always called, this hack is no longer
required and prepare_fb may fail without consequences.

Changes since v1:
- Clean up a few fb tracking warnings by changing plane->fb to
  plane->state->fb.
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Ander Conselvan de Oliveira <conselvan2@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

Mmio register access after dc6/dc5 entry is not allowed when
DC6 power states are enabled according to bspec (bspec-id 0527),
so enabling dc6 as the last call in suspend flow.

Addtional note from Imre:

Currently we keep DC6 enabled during modesets and DPAUX transfers, which
is not allowed according to the specification. This can lead at least to
PLL locking failures, DPAUX timeouts and prevent deeper package power
states (PC9/10). Fix this for now by enabling DC6 only when we know the
above events (modeset, DPAUX) can't happen.

This a temporary solution as some issues are still unsolved as described
in [1] and [2], we'll address those as a follow-up.

[1]
http://lists.freedesktop.org/archives/intel-gfx/2015-October/077669.html
[2]
http://lists.freedesktop.org/archives/intel-gfx/2015-October/077787.html

v1: Initial version.

v2: Based on review comment from Daniel,
- created a seperate patch for csr uninitialization set call.

v3: Rebased on top of latest code.

Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Damien Lespiau <damien.lespiau@intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Cc: Sunil Kamath <sunil.kamath@intel.com>
Signed-off-by: Animesh Manna <animesh.manna@intel.com>
Signed-off-by: Vathsala Nagaraju <vathsala.nagaraju@intel.com>
Signed-off-by: Rajneesh Bhardwaj <rajneesh.bhardwaj@intel.com>
Acked-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

We still had one lingering RMW in ivb_sprite_disable(), all the other
RMWs were killed off from the sprite code some time ago. Kill the
straggler too.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

This was accidentally lost in

commit 75d04a37
Author: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Date:   Tue Apr 28 17:56:17 2015 +0300

    drm/i915/gtt: Allocate va range only if vma is not bound

While at it implement an improved version suggested by Chris which
avoids the double-bind irrespective of what type of bind is done
first.

Note that this exact bug was already addressed in

commit d0e30adc
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Wed Jul 29 20:02:48 2015 +0100

    drm/i915: Mark PIN_USER binding as GLOBAL_BIND without the aliasing ppgtt

but the problem is still that originally in

commit 0875546c
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Mon Apr 20 09:04:05 2015 -0700

    drm/i915: Fix up the vma aliasing ppgtt binding

if forgotten to take into account there case where we have a
GLOBAL_BIND before a LOCAL_BIND. This patch here fixes that.

v2: Pimp commit message and revert the partial fix.

v3: Split into two functions to specialize on aliasing_ppgtt y/n.

v4: WARN_ON for paranoia in the init sequence, since the ggtt probe
and aliasing ppgtt setup are far apart.

v5: Style nits.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Michel Thierry <michel.thierry@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://mid.gmane.org/1444911781-32607-1-git-send-email-daniel.vetter@ffwll.chReviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

Since SKL has universal planes, we should configure the sprite planes
and the primary plane the same.  For the primary plane we do enable
the pipe gamma on the plane so do the same for the non-primary planes.

Without this, the pipe CRC values will be different for something
displayed on the primary plane and something displayed on a sprite
plane when the ARGB8888 format is used.
Signed-off-by: Bob Paauwe <bob.j.paauwe@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

Extend this to SKL and BXT as it's needed for these platforms as well.

v2: Change if condition to HAS_DDI() instead of listing each platform
Signed-off-by: Bob Paauwe <bob.j.paauwe@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

There's plenty of drm/i915 related hardware and software documentation,
and firmware downloads for the latest platforms.

Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

Some registers are, naturally, lost in gpu reset/suspend cycle.
And some registers, for example in display domain, are not subject
to gpu reset so they retain their contents.

As hang recovery triggers a reset, recoverable gpu hang can currently
flush out essential workarounds and cause havoc later on.

When register GEN8_GARBNTL is missing the WaEnableGapsTsvCreditFix:skl,
it can cause random system hangs [1]. This workaround was added in:
commit 245d9667 ("drm/i915:skl: Add WaEnableGapsTsvCreditFix")
But another set of system hangs were observed and the failure pattern
indicated that there was random gpu hang preceding the system hang [2].
This lead to the realization that we lose this workaround and BDW_SCRATCH1
on reset.

Add these workarounds setup in display init to skl/bxt ring init
where LRI workarounds are also setup. This way their setup is not
dependent on display side init.

References: [1] https://bugs.freedesktop.org/show_bug.cgi?id=90854
References: [2] https://bugs.freedesktop.org/show_bug.cgi?id=92315Reported-by: Tomi Sarvela <tomix.p.sarvela@intel.com>
Cc: Tomi Sarvela <tomix.p.sarvela@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Tested-by: Tomi Sarvela <tomix.p.sarvela@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

Since the remove of the pin-ioctl, we only care about not changing the
cache level on buffers pinned to the hardware as indicated by
obj->pin_display. By knowing that only objects pinned to the hardware
will have an elevated vma->pin_count, so we can coallesce many of the
linear walks over the obj->vma_list.

v2: Try and retrospectively add comments explaining the steps in
rebinding the active VMA.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

This is a squash of the following commits:

Revert "drm/i915: Drop intel_update_sprite_watermarks"
This reverts commit 47c99438.

Revert "drm/i915/ivb: Move WaCxSRDisabledForSpriteScaling w/a to atomic check"
This reverts commit 7809e5ae.

Revert "drm/i915/skl: Eliminate usage of pipe_wm_parameters from SKL-style WM (v3)"
This reverts commit 3a05f5e2.

With these reverts, SKL finally stops failing every single FBC test
with FIFO underrun error messages. After some brief testing, it also
seems that this commit prevents the machine from completely freezing
when we run igt/kms_fbc_crc (see fd.o #92355).

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=92355
Cc: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

Keep single 'lvds_reg' and 'lvds' variable around in
intel_lvds_init(), and read it just once at the start.

Also intel_lvds_get_config() doesn't need to figure out which reg to use
since it can just consult lvds_encoder->reg.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

Drop some useless 'reg' variables when we only use them once.

v2: A few more, including a few variable moves
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

Parametrize the SWF registers. This also fixes the register offsets,
which were mostly garbage in the old defines.

Also save/restore only as many SWF registers that each platform has.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

The PIPE_FRMCOUNT_GM45 and PIPE_FLIPCOUNT_GM45 names have bothered me
for a long time. The work equally well for ELK and onwards, so let's
s/GM45/G4X/.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

A few register mask defines were missing the '0x' from hex numbers. Or
at least I assume those were meant to be hex numbers. Put the '0x' in
place.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

Always put parens around macro argument evaluations.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

v2: Keep using the same registers (PCH_*) instead of accidentally
    starting to use the other ones (BXT_*)2) (Jesse)
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

We should serialise access to the intel_crtc->unpin_work through the
dev->event_lock spinlock. It should not be possible for it to disappear
without severe error as the mmio_flip worker has not tagged the
unpin_work pending flip-completion. Similarly if the error exists, just
taking the unpin_work whilst holding the spinlock and then using it
unserialised just masks the race. (It is supposed to be valid as the
unpin_work exists until the flip completion interrupt which should not
fire until we flush the mmio writes to update the display base which is
the last time we access the unpin_work from the kthread.)

References: https://bugs.freedesktop.org/show_bug.cgi?id=92335Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

i915 expects the OpRegion to be cached (i.e. not __iomem), so explicitly
map it with memremap rather than the implied cache setting of
acpi_os_ioremap().

Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: intel-gfx@lists.freedesktop.org
Cc: David Airlie <airlied@linux.ie>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

Commit 599bbb9d ("drm/i915: i915 cannot provide switcher services.")
removed all remaining vga_switcheroo symbols from intel_acpi.c but left
the include. Drop it.
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

It's been reported that the atomic watermark series triggers some
regressions on SKL, which we haven't been able to track down yet.  Let's
temporarily revert these patches while we track down the root cause.

This commit squashes the reverts of:
  76305b1a drm/i915: Calculate watermark configuration during atomic check (v2)
  a4611e44 drm/i915: Don't set plane visible during HW readout if CRTC is off
  a28170f3 drm/i915: Calculate ILK-style watermarks during atomic check (v3)
  de4a9f83 drm/i915: Calculate pipe watermarks into CRTC state (v3)
  de165e0b drm/i915: Refactor ilk_update_wm (v3)

Reference: http://lists.freedesktop.org/archives/intel-gfx/2015-October/077190.html
Cc: "Zanoni, Paulo R" <paulo.r.zanoni@intel.com>
Cc: "Vetter, Daniel" <daniel.vetter@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

When submitting semaphores in execlist mode the hang checker crashes in this
function because it is only runnable in ring submission mode. The reason this
is of particular interest to the TDR patch series is because we use semaphores
as a mean to induce hangs during testing (which is the recommended way to
induce hangs for gen8+). It's not clear how this is supposed to work in
execlist mode since:

1. This function requires a ring buffer.

2. Retrieving a ring buffer in execlist mode requires us to retrieve the
corresponding context, which we get from a request.

3. Retieving a request from the hang checker is not straight-forward since that
requires us to grab the struct_mutex in order to synchronize against the
request retirement thread.

4. Grabbing the struct_mutex from the hang checker is nothing that we will do
since that puts us at risk of deadlock since a hung thread might be holding the
struct_mutex already.

Therefore it's not obvious how we're supposed to deal with this. For now, we're
doing an early exit from this function, which avoids any kernel panic situation
when running our own internal TDR ULT.

* v2: (Chris Wilson)
Turned the execlist mode check into a ringbuffer NULL check to make it more
submission mode agnostic and less of a layering violation.
Signed-off-by: Tomas Elf <tomas.elf@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>