We notice two identical copies of the shadow register table and
following from that removal can also unify CHV and Gen9 write
mmio functions and macros into a single implementation.

v2: Name fwtable consistently and use HAS_FWTABLE. (Joonas Lahtinen)
Signed-off-by: NTvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>

It is now obvious VLV, CHV and Gen9 mmio read fcuntions are
completely identical so we can remove the three copies and
just keep the newly named generic implementation.

v2: Use fwtable naming consistently. (Joonas Lahtinen)
Signed-off-by: NTvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: NJoonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>

Remove some macros which are now obviously identical.

v2: Added HAS_FWTABLE macro and simplified intel_uncore_forcewake_for_read.
    (Joonas Lahtinen)
Signed-off-by: NTvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>

If we store this in the uncore structure we are on a good way to
show more commonality between the per-platform implementations.

v2: Constify table pointer and correct coding style. (Chris Wilson)
v3: Rebase.
Signed-off-by: NTvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>

If we insert blitter forcewake domain entries in the range
table we can eliminate that special case and simplify the
code in a few macros. This will enable more unification later.
Signed-off-by: NTvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>

Instead of the existing linear seach, now that we have sorted
range tables, we can do a binary search on them for some
potential miniscule performance gain, but more importantly
for elegance and code size. Hopefully the perfomance gain is
sufficient to offset the function calls which were not there
before.

v2: Removed const cast away.
Signed-off-by: NTvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>

Sorting the tables (verified at runtime to help during
development) is another prerequisite for interesting
work which will follow.

v2:
 * Remove const away cast and improve comments. (Chris Wilson)
 * Check tables only when debug option is enabled.

v3: Use IS_ENABLED. (Chris Wilson)
Signed-off-by: NTvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>

Move finding the correct forcewake domains to take for
register access from code to a mapping table. This will
allow more interesting work in the following patches
and is easier to review if singled out early.
Signed-off-by: NTvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>

Once we know we need to take new forcewakes, that being
a slow operation, it does not make sense to inline that
code into every mmio accessor.

Move it to a separate function and save some code.

v2: Be explicit with noinline and remove stale comment.
    (Chris Wilson)
Signed-off-by: NTvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>

There are current places in the code, and there will be more in the
future, which iterate the forcewake domains to find out which ones
are currently active.

To save them from doing this iteration, we can cheaply keep a mask
of active domains in dev_priv->uncore.fw_domains_active.

This has no cost in terms of object size, even manages to shrink it
overall by 368 bytes on my config.
Signed-off-by: NTvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: "Paneri, Praveen" <praveen.paneri@intel.com>
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>

They are completely identical to gen6_write* ones.
Signed-off-by: NTvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: NJoonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>

Fix sparse warning:

drivers/gpu/drm/i915/intel_device_info.c:195:31: warning: Variable
length array is used.

In truth the array does have constant length, but sparse is too dumb to
realize. This is a bit ugly, but silence the warning no matter what.

Fixes: 91bedd34 ("drm/i915/bdw: Check for slice, subslice and EU count for BDW")
Reviewed-by: NJoonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: NJani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1475574853-4178-1-git-send-email-jani.nikula@intel.com

Move the outcast intel_fbdev_output_poll_changed() stub for
CONFIG_DRM_FBDEV_EMULATION=n next to its friends.
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NJani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1475567628-5529-1-git-send-email-jani.nikula@intel.com

When decoding the semaphores inside hangcheck, we need to use the hw-id
and not the local array index.

Fixes: de1add36 ("drm/i915: Decouple execbuf uAPI ...")
Testcase: igt/gem_exec_whisper/hang # gen6-7
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: stable@vger.kernel.org
Reviewed-by: NJoonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161003124516.12388-3-chris@chris-wilson.co.uk

We use obj->phys_handle to choose the pread/pwrite path, but as
obj->phys_handle is a union with obj->userptr, we then mistakenly use
the phys_handle path for userptr objects within pread/pwrite.

Testcase: igt/gem_userptr_blits/forbidden-operations
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=97519Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Cc: stable@vger.kernel.org
Reviewed-by: NJoonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161003124516.12388-2-chris@chris-wilson.co.uk

When we enable the per-register access mmiodebug, it is to detect which
access is illegal. Reporting on earlier untraced access outside of the
mmiodebug does not help debugging (as the suspicion is immediately put
upon the current register which is not at fault)!

References: https://bugs.freedesktop.org/show_bug.cgi?id=97985Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: NMika Kuoppala <mika.kuoppala@intel.com>
Cc: stable@vger.kernel.org
Link: http://patchwork.freedesktop.org/patch/msgid/20161003124516.12388-1-chris@chris-wilson.co.uk

We were previously adding all the planes owned by the CRTC even when
the ddb partitioning didn't change for them. As a consequence, a lot
of functions were being called when we were just moving the cursor
around the screen, such as skylake_update_primary_plane().

This was causing flickering on the primary plane when moving the
cursor. I'm not 100% sure which operation caused the flickering, but
we were writing to a lot of registers, so it could be any of these
writes. With this patch, just moving the mouse won't add the primary
plane to the commit since it won't trigger a change in DDB
partitioning.

v2: Use skl_ddb_entry_equal() (Lyude).
v3: Change Reported-and-bisected-by: to Reported-by: for checkpatch

Fixes: 05a76d3d ("drm/i915/skl: Ensure pipes with changed wms get added to the state")
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=97888
Cc: Mike Lothian <mike@fireburn.co.uk>
Cc: stable@vger.kernel.org
Reported-by: NMike Lothian <mike@fireburn.co.uk>
Signed-off-by: NPaulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: NLyude <cpaul@redhat.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1475177808-29955-1-git-send-email-paulo.r.zanoni@intel.com

This reverts 'commit 3708d5e0 ("drm/i915: start adding dp mst audio")'
because it breaks MST multi-monitor setups on some platforms.

Fixes: 3708d5e0 ("drm/i915: start adding dp mst audio")
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=97907Signed-off-by: NDhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Reported-by: NKim Lidström <kim@dxtr.im>
Cc: Libin Yang <libin.yang@linux.intel.com>
Cc: Lyude <cpaul@redhat.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: NJani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1475132104-2754-1-git-send-email-dhinakaran.pandiyan@intel.com

DPLL_SDVO_HIGH_SPEED must be set for SDVO/HDMI/DP, but nowhere is it
forbidden to set it for LVDS/CRT as well. So let's also set it on
CRT to make it possible to share the DPLL between HDMI and CRT.

What that bit apparently does is enable the x5 clock to the port,
which then pumps out the bits on both edges of the clock. The DAC
doesn't need that clock since it's not pumping out bits, but I don't
think it hurts to have the DPLL output that clock anyway.

This is fairly important on IVB since it has only two DPLLs with three
pipes. So trying to drive three or more PCH ports with three pipes
is only possible when at least one of the DPLLs gets shared between
two of the pipes.

SNB doesn't really need to do this since it has only two pipes. It could
be done to avoid enabling the second DPLL at all in certain cases, but
I'm not sure that's such a huge win. So let's not do it for SNB, at
least for now. On ILK it never makes sense as the DPLLs can't be shared.

v2: Just always enable the high speed clock to keep things simple (Daniel)
    Beef up the commit message a bit (Daniel)

Cc: Nick Yamane <nick.diego@gmail.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: stable@vger.kernel.org
Tested-by: NNick Yamane <nick.diego@gmail.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=97204Signed-off-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1474878646-17711-1-git-send-email-ville.syrjala@linux.intel.comReviewed-by: NAnder Conselvan de Oliveira <conselvan2@gmail.com>

Replace dev with dev_priv and INTEL_INFO with INTEL_GEN

v1:
* Rebased on drm-nightly (Jani Nikula)
* Separated from the link training patch series
Signed-off-by: NManasi Navare <manasi.d.navare@intel.com>
Reviewed-by: NMika Kahola <mika.kahola@intel.com>
Signed-off-by: NJani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1475019413-19811-1-git-send-email-manasi.d.navare@intel.com

a277ca7d should've been a no-functional-change commit, but it
removed the initialization of the dpll_hw_state for HDMI outputs,
resulting in state mismatches and a failed modeset with blank
screen. Fix this by reinstating the dpll_hw_state initialization.

v2:
- Make bxt_ddi_hdmi_set_dpll_hw_state() static.

Cc: Manasi Navare <manasi.d.navare@intel.com>
Cc: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Cc: Durgadoss R <durgadoss.r@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Fixes: a277ca7d ("drm/i915: Split bxt_ddi_pll_select()")
Signed-off-by: NImre Deak <imre.deak@intel.com>
Reviewed-by: NAnder Conselvan de Oliveira <conselvan2@gmail.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1474901671-22719-1-git-send-email-imre.deak@intel.com

We just removed the implementation for all the pre-production
workarounds, so now tell the user that we expect his machine to not
work properly. Also convert this to DRM_ERROR so we can more easily
spot these problems in bug reports and CI/QA runs.

Cc: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: NPaulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: NJani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1474891672-23414-2-git-send-email-jani.nikula@intel.com

Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: NPaulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: NJani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1474891672-23414-1-git-send-email-jani.nikula@intel.com

When i915_fbc_status is read while fbc is disabled,
it reports compressing to be true, which is confusing.
Report compressing only when fbc is enabled.

v2 (from Paulo): commit message capitalization.
Signed-off-by: Nvathsala nagaraju <vathsala.nagaraju@intel.com>
Reviewed-by: NPaulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: NPaulo Zanoni <paulo.r.zanoni@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1474534193-9527-1-git-send-email-vathsala.nagaraju@intel.com

Now that this code is part of the compute stage we can return -EINVAL
to prevent the modeset instead of giving a WARN and trying anyway.

v2:
 - Fix typo (Paul Menzel).
 - Add MISSING_CASE() (Ville, Maarten).
Reported-by: NLyude <cpaul@redhat.com>
Reviewed-by: NMaarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: NPaulo Zanoni <paulo.r.zanoni@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1474578035-424-10-git-send-email-paulo.r.zanoni@intel.com

This should affect linear and X tiled planes on really small htotal
cases. It doesn't seem to be a very feasible case, but let's implement
it since it's on the specification and it's better to have it and
never need than not have it and realize we needed it.
Reviewed-by: NLyude <cpaul@redhat.com>
Reviewed-by: NMaarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: NPaulo Zanoni <paulo.r.zanoni@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1474578035-424-9-git-send-email-paulo.r.zanoni@intel.com

We forgot the "res_blocks += y_tile_minimum" that's described on step
V of our documentation.

Again, this should only affect the Y tiling cases.

It looks like the relevant code was introduced in 0fda6568, but
there's always the possibility that it matched our specification when
it was introduced, and then the specification changed while the code
stayed the same. So we can't really say this was a regression, but
let's try to add a "Fixes" tag anyway to help backporting.

v2: Try to add a "Fixes" tag (Maarten).

Fixes: 0fda6568 ("drm/i915/skl: Update watermarks for Y tiling")
Cc: stable@vger.kernel.org
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: NLyude <cpaul@redhat.com>
Reviewed-by: NMaarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: NPaulo Zanoni <paulo.r.zanoni@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1474578035-424-8-git-send-email-paulo.r.zanoni@intel.com

The confusing thing is that plane_blocks_per_line is listed as part of
the method 2 calculation but is also used for other things. We
calculated it in two different places and different ways: one inside
skl_wm_method2() and the other inside skl_compute_plane_wm(). The
skl_wm_method2() implementation is the one that matches the
specification.

With this patch we fix the skl_compute_plane_wm() calculation and just
pass it as a parameter to skl_wm_method2(). We also take care to not
modify the value of plane_bytes_per_line since we're going to rely on
it having a correct value in later patches.

This should affect the watermarks for Linear and Y-tiled.

From my analysis, it looks like the two plane_blocks_per_line
variables got out of sync on 0fda6568, but we can't really say
that commit was a regression, it looks like just an incomplete fix.
There's always the possibility that 0fda6568 matched our
specification at that time, and then later the specification changed.

v2: Try to add a "Fixes" tag (Maarten).

Fixes: 0fda6568 ("drm/i915/skl: Update watermarks for Y tiling")
Cc: stable@vger.kernel.org
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: NLyude <cpaul@redhat.com>
Reviewed-by: NMaarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: NPaulo Zanoni <paulo.r.zanoni@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1474578035-424-7-git-send-email-paulo.r.zanoni@intel.com

During watermarks calculations, this value is used in 3 different
places. Only one of them was not using a hardcoded 4. Move the code up
so everybody can benefit from the actual value.

This should only help on situations with Y tiling + 90/270 rotation +
1 or 2 bpp or NV12.

Cc: stable@vger.kernel.org
Reviewed-by: NMaarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: NPaulo Zanoni <paulo.r.zanoni@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1474578035-424-6-git-send-email-paulo.r.zanoni@intel.com

Bspec says:
  "The mailbox response data may not account for memory read latency.
   If the mailbox response data for level 0 is 0us, add 2 microseconds
   to the result for each valid level."

This means we should only do the +2 in case wm[0] == 0, not always.

So split the sanitizing implementation from the WA implementation and
fix the WA implementation.

v2: Add Fixes tag (Maarten).

Fixes: 367294be ("drm/i915/gen9: Add 2us read latency to WM level")
Cc: stable@vger.kernel.org
Cc: Vandana Kannan <vandana.kannan@intel.com>
Reviewed-by: NMaarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: NPaulo Zanoni <paulo.r.zanoni@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1474578035-424-5-git-send-email-paulo.r.zanoni@intel.com

According to BSpec, it's the "core CPUs" that need the code, which
means SKL and KBL, but not BXT.

I don't have a KBL to test this patch on it.

v2: Only SKL should have I915_SAGV_NOT_CONTROLLED.

Cc: stable@vger.kernel.org
Reviewed-by: NMaarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: NPaulo Zanoni <paulo.r.zanoni@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1474578035-424-4-git-send-email-paulo.r.zanoni@intel.com

And use it to move knowledge about the SAGV-supporting platforms from
the callers to the SAGV code.

We'll add more platforms to intel_has_sagv(), so IMHO it makes more
sense to move all this to a single function instead of patching all
the callers every time we add SAGV support to a new platform.

v2: Move I915_SAGV_NOT_CONTROLLED to the new function (Lyude).

Cc: stable@vger.kernel.org
Reviewed-by: NMaarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: NPaulo Zanoni <paulo.r.zanoni@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1474578035-424-3-git-send-email-paulo.r.zanoni@intel.com

The plan is to introduce intel_has_sagv() and then use it to discover
which platforms actually support it.

I thought about keeping the functions with their current skl names,
but found two problems: (i) skl_has_sagv() would become a very
confusing name, and (ii) intel_atomic_commit_tail() doesn't seem to be
calling any functions whose name start with a platform name, so the
"intel_" naming scheme seems make more sense than the "firstplatorm_"
naming scheme here.

Cc: stable@vger.kernel.org
Reviewed-by: NLyude <cpaul@redhat.com>
Reviewed-by: NMaarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: NPaulo Zanoni <paulo.r.zanoni@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1474578035-424-2-git-send-email-paulo.r.zanoni@intel.com

Pre-production hardware is not supported.
Reviewed-by: NPaulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: NJani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/e5433e6430dcfd941209c4d8103035ddb13d17b4.1474034059.git.jani.nikula@intel.com

Pre-production hardware is not supported.
Reviewed-by: NPaulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: NJani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/0633a02177195703502ef2396aab03efc0314334.1474034059.git.jani.nikula@intel.com

Pre-production hardware is not supported.
Reviewed-by: NPaulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: NJani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/d28d21ceddeec226b5d1a20a7382bee9a72709a4.1474034059.git.jani.nikula@intel.com

Pre-production hardware is not supported.
Reviewed-by: NPaulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: NJani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/ed7b784306b35fa5215b9c04de79a2bc48585503.1474034059.git.jani.nikula@intel.com

Pre-production hardware is not supported.
Reviewed-by: NPaulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: NJani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/7929af62a68504c84038a8db1625bd96ebaa9e6f.1474034059.git.jani.nikula@intel.com

We never remembered to set it (so it was zero), but this was not a
problem in the past due to the way handled the hardware registers.
Unfortunately we changed how we set the hardware and forgot to set
intel_crtc->dspaddr_offset.

This started to reflect on a few kms_frontbuffer_tracking subtests
that relied on page flips with CRTCs that don't point to the x:0,y:0
coordinates of the frontbuffer. After the page flip the CRTC was
showing the x:0,y:0 coordinate of the frontbuffer instead of
x:500,y:500. This problem is present even if we don't enable FBC or
PSR.

While trying to bisect it I realized that the first bad commit
actually just gives me a black screen for the mentioned tests instead
of showing the wrong x:0,y:0 offsets. A few commits later the black
screen problem goes away and we get to the point where the code is
today, but I'll consider the black screen as the first bad commit
since it's the point where the IGT subtests start to fail.

Fixes: 6687c906 ("drm/i915: Rewrite fb rotation GTT handling")
Testcase: kms_frontbuffer_tracking/fbc-1p-primscrn-shrfb-pgflip-blt
Testcase: kms_frontbuffer_tracking/fbc-1p-primscrn-shrfb-evflip-blt
Testcase: kms_frontbuffer_tracking/fbc-1p-shrfb-fliptrack
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Sivakumar Thulasimani <sivakumar.thulasimani@intel.com>
Cc: drm-intel-fixes@lists.freedesktop.org
Signed-off-by: NPaulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1471644203-23463-1-git-send-email-paulo.r.zanoni@intel.com

Ever since I started working on FBC I was already aware that FBC can
really amplify the FIFO underrun symptoms. On systems where FIFO
underruns were harmless error messages, enabling FBC would cause the
underruns to give black screens.

We recently tried to enable FBC on Haswell and got reports of a system
that would hang after some hours of uptime, and the first bad commit
was the one that enabled FBC. We also observed that this system had
FIFO underrun error messages on its dmesg. Although we don't have any
evidence that fixing the underruns would solve the bug and make FBC
work properly on this machine, IMHO it's better if we minimize the
amount of possible problems by just giving up FBC whenever we detect
an underrun.

v2: New version, different implementation and commit message.
v3: Clarify the fact that we run from an IRQ handler (Chris).
v4: Also add the underrun_detected check at can_choose() to avoid
    misleading dmesg messages (DK).
v5: Fix Engrish, use READ_ONCE on the unlocked read (Chris).

Cc: Stefan Richter <stefanr@s5r6.in-berlin.de>
Cc: Lyude <cpaul@redhat.com>
Cc: stevenhoneyman@gmail.com <stevenhoneyman@gmail.com>
Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NPaulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: NDhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1473773937-19758-1-git-send-email-paulo.r.zanoni@intel.com