If we leave obj->pages set to NULL before attempting to deswizzle them,
then an OOPS is well deserved.

Fixes regression introduced in commit 9da3da66
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Fri Jun 1 15:20:22 2012 +0100

    drm/i915: Replace the array of pages with a scatterlist

Reported-and-tested-by: Krzysztof Kolasa
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: NJesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

-ENOSPC can happen if userspace is being simplistic and tries to map a
too big object. To aid further spurious WARN debugging, also print out
the error code.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=56017Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

This magic brings stability to HSW CRW machines.
Signed-off-by: NRodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: NBen Widawsky <ben@bwidawsk.net>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

The intention was to allow the caller to avoid a failure to queue a
request having already written commands to the ring. However, this is a
moot point as the i915_add_request() can fail for other reasons than a
mere allocation failure and those failure cases are more likely than
ENOMEM. So the overlay code already had to handle i915_add_request()
failures, and due to

commit 3bb73aba
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Fri Jul 20 12:40:59 2012 +0100

    drm/i915: Allow late allocation of request for i915_add_request()

the error handling code in intel_overlay.c was subject to causing
double-frees, as found by coverity.

Rather than further complicate i915_add_request() and callers, realise
the battle is lost and adapt intel_overlay.c to take advantage of the
late allocation of requests.

v2: Handle callers passing in a NULL seqno.
v3: Ditto. This time for sure.
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: NJani Nikula <jani.nikula@intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

starting an old X server causes a kernel BUG since commit 1b50247a:

------------[ cut here ]------------
kernel BUG at drivers/gpu/drm/i915/i915_gem.c:3661!
invalid opcode: 0000 [#1] SMP
Modules linked in: snd_seq_dummy snd_seq_oss snd_seq_midi_event snd_seq snd_seq_device snd_pcm_oss snd_mixer_oss uvcvideo
+videobuf2_core videodev videobuf2_vmalloc videobuf2_memops uhci_hcd ath9k mac80211 snd_hda_codec_realtek ath9k_common microcode
+ath9k_hw psmouse serio_raw sg ath cfg80211 atl1c lpc_ich mfd_core ehci_hcd snd_hda_intel snd_hda_codec snd_hwdep snd_pcm rtc_cmos
+snd_timer snd evdev eeepc_laptop snd_page_alloc sparse_keymap

Pid: 2866, comm: X Not tainted 3.5.6-rc1-eeepc #1 ASUSTeK Computer INC. 1005HA/1005HA
EIP: 0060:[<c12dc291>] EFLAGS: 00013297 CPU: 0
EIP is at i915_gem_entervt_ioctl+0xf1/0x110
EAX: f5941df4 EBX: f5940000 ECX: 00000000 EDX: 00020000
ESI: f5835400 EDI: 00000000 EBP: f51d7e38 ESP: f51d7e20
 DS: 007b ES: 007b FS: 00d8 GS: 0033 SS: 0068
CR0: 8005003b CR2: b760e0a0 CR3: 351b6000 CR4: 000007d0
DR0: 00000000 DR1: 00000000 DR2: 00000000 DR3: 00000000
DR6: ffff0ff0 DR7: 00000400
Process X (pid: 2866, ti=f51d6000 task=f61af8d0 task.ti=f51d6000)
Stack:
 00000001 00000000 f5835414 f51d7e84 f5835400 f54f85c0 f51d7f10 c12b530b
 00000001 c151b139 c14751b6 c152e030 00000b32 00006459 00000059 0000e200
 00000001 00000000 00006459 c159ddd0 c12dc1a0 ffffffea 00000000 00000000
Call Trace:
 [<c12b530b>] drm_ioctl+0x2eb/0x440
 [<c12dc1a0>] ? i915_gem_init+0xe0/0xe0
 [<c1052b2b>] ? enqueue_hrtimer+0x1b/0x50
 [<c1053321>] ? __hrtimer_start_range_ns+0x161/0x330
 [<c10530b3>] ? lock_hrtimer_base+0x23/0x50
 [<c1053163>] ? hrtimer_try_to_cancel+0x33/0x70
 [<c12b5020>] ? drm_version+0x90/0x90
 [<c10ca171>] vfs_ioctl+0x31/0x50
 [<c10ca2e4>] do_vfs_ioctl+0x64/0x510
 [<c10535de>] ? hrtimer_nanosleep+0x8e/0x100
 [<c1052c20>] ? update_rmtp+0x80/0x80
 [<c10ca7c9>] sys_ioctl+0x39/0x60
 [<c1433949>] syscall_call+0x7/0xb
Code: 83 c4 0c 5b 5e 5f 5d c3 c7 44 24 04 2c 05 53 c1 c7 04 24 6f ef 47 c1 e8 6e e0 fd ff c7 83 38 1e 00 00 00 00 00 00 e9 3f ff ff
+ff <0f> 0b eb fe 0f 0b eb fe 8d b4 26 00 00 00 00 0f 0b eb fe 8d b6
EIP: [<c12dc291>] i915_gem_entervt_ioctl+0xf1/0x110 SS:ESP 0068:f51d7e20
---[ end trace dd332ec083cbd513 ]---

The crash happens here in i915_gem_entervt_ioctl() :

    3659          BUG_ON(!list_empty(&dev_priv->mm.active_list));
    3660          BUG_ON(!list_empty(&dev_priv->mm.flushing_list));
 -> 3661          BUG_ON(!list_empty(&dev_priv->mm.inactive_list));
    3662          mutex_unlock(&dev->struct_mutex);

Quoting Chris :
  "That BUG_ON there is silly and can simply be removed. The check is to
   verify that no batches were submitted to the kernel whilst the UMS/GEM
   client was suspended - to which the BUG_ONs are a crude approximation.
   Furthermore, the checks are too late, since it means we attempted to
   program the hardware whilst it was in an invalid state, the BUG_ONs are
   the least of your concerns at that point."

Note that this regression has been introduced in

commit 1b50247a
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Tue Apr 24 15:47:30 2012 +0100

    drm/i915: Remove the list of pinned inactive objects

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NWilly Tarreau <w@1wt.eu>
[danvet: Added note about the regressing commit and cc: stable.]
Cc: stable@vger.kernel.org
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

Falling into default case in vmi915_gem_fault is a bug. Be more
verbose about it.
Signed-off-by: NMika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

Subsequent threads returning EBUSY from vm_insert_pfn() was not handled
correctly. As a result concurrent access from new threads to
mmapped data caused SIGBUS.

Note that this fixes i-g-t/tests/gem_threaded_tiled_access.
Tested-by: NMika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: NDmitry Rogozhkin <dmitry.v.rogozhkin@intel.com>
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

On the EINVAL case we don't release struct_mutex. It should be safe to
grab the lock after checking the parameters, which also resolves the
issues.
Signed-off-by: NBen Widawsky <ben@bwidawsk.net>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

Signed-off-by: NBen Widawsky <ben@bwidawsk.net>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

By providing a callback for when we need to bind the pages, and then
release them again later, we can shorten the amount of time we hold the
foreign pages mapped and pinned, and importantly the dmabuf objects then
behave as any other normal object with respect to the shrinker and
memory management.
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: NBen Widawsky <ben@bwidawsk.net>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

Rather than have multiple data structures for describing our page layout
in conjunction with the array of pages, we can migrate all users over to
a scatterlist.

One major advantage, other than unifying the page tracking structures,
this offers is that we replace the vmalloc'ed array (which can be up to
a megabyte in size) with a chain of individual pages which helps reduce
memory pressure.

The disadvantage is that we then do not have a simple array to iterate,
or to access randomly. The common case for this is in the relocation
processing, which will typically fit within a single scatterlist page
and so be almost the same cost as the simple array. For iterating over
the array, the extra function call could be optimised away, but in
reality is an insignificant cost of either binding the pages, or
performing the pwrite/pread.

v2: Fix drm_clflush_sg() to not invoke wbinvd as well! And fix the
trivial compile error from rebasing.
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

By using the recently introduced pinning of pages, we can safely drop
the mutex in the knowledge that the pages are not going to disappear
beneath us, and so we can simplify the code for iterating over the pages.
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: NBen Widawsky <ben@bwidawsk.net>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

By using the recently introduced pinning of pages, we can safely drop
the mutex in the knowledge that the pages are not going to disappear
jeneath us, and so we can simplify the code for iterating over the pages.

Note: The old code had such complicated page refcounting since it used
obj->pages as a micro-optimization if it's there, but that could
(before this patch) disappear when we drop the dev->struct_mutex.
Hence some manual page refcounting was required for the slow path,
complicated by the fact that pages returned by shmem_read_mapping_page
already have a pageref, which needs to be dropped again.
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: NBen Widawsky <ben@bwidawsk.net>
[danvet: Added note to explain the question Ben raised in review.]
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

We need to refcount our pages in order to prevent reaping them at
inopportune times, such as when they currently vmapped or exported to
another driver. However, we also wish to keep the lazy deallocation of
our pages so we need to take a pin/unpinned approach rather than a
simple refcount.
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: NBen Widawsky <ben@bwidawsk.net>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

In order to specialise functions depending upon the type of object, we
can attach vfuncs to each object via a new ->ops pointer.

For instance, this will be used in future patches to only bind pages from
a dma-buf for the duration that the object is used by the GPU - and so
prevent them from pinning those pages for the entire of the object.

v2: Bonus comments.
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: NBen Widawsky <ben@bwidawsk.net>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

Pin-leaks persist and we get the perennial bug reports of machine
lockups to the BUG_ON(pin_count==MAX). If we instead loudly report that
the object cannot be pinned at that time it should prevent the driver from
locking up, and hopefully restore a semblance of working whilst still
leaving us a OOPS to debug.
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Cc: stable@vger.kernel.org
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

When I pulled-in today's drm-intel-next into linux-next (next-20120824)
I saw this build-breakage:

drivers/gpu/drm/i915/i915_gem.c: In function 'i915_gem_object_get_pages_gtt':
drivers/gpu/drm/i915/i915_gem.c:1778:40: error: '__GFP_NO_KSWAPD' undeclared (first use in this function)
drivers/gpu/drm/i915/i915_gem.c:1778:40: note: each undeclared identifier is reported only once for each function it appears in

This is caused by commit ba099ef165f8 ("mm: remove __GFP_NO_KSWAPD")
and commit b6beae2c2014 ("mm: remove __GFP_NO_KSWAPD fixes") in
linux-next (next-20120824).

Fix this by removing __GFP_NO_KSWAPD from drm/i915 driver.
Signed-off-by: NSedat Dilek <sedat.dilek@gmail.com>
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

The principal use for set-to-domain is for userspace to serialise
operations with a particular buffer, for example to maintain coherency
with a CPU map or to ratelimit its rendering by waiting on all previous
operations before continuing. As such we tend to hold the struct_mutex
for long periods during the synchronisation and so cause contention
issues with other users of the graphics device, even for independent
operations as memory management. An example is the contention between
compiz and X which causes jitter in the display and a drop in peak
throughput.

The ultimate solution would be a set of fine grained locks and lockless
operations, but an intermediate step is to first attempt the
synchronisation for set-to-domain without holding the mutex. This
introduces a number of race conditions, so we limit it use to the ioctl
periphery where we have no dependent state and can safely complete with
a locked synchronisation afterwards.
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

Move the wait-for-rendering logic around in the file so that we can
group it together with the subsequent variations. The general goal is to
have the lower level routines clustered together and then the higher
level logic building upon those low level routines that came before.
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

As we wish to create specialised object constructions in the near
future that share the same basic GEM object struct, export the default
initializer.
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

If the object has no backing shmemfs filp, then we obviously cannot
perform a truncation operation upon it.
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

Avoid stalling and waiting for the GPU by checking to see if there is
sufficient inactive space in the aperture for us to bind the buffer
prior to writing through the GTT. If there is inadequate space we will
have to stall waiting for the GPU, and incur overheads moving objects
about. Instead, only incur the clflush overhead on the target object by
writing through shmem.
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

Given the persistence of an offset for the lifetime of an object, itis
easy to contemplate how the mmap space becomes badly fragmented to the
point that further allocations fail with ENOSPC. Our only recourse at
this point is to try to purge the objects to release some space and
reattempt the allocation.

References: https://bugs.freedesktop.org/show_bug.cgi?id=39552Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

A pair of universally true checks that just need to be put in the right
place depending on where in the patch sequence you go. Note that
i915_gem_object_put_pages_gtt() already gains the
BUG_ON(obj->gtt_space), but on reflection that needed to migrate to
put_pages().
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

When dealing with a working set larger than the GATT, or even the
mappable aperture when touching through the GTT, we end up with evicting
objects only to rebind them at a new offset again later. Moving an
object into and out of the GTT requires clflushing the pages, thus
causing a double-clflush penalty for rebinding.

To avoid having to clflush on rebinding, we can track the pages as they
are evicted from the GTT and only relinquish those pages on memory
pressure.

As usual, if it were not for the handling of out-of-memory condition and
having to manually shrink our own bo caches, it would be a net reduction
of code. Alas.

Note: The patch also contains a few changes to the last-hope
evict_everything logic in i916_gem_execbuffer.c - we no longer try to
only evict the purgeable stuff in a first try (since that's superflous
and only helps in OOM corner-cases, not fragmented-gtt trashing
situations).

Also, the extraction of the get_pages retry loop from bind_to_gtt (and
other callsites) to get_pages should imo have been a separate patch.

v2: Ditch the newly added put_pages (for unbound objects only) in
i915_gem_reset. A quick irc discussion hasn't revealed any important
reason for this, so if we need this, I'd like to have a git blame'able
explanation for it.

v3: Undo the s/drm_malloc_ab/kmalloc/ in get_pages that Chris noticed.
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
[danvet: Split out code movements and rant a bit in the commit message
with a few Notes. Done v2]
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

Prep work to make Chris Wilson's unbound tracking patch a bit easier
to read. Alas, I'd have preferred that moving the page allocation
retry loop from bind to get_pages would have been a separate patch,
too. But that looks like real work ;-)
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

After reset we unconditionally reinitialize lists. If the context switch
hasn't yet completed before the suspend, the default context object will
end up on lists that are going to go away when we resume.

The patch forces the context switch to be synchronous before suspend
assuring that the active/inactive tracking is correct at the time of
resume.

References: https://bugs.freedesktop.org/show_bug.cgi?id=52429Tested-by: NGuang A Yang <guang.a.yang@intel.com>
Signed-off-by: NBen Widawsky <ben@bwidawsk.net>
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

Avoid the forcewake overhead when simply retiring requests, as often the
last seen seqno is good enough to satisfy the retirment process and will
be promptly re-run in any case. Only ensure that we force the coherent
seqno read when we are explicitly waiting upon a completion event to be
sure that none go missing, and also for when we are reporting seqno
values in case of error or debugging.

This greatly reduces the load for userspace using the busy-ioctl to
track active buffers, for instance halving the CPU used by X in pushing
the pixels from a software render (flash). The effect will be even more
magnified with userptr and so providing a zero-copy upload path in that
instance, or in similar instances where X is simply compositing DRI
buffers.

v2: Reverse the polarity of the tachyon stream. Daniel suggested that
'force' was too generic for the parameter name and that 'lazy_coherency'
better encapsulated the semantics of it being an optimization and its
purpose. Also notice that gen6_get_seqno() is only used by gen6/7
chipsets and so the test for IS_GEN6 || IS_GEN7 is redundant in that
function.
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: NDaniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

By selecting the cache level (essentially whether or not the CPU snoops
any updates to the bo, and on more recent machines whether it resides
inside the CPU's last-level-cache) a userspace driver is able to then
manage all of its memory within buffer objects, if it so desires. This
enables the userspace driver to accelerate uploads and more importantly
downloads from the GPU and to able to mix CPU and GPU rendering/activity
efficiently.
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
[danvet: Added code comment about where we plan to stuff platform
specific cacheing control bits in the ioctl struct.]
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

Several functions of the GPU have the restriction that differing memory
domains cannot be placed next to each other (as the GPU may prefetch
beyond the end of one domain and hang as it crosses into the other
domain). We use the facility of the drm_mm to mark ranges with a
particular color that corresponds to the cache attributes of those pages
in order to prevent allocating adjacent blocks of differing memory
types.

v2: Rebase ontop of drm_mm coloring v2.
v3: Fix rebinding existing gtt_space and add a verification routine.
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

As suggested by Daniel, rip out the independent timers for device and
crtc busyness and integrate the manual powermanagement of the display
engine into the GEM core and its request tracking. The benefits are that
the code is a lot smaller, fewer moving parts and should fit more neatly
into the overall activity tracking of the driver.

v2: Complete overhaul and removal of the racy timers and workers.
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

By moving the function to intel_ringbuffer and currying the appropriate
parameter, hopefully we make the callsites easier to read and
understand.
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

Rely instead on the insertion of the implicit flush before the seqno
breadcrumb.
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

As the flush is either performed explictly immediately after the
execbuffer dispatch, or before the serialisation of last_fenced_seqno we
can forgo the explict i915_gem_flush_ring().
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

This is now handled by a global flag to ensure we emit a flush before
the next serialisation point (if we failed to queue one previously).
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

As we guarantee to emit a flush before emitting the breadcrumb or
the next batchbuffer, there is no further need for the flushing list.
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

As we always flush the GPU cache prior to emitting the breadcrumb, we no
longer have to worry about the deferred flush causing the
pending_gpu_write to be delayed. So we can instead utilize the known
last_write_seqno to hopefully minimise the wait times.
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

As we move to lazily clearing the GPU write domain only when the buffer
becomes inactive, this leaves a window of opportunity for
i915_gem_object_pin_to_display_plane() to detect a seemingly
inconsistent value. This function is special as it tries to pipeline the
operation to avoid the stall and so may not retires the buffer and we
may not get the opportunity to clear the write domain. However, we know
all is good, so drop the assertion.
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

Request preallocation was added to i915_add_request() in order to
support the overlay. However, not all users care and can quite happily
ignore the failure to allocate the request as they will simply repeat
the request in the future.

By pushing the allocation down into i915_add_request(), we can then
remove some rather ugly error handling in the callers.

v2: Nullify request->file_priv otherwise we chase a garbage pointer
when retiring requests.
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

The intention is to help select which engine to use for copies with
interoperating clients - such as a GL client making a request to the X
server to perform a SwapBuffers, which may require copying from the
active GL back buffer to the X front buffer.

We choose to report a mask of the active rings to future proof the
interface against any changes which may allow for the object to reside
upon multiple rings.
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
[danvet: bikeshed away the write ring mask and add the explanation
Chris sent in a follow-up mail why we decided to use masks.]
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>