The workaround itself applies to gen7 only (according to the docs) and
as Eric Anholt points out shouldn't be required since we don't use HW
scheduling features, and therefore arbitration. Though since it is a
small, and simple addition, and we don't really understand the issue,
just do it.

FWIW, I eventually want to play with some of the arbitration stuff, and
I'd hate to forget about this.
Signed-off-by: NBen Widawsky <ben@bwidawsk.net>

This way round we don't introduce and ugly layering violations and use
the interface as I planned to use it.
Signed-Off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

Implement the context switch code as well as the interfaces to do the
context switch. This patch also doesn't match 1:1 with the RFC patches.
The main difference is that from Daniel's responses the last context
object is now stored instead of the last context. This aids in allows us
to free the context data structure, and context object independently.

There is room for optimization: this code will pin the context object
until the next context is active. The optimal way to do it is to
actually pin the object, move it to the active list, do the context
switch, and then unpin it. This allows the eviction code to actually
evict the context object if needed.

The context switch code is missing workarounds, they will be implemented
in future patches.

v2: actually do obj->dirty=1 in switch (daniel)
Modified comment around above
Remove flags to context switch (daniel)
Move mi_set_context code to i915_gem_context.c (daniel)
Remove seqno , use lazy request instead (daniel)

v3: use i915_gem_request_next_seqno instead of
      outstanding_lazy_request (Daniel)
remove id's from trace events (Daniel)
Put the context BO in the instruction domain (Daniel)
Don't unref the BO is context switch fails (Chris)
Signed-off-by: NBen Widawsky <ben@bwidawsk.net>

Invent an abstraction for a hw context which is passed around through
the core functions. The main bit a hw context holds is the buffer object
which backs the context. The rest of the members are just helper
functions. Specifically the ring member, which could likely go away if
we decide to never implement whatever other hw context support exists.

Of note here is the introduction of the 64k alignment constraint for the
BO. If contexts become heavily used, we should consider tweaking this
down to 4k. Until the contexts are merged and tested a bit though, I
think 64k is a nice start (based on docs).

Since we don't yet switch contexts, there is really not much complexity
here. Creation/destruction works pretty much as one would expect. An idr
is used to generate the context id numbers which are unique per file
descriptor.

v2: add DRM_DEBUG_DRIVERS to distinguish ENOMEM failures (ben)
convert a BUG_ON to WARN_ON, default destruction is still fatal (ben)
Signed-off-by: NBen Widawsky <ben@bwidawsk.net>

Very basic code for context setup/destruction in the driver.

Adds the file i915_gem_context.c This file implements HW context
support. On gen5+ a HW context consists of an opaque GPU object which is
referenced at times of context saves and restores.  With RC6 enabled,
the context is also referenced as the GPU enters and exists from RC6
(GPU has it's own internal power context, except on gen5).  Though
something like a context does exist for the media ring, the code only
supports contexts for the render ring.

In software, there is a distinction between contexts created by the
user, and the default HW context. The default HW context is used by GPU
clients that do not request setup of their own hardware context. The
default context's state is never restored to help prevent programming
errors. This would happen if a client ran and piggy-backed off another
clients GPU state.  The default context only exists to give the GPU some
offset to load as the current to invoke a save of the context we
actually care about. In fact, the code could likely be constructed,
albeit in a more complicated fashion, to never use the default context,
though that limits the driver's ability to swap out, and/or destroy
other contexts.

All other contexts are created as a request by the GPU client. These
contexts store GPU state, and thus allow GPU clients to not re-emit
state (and potentially query certain state) at any time. The kernel
driver makes certain that the appropriate commands are inserted.

There are 4 entry points into the contexts, init, fini, open, close.
The names are self-explanatory except that init can be called during
reset, and also during pm thaw/resume. As we expect our context to be
preserved across these events, we do not reinitialize in this case.

As Adam Jackson pointed out, The cutoff of 1MB where a HW context is
considered too big is arbitrary. The reason for this is even though
context sizes are increasing with every generation, they have yet to
eclipse even 32k. If we somehow read back way more than that, it
probably means BIOS has done something strange, or we're running on a
platform that wasn't designed for this.

v2: rename load/unload to init/fini (daniel)
remove ILK support for get_size() (indirectly daniel)
add HAS_HW_CONTEXTS macro to clarify supported platforms (daniel)
added comments (Ben)
Signed-off-by: NBen Widawsky <ben@bwidawsk.net>