[Intel-gfx] [CI] drm/i915/gt: Keep a no-frills swappable copy of the default context state

[Dave Airlie]

On Thu, 30 Apr 2020 at 03:24, Chris Wilson <chris at chris-wilson.co.uk> wrote:
>
> We need to keep the default context state around to instantiate new
> contexts (aka golden rendercontext), and we also keep it pinned while
> the engine is active so that we can quickly reset a hanging context.
> However, the default contexts are large enough to merit keeping in
> swappable memory as opposed to kernel memory, so we store them inside
> shmemfs. Currently, we use the normal GEM objects to create the default
> context image, but we can throw away all but the shmemfs file.
>
> This greatly simplifies the tricky power management code which wants to
> run underneath the normal GT locking, and we definitely do not want to
> use any high level objects that may appear to recurse back into the GT.
> Though perhaps the primary advantage of the complex GEM object is that
> we aggressively cache the mapping, but here we are recreating the
> vm_area everytime time we unpark. At the worst, we add a lightweight
> cache, but first find a microbenchmark that is impacted.
>
> Having started to create some utility functions to make working with
> shmemfs objects easier, we can start putting them to wider use, where
> GEM objects are overkill, such as storing persistent error state.
>
The current DG1 patchset is missing something around here.

The driver creates in intel_lrc.c the contexts in LMEM, but then it
gets to this patch and gets an EBUSY trying to pin_map it.

Dave.