drm/i915: Keep contexts pinned until after the next kernel context switch
We need to keep the context image pinned in memory until after the GPU has finished writing into it. Since it continues to write as we signal the final breadcrumb, we need to keep it pinned until the request after it is complete. Currently we know the order in which requests execute on each engine, and so to remove that presumption we need to identify a request/context-switch we know must occur after our completion. Any request queued after the signal must imply a context switch, for simplicity we use a fresh request from the kernel context. The sequence of operations for keeping the context pinned until saved is: - On context activation, we preallocate a node for each physical engine the context may operate on. This is to avoid allocations during unpinning, which may be from inside FS_RECLAIM context (aka the shrinker) - On context deactivation on retirement of the last active request (which is before we know the context has been saved), we add the preallocated node onto a barrier list on each engine - On engine idling, we emit a switch to kernel context. When this switch completes, we know that all previous contexts must have been saved, and so on retiring this request we can finally unpin all the contexts that were marked as deactivated prior to the switch. We can enhance this in future by flushing all the idle contexts on a regular heartbeat pulse of a switch to kernel context, which will also be used to check for hung engines. v2: intel_context_active_acquire/_release Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk> Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com> Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com> Link: https://patchwork.freedesktop.org/patch/msgid/20190614164606.15633-1-chris@chris-wilson.co.uk
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