[Intel-gfx] [PATCH v10] drm/i915: Extend LRC pinning to cover GPU context writeback

[Chris Wilson]

On Wed, Jan 13, 2016 at 04:19:45PM +0000, Nick Hoath wrote:
> +	if (ctx->engine[ring->id].dirty) {
> +		struct drm_i915_gem_request *req = NULL;
> +
> +		/**
> +		 * If there is already a request pending on
> +		 * this ring, wait for that to complete,
> +		 * otherwise create a switch to idle request
> +		 */
> +		if (list_empty(&ring->request_list)) {
> +			int ret;
> +
> +			ret = i915_gem_request_alloc(
> +					ring,
> +					ring->default_context,
> +					&req);
> +			if (!ret)
> +				i915_add_request(req);
> +			else
> +				DRM_DEBUG("Failed to ensure context saved");
> +		} else {
> +			req = list_first_entry(
> +					&ring->request_list,
> +					typeof(*req), list);
> +		}
> +		if (req) {
> +			ret = i915_wait_request(req);
> +			if (ret != 0) {
> +				/**
> +				 * If we get here, there's probably been a ring
> +				 * reset, so we just clean up the dirty flag.&
> +				 * pin count.
> +				 */
> +				ctx->engine[ring->id].dirty = false;
> +				__intel_lr_context_unpin(
> +					ring,
> +					ctx);
> +			}
> +		}

If you were to take a lr_context_pin on the last_context, and only
release that pin when you change to a new context, you do not need to
introduce a blocking context-close, nor do you need to introduce the
usage of default_context.

(lr_context_pin should take a reference on the ctx to prevent early
freeeing ofc).

The code at that point starts to look v.v.similar to legacy, right down
to the need to use a GPU reset during shutdown to prevent writing back
the context image. (Which you still currently need to get rid of the
default context now.)
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre