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

[Nick Hoath <nicholas.hoath@intel.com>]

On 14/01/2016 11:36, Chris Wilson wrote:
> 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

That what this patch does.

> introduce a blocking context-close, nor do you need to introduce the
> usage of default_context.

The use of default_context here is to stop a context hanging around 
after it is no longer needed.

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

You can't clear the reference on the ctx in an interrupt context.

>
> 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
>

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