Re: [PATCH 16/38] drm/i915: Introduce a context barrier callback

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 01/03/2019 14:03, Chris Wilson wrote:
> In the next patch, we will want to update live state within a context.
> As this state may be in use by the GPU and we haven't been explicitly
> tracking its activity, we instead attach it to a request we send down
> the context setup with its new state and on retiring that request
> cleanup the old state as we then know that it is no longer live.
> 
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> ---
>   drivers/gpu/drm/i915/i915_gem_context.c       |  74 +++++++++++++
>   .../gpu/drm/i915/selftests/i915_gem_context.c | 103 ++++++++++++++++++
>   2 files changed, 177 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/i915_gem_context.c b/drivers/gpu/drm/i915/i915_gem_context.c
> index 3b5145b30d85..91926a407548 100644
> --- a/drivers/gpu/drm/i915/i915_gem_context.c
> +++ b/drivers/gpu/drm/i915/i915_gem_context.c
> @@ -707,6 +707,80 @@ last_request_on_engine(struct i915_timeline *timeline,
>   	return NULL;
>   }
>   
> +struct context_barrier_task {
> +	struct i915_active base;
> +	void (*task)(void *data);
> +	void *data;
> +};
> +
> +static void cb_retire(struct i915_active *base)
> +{
> +	struct context_barrier_task *cb = container_of(base, typeof(*cb), base);
> +
> +	if (cb->task)
> +		cb->task(cb->data);
> +
> +	i915_active_fini(&cb->base);
> +	kfree(cb);
> +}
> +
> +I915_SELFTEST_DECLARE(static unsigned long context_barrier_inject_fault);
> +static int context_barrier_task(struct i915_gem_context *ctx,
> +				unsigned long engines,

I'm in two minds about usefulness of intel_engine_mask_t.

> +				void (*task)(void *data),
> +				void *data)
> +{
> +	struct drm_i915_private *i915 = ctx->i915;
> +	struct context_barrier_task *cb;
> +	struct intel_context *ce;
> +	intel_wakeref_t wakeref;
> +	int err = 0;
> +
> +	lockdep_assert_held(&i915->drm.struct_mutex);
> +	GEM_BUG_ON(!task);
> +
> +	cb = kmalloc(sizeof(*cb), GFP_KERNEL);
> +	if (!cb)
> +		return -ENOMEM;
> +
> +	i915_active_init(i915, &cb->base, cb_retire);
> +	i915_active_acquire(&cb->base);
> +
> +	wakeref = intel_runtime_pm_get(i915);
> +	list_for_each_entry(ce, &ctx->active_engines, active_link) {
> +		struct intel_engine_cs *engine = ce->engine;
> +		struct i915_request *rq;
> +
> +		if (!(ce->engine->mask & engines))
> +			continue;
> +
> +		if (I915_SELFTEST_ONLY(context_barrier_inject_fault &
> +				       engine->mask)) {
> +			err = -ENXIO;
> +			break;
> +		}
> +
> +		rq = i915_request_alloc(engine, ctx);
> +		if (IS_ERR(rq)) {
> +			err = PTR_ERR(rq);
> +			break;
> +		}
> +
> +		err = i915_active_ref(&cb->base, rq->fence.context, rq);
> +		i915_request_add(rq);
> +		if (err)
> +			break;
> +	}
> +	intel_runtime_pm_put(i915, wakeref);
> +
> +	cb->task = err ? NULL : task; /* caller needs to unwind instead */
> +	cb->data = data;
> +
> +	i915_active_release(&cb->base);
> +
> +	return err;
> +}
> +
>   int i915_gem_switch_to_kernel_context(struct drm_i915_private *i915,
>   				      unsigned long mask)
>   {
> diff --git a/drivers/gpu/drm/i915/selftests/i915_gem_context.c b/drivers/gpu/drm/i915/selftests/i915_gem_context.c
> index 7ae5033457b6..4f7c04247354 100644
> --- a/drivers/gpu/drm/i915/selftests/i915_gem_context.c
> +++ b/drivers/gpu/drm/i915/selftests/i915_gem_context.c
> @@ -1594,10 +1594,113 @@ static int igt_switch_to_kernel_context(void *arg)
>   	return err;
>   }
>   
> +static void mock_barrier_task(void *data)
> +{
> +	unsigned int *counter = data;
> +
> +	++*counter;
> +}
> +
> +static int mock_context_barrier(void *arg)
> +{
> +#undef pr_fmt
> +#define pr_fmt(x) "context_barrier_task():" # x
> +	struct drm_i915_private *i915 = arg;
> +	struct i915_gem_context *ctx;
> +	struct i915_request *rq;
> +	intel_wakeref_t wakeref;
> +	unsigned int counter;
> +	int err;
> +
> +	/*
> +	 * The context barrier provides us with a callback after it emits
> +	 * a request; useful for retiring old state after loading new.
> +	 */
> +
> +	mutex_lock(&i915->drm.struct_mutex);
> +
> +	ctx = mock_context(i915, "mock");
> +	if (IS_ERR(ctx)) {
> +		err = PTR_ERR(ctx);
> +		goto unlock;
> +	}
> +
> +	counter = 0;
> +	err = context_barrier_task(ctx, 0, mock_barrier_task, &counter);
> +	if (err) {
> +		pr_err("Failed at line %d, err=%d\n", __LINE__, err);
> +		goto out;
> +	}
> +	if (counter == 0) {
> +		pr_err("Did not retire immediately with 0 engines\n");
> +		err = -EINVAL;
> +		goto out;
> +	}
> +
> +	counter = 0;
> +	err = context_barrier_task(ctx, -1, mock_barrier_task, &counter);
> +	if (err) {
> +		pr_err("Failed at line %d, err=%d\n", __LINE__, err);
> +		goto out;
> +	}
> +	if (counter == 0) {
> +		pr_err("Did not retire immediately for all inactive engines\n");

Why would this one retire immediately? It will send requests down the 
pipe, no? So don't you actually need to wait for the tracker to be 
signalled and that counter == num_engines?

> +		err = -EINVAL;
> +		goto out;
> +	}
> +
> +	rq = ERR_PTR(-ENODEV);
> +	with_intel_runtime_pm(i915, wakeref)
> +		rq = i915_request_alloc(i915->engine[RCS], ctx);
> +	if (IS_ERR(rq)) {
> +		pr_err("Request allocation failed!\n");
> +		goto out;
> +	}
> +	i915_request_add(rq);

Doesn't this need to go under the wakeref as well?

> +	GEM_BUG_ON(list_empty(&ctx->active_engines));
> +
> +	counter = 0;
> +	context_barrier_inject_fault = BIT(RCS);
> +	err = context_barrier_task(ctx, -1, mock_barrier_task, &counter);
> +	context_barrier_inject_fault = 0;
> +	if (err == -ENXIO)
> +		err = 0;
> +	else
> +		pr_err("Did not hit fault injection!\n");
> +	if (counter != 0) {
> +		pr_err("Invoked callback on error!\n");
> +		err = -EIO;
> +	}
> +	if (err)
> +		goto out;
> +
> +	counter = 0;
> +	err = context_barrier_task(ctx, -1, mock_barrier_task, &counter);
> +	if (err) {
> +		pr_err("Failed at line %d, err=%d\n", __LINE__, err);
> +		goto out;
> +	}
> +	mock_device_flush(i915);
> +	if (counter == 0) {
> +		pr_err("Did not retire on each active engines\n");
> +		err = -EINVAL;
> +		goto out;
> +	}

This one is inline with my understanding, and the context_barrier_task 
arguments are the same as the one above.. hm.. I am confused.

> +
> +out:
> +	mock_context_close(ctx);
> +unlock:
> +	mutex_unlock(&i915->drm.struct_mutex);
> +	return err;
> +#undef pr_fmt
> +#define pr_fmt(x) x
> +}
> +
>   int i915_gem_context_mock_selftests(void)
>   {
>   	static const struct i915_subtest tests[] = {
>   		SUBTEST(igt_switch_to_kernel_context),
> +		SUBTEST(mock_context_barrier),
>   	};
>   	struct drm_i915_private *i915;
>   	int err;
> 

Regards,

Tvrtko
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