Re: [Intel-gfx] [PATCH 3/4] drm/i915: Drop the CONTEXT_CLONE API

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

On 19/03/2021 22:38, Jason Ekstrand wrote:
> This API allows one context to grab bits out of another context upon
> creation.  It can be used as a short-cut for setparam(getparam()) for
> things like I915_CONTEXT_PARAM_VM.  However, it's never been used by any
> real userspace.  It's used by a few IGT tests and that's it.  Since it
> doesn't add any real value (most of the stuff you can CLONE you can copy
> in other ways), drop it.

No complaints to remove if it ended up unused outside IGT. Latter is a 
_big_ problem though, since it is much more that a few IGT tests. So I 
really think there really needs to be an evaluation and a plan for that 
(we don't want to lose 50% of the coverage over night).

> There is one thing that this API allows you to clone which you cannot
> clone via getparam/setparam: timelines.  However, timelines are an
> implementation detail of i915 and not really something that needs to be

Not really true timelines are i915 implementation detail. They are in 
fact a dma-fence context:seqno concept, nothing more that than. I think 
you are probably confusing struct intel_timeline with the timeline 
wording in the uapi. Former is i915 implementation detail, but 
context:seqno are truly userspace timelines.

But again, no objection to removing unused uapi in principle. Narrative 
has to be accurate and test coverage not lost though.

Regards,

Tvrtko

> exposed to userspace.  Also, sharing timelines between contexts isn't
> obviously useful and supporting it has the potential to complicate i915
> internally.  It also doesn't add any functionality that the client can't
> get in other ways.  If a client really wants a shared timeline, they can
> use a syncobj and set it as an in and out fence on every submit.
> 
> Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
> Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> ---
>   drivers/gpu/drm/i915/gem/i915_gem_context.c | 199 +-------------------
>   include/uapi/drm/i915_drm.h                 |  16 +-
>   2 files changed, 6 insertions(+), 209 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_context.c b/drivers/gpu/drm/i915/gem/i915_gem_context.c
> index d28ac79de7573..f88bac19333ec 100644
> --- a/drivers/gpu/drm/i915/gem/i915_gem_context.c
> +++ b/drivers/gpu/drm/i915/gem/i915_gem_context.c
> @@ -1983,207 +1983,14 @@ static int create_setparam(struct i915_user_extension __user *ext, void *data)
>   	return ctx_setparam(arg->fpriv, arg->ctx, &local.param);
>   }
>   
> -static int clone_engines(struct i915_gem_context *dst,
> -			 struct i915_gem_context *src)
> +static int invalid_ext(struct i915_user_extension __user *ext, void *data)
>   {
> -	struct i915_gem_engines *clone, *e;
> -	bool user_engines;
> -	unsigned long n;
> -
> -	e = __context_engines_await(src, &user_engines);
> -	if (!e)
> -		return -ENOENT;
> -
> -	clone = alloc_engines(e->num_engines);
> -	if (!clone)
> -		goto err_unlock;
> -
> -	for (n = 0; n < e->num_engines; n++) {
> -		struct intel_engine_cs *engine;
> -
> -		if (!e->engines[n]) {
> -			clone->engines[n] = NULL;
> -			continue;
> -		}
> -		engine = e->engines[n]->engine;
> -
> -		/*
> -		 * Virtual engines are singletons; they can only exist
> -		 * inside a single context, because they embed their
> -		 * HW context... As each virtual context implies a single
> -		 * timeline (each engine can only dequeue a single request
> -		 * at any time), it would be surprising for two contexts
> -		 * to use the same engine. So let's create a copy of
> -		 * the virtual engine instead.
> -		 */
> -		if (intel_engine_is_virtual(engine))
> -			clone->engines[n] =
> -				intel_execlists_clone_virtual(engine);
> -		else
> -			clone->engines[n] = intel_context_create(engine);
> -		if (IS_ERR_OR_NULL(clone->engines[n])) {
> -			__free_engines(clone, n);
> -			goto err_unlock;
> -		}
> -
> -		intel_context_set_gem(clone->engines[n], dst);
> -	}
> -	clone->num_engines = n;
> -	i915_sw_fence_complete(&e->fence);
> -
> -	/* Serialised by constructor */
> -	engines_idle_release(dst, rcu_replace_pointer(dst->engines, clone, 1));
> -	if (user_engines)
> -		i915_gem_context_set_user_engines(dst);
> -	else
> -		i915_gem_context_clear_user_engines(dst);
> -	return 0;
> -
> -err_unlock:
> -	i915_sw_fence_complete(&e->fence);
> -	return -ENOMEM;
> -}
> -
> -static int clone_flags(struct i915_gem_context *dst,
> -		       struct i915_gem_context *src)
> -{
> -	dst->user_flags = src->user_flags;
> -	return 0;
> -}
> -
> -static int clone_schedattr(struct i915_gem_context *dst,
> -			   struct i915_gem_context *src)
> -{
> -	dst->sched = src->sched;
> -	return 0;
> -}
> -
> -static int clone_sseu(struct i915_gem_context *dst,
> -		      struct i915_gem_context *src)
> -{
> -	struct i915_gem_engines *e = i915_gem_context_lock_engines(src);
> -	struct i915_gem_engines *clone;
> -	unsigned long n;
> -	int err;
> -
> -	/* no locking required; sole access under constructor*/
> -	clone = __context_engines_static(dst);
> -	if (e->num_engines != clone->num_engines) {
> -		err = -EINVAL;
> -		goto unlock;
> -	}
> -
> -	for (n = 0; n < e->num_engines; n++) {
> -		struct intel_context *ce = e->engines[n];
> -
> -		if (clone->engines[n]->engine->class != ce->engine->class) {
> -			/* Must have compatible engine maps! */
> -			err = -EINVAL;
> -			goto unlock;
> -		}
> -
> -		/* serialises with set_sseu */
> -		err = intel_context_lock_pinned(ce);
> -		if (err)
> -			goto unlock;
> -
> -		clone->engines[n]->sseu = ce->sseu;
> -		intel_context_unlock_pinned(ce);
> -	}
> -
> -	err = 0;
> -unlock:
> -	i915_gem_context_unlock_engines(src);
> -	return err;
> -}
> -
> -static int clone_timeline(struct i915_gem_context *dst,
> -			  struct i915_gem_context *src)
> -{
> -	if (src->timeline)
> -		__assign_timeline(dst, src->timeline);
> -
> -	return 0;
> -}
> -
> -static int clone_vm(struct i915_gem_context *dst,
> -		    struct i915_gem_context *src)
> -{
> -	struct i915_address_space *vm;
> -	int err = 0;
> -
> -	if (!rcu_access_pointer(src->vm))
> -		return 0;
> -
> -	rcu_read_lock();
> -	vm = context_get_vm_rcu(src);
> -	rcu_read_unlock();
> -
> -	if (!mutex_lock_interruptible(&dst->mutex)) {
> -		__assign_ppgtt(dst, vm);
> -		mutex_unlock(&dst->mutex);
> -	} else {
> -		err = -EINTR;
> -	}
> -
> -	i915_vm_put(vm);
> -	return err;
> -}
> -
> -static int create_clone(struct i915_user_extension __user *ext, void *data)
> -{
> -	static int (* const fn[])(struct i915_gem_context *dst,
> -				  struct i915_gem_context *src) = {
> -#define MAP(x, y) [ilog2(I915_CONTEXT_CLONE_##x)] = y
> -		MAP(ENGINES, clone_engines),
> -		MAP(FLAGS, clone_flags),
> -		MAP(SCHEDATTR, clone_schedattr),
> -		MAP(SSEU, clone_sseu),
> -		MAP(TIMELINE, clone_timeline),
> -		MAP(VM, clone_vm),
> -#undef MAP
> -	};
> -	struct drm_i915_gem_context_create_ext_clone local;
> -	const struct create_ext *arg = data;
> -	struct i915_gem_context *dst = arg->ctx;
> -	struct i915_gem_context *src;
> -	int err, bit;
> -
> -	if (copy_from_user(&local, ext, sizeof(local)))
> -		return -EFAULT;
> -
> -	BUILD_BUG_ON(GENMASK(BITS_PER_TYPE(local.flags) - 1, ARRAY_SIZE(fn)) !=
> -		     I915_CONTEXT_CLONE_UNKNOWN);
> -
> -	if (local.flags & I915_CONTEXT_CLONE_UNKNOWN)
> -		return -EINVAL;
> -
> -	if (local.rsvd)
> -		return -EINVAL;
> -
> -	rcu_read_lock();
> -	src = __i915_gem_context_lookup_rcu(arg->fpriv, local.clone_id);
> -	rcu_read_unlock();
> -	if (!src)
> -		return -ENOENT;
> -
> -	GEM_BUG_ON(src == dst);
> -
> -	for (bit = 0; bit < ARRAY_SIZE(fn); bit++) {
> -		if (!(local.flags & BIT(bit)))
> -			continue;
> -
> -		err = fn[bit](dst, src);
> -		if (err)
> -			return err;
> -	}
> -
> -	return 0;
> +	return -EINVAL;
>   }
>   
>   static const i915_user_extension_fn create_extensions[] = {
>   	[I915_CONTEXT_CREATE_EXT_SETPARAM] = create_setparam,
> -	[I915_CONTEXT_CREATE_EXT_CLONE] = create_clone,
> +	[I915_CONTEXT_CREATE_EXT_CLONE] = invalid_ext,
>   };
>   
>   static bool client_is_banned(struct drm_i915_file_private *file_priv)
> diff --git a/include/uapi/drm/i915_drm.h b/include/uapi/drm/i915_drm.h
> index 4c4b9254def1b..33ef78cb1deb7 100644
> --- a/include/uapi/drm/i915_drm.h
> +++ b/include/uapi/drm/i915_drm.h
> @@ -1841,20 +1841,10 @@ struct drm_i915_gem_context_create_ext_setparam {
>   	struct drm_i915_gem_context_param param;
>   };
>   
> -struct drm_i915_gem_context_create_ext_clone {
> +/* This API has been removed.  On the off chance someone somewhere has
> + * attempted to use it, never re-use this extension number.
> + */
>   #define I915_CONTEXT_CREATE_EXT_CLONE 1
> -	struct i915_user_extension base;
> -	__u32 clone_id;
> -	__u32 flags;
> -#define I915_CONTEXT_CLONE_ENGINES	(1u << 0)
> -#define I915_CONTEXT_CLONE_FLAGS	(1u << 1)
> -#define I915_CONTEXT_CLONE_SCHEDATTR	(1u << 2)
> -#define I915_CONTEXT_CLONE_SSEU		(1u << 3)
> -#define I915_CONTEXT_CLONE_TIMELINE	(1u << 4)
> -#define I915_CONTEXT_CLONE_VM		(1u << 5)
> -#define I915_CONTEXT_CLONE_UNKNOWN -(I915_CONTEXT_CLONE_VM << 1)
> -	__u64 rsvd;
> -};
>   
>   struct drm_i915_gem_context_destroy {
>   	__u32 ctx_id;
> 
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