Re: [PATCH 20/26] drm/i915/guc: Implement no mid batch preemption for multi-lrc

[Matthew Brost <matthew.brost@intel.com>]

On Mon, Oct 11, 2021 at 04:32:03PM -0700, John Harrison wrote:
> On 10/4/2021 15:06, Matthew Brost wrote:
> > For some users of multi-lrc, e.g. split frame, it isn't safe to preempt
> > mid BB. To safely enable preemption at the BB boundary, a handshake
> > between to parent and child is needed. This is implemented via custom
> between to parent -> between parent
> > emit_bb_start & emit_fini_breadcrumb functions and enabled via by
> via by -> by
> 
> I'm also not seeing any mention of the forced re-group behavioural change in
> either the comments or commit description.
> 

Will do all of the above + mention fixing the PD size.

> > default if a context is configured by set parallel extension.
> > 
> > v2:
> >   (John Harrison)
> >    - Fix a few comments wording
> >    - Add struture for parent page layout
> > 
> > Signed-off-by: Matthew Brost <matthew.brost@intel.com>
> > ---
> >   drivers/gpu/drm/i915/gt/intel_context.c       |   2 +-
> >   drivers/gpu/drm/i915/gt/intel_context_types.h |   2 +
> >   drivers/gpu/drm/i915/gt/uc/intel_guc_fwif.h   |   2 +-
> >   .../gpu/drm/i915/gt/uc/intel_guc_submission.c | 330 +++++++++++++++++-
> >   4 files changed, 324 insertions(+), 12 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/gt/intel_context.c b/drivers/gpu/drm/i915/gt/intel_context.c
> > index 3b340eb59ada..ee84259959d0 100644
> > --- a/drivers/gpu/drm/i915/gt/intel_context.c
> > +++ b/drivers/gpu/drm/i915/gt/intel_context.c
> > @@ -569,7 +569,7 @@ void intel_context_bind_parent_child(struct intel_context *parent,
> >   	GEM_BUG_ON(intel_context_is_child(child));
> >   	GEM_BUG_ON(intel_context_is_parent(child));
> > -	parent->parallel.number_children++;
> > +	parent->parallel.child_index = parent->parallel.number_children++;
> >   	list_add_tail(&child->parallel.child_link,
> >   		      &parent->parallel.child_list);
> >   	child->parallel.parent = parent;
> > diff --git a/drivers/gpu/drm/i915/gt/intel_context_types.h b/drivers/gpu/drm/i915/gt/intel_context_types.h
> > index 1d880303a7e4..95a5b94b4ece 100644
> > --- a/drivers/gpu/drm/i915/gt/intel_context_types.h
> > +++ b/drivers/gpu/drm/i915/gt/intel_context_types.h
> > @@ -250,6 +250,8 @@ struct intel_context {
> >   		struct i915_request *last_rq;
> >   		/** @number_children: number of children if parent */
> >   		u8 number_children;
> > +		/** @child_index: index into child_list if child */
> > +		u8 child_index;
> >   		/** @guc: GuC specific members for parallel submission */
> >   		struct {
> >   			/** @wqi_head: head pointer in work queue */
> > diff --git a/drivers/gpu/drm/i915/gt/uc/intel_guc_fwif.h b/drivers/gpu/drm/i915/gt/uc/intel_guc_fwif.h
> > index a00eeddc1449..663950d3badc 100644
> > --- a/drivers/gpu/drm/i915/gt/uc/intel_guc_fwif.h
> > +++ b/drivers/gpu/drm/i915/gt/uc/intel_guc_fwif.h
> > @@ -181,7 +181,7 @@ struct guc_process_desc {
> >   	u32 wq_status;
> >   	u32 engine_presence;
> >   	u32 priority;
> > -	u32 reserved[30];
> > +	u32 reserved[36];
> Not seeing the promised explanation of this bug fix.
> 

Will add in commit message.

> >   } __packed;
> >   #define CONTEXT_REGISTRATION_FLAG_KMD	BIT(0)
> > diff --git a/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c b/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
> > index 12ee8ca76249..f28e36aa77c2 100644
> > --- a/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
> > +++ b/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
> > @@ -11,6 +11,7 @@
> >   #include "gt/intel_context.h"
> >   #include "gt/intel_engine_pm.h"
> >   #include "gt/intel_engine_heartbeat.h"
> > +#include "gt/intel_gpu_commands.h"
> >   #include "gt/intel_gt.h"
> >   #include "gt/intel_gt_irq.h"
> >   #include "gt/intel_gt_pm.h"
> > @@ -368,10 +369,16 @@ static inline struct i915_priolist *to_priolist(struct rb_node *rb)
> >   /*
> >    * When using multi-lrc submission an extra page in the context state is
> > - * reserved for the process descriptor and work queue.
> > + * reserved for the process descriptor, work queue, and handshake between the
> > + * parent + childlren contexts to insert safe preemption points between each set
> > + * of BBs.
> >    *
> >    * The layout of this page is below:
> >    * 0						guc_process_desc
> > + * + sizeof(struct guc_process_desc)		child go
> > + * + CACHELINE_BYTES				child join[0]
> > + * ...
> > + * + CACHELINE_BYTES				child join[n - 1]
> >    * ...						unused
> >    * PAGE_SIZE / 2				work queue start
> >    * ...						work queue
> > @@ -379,7 +386,25 @@ static inline struct i915_priolist *to_priolist(struct rb_node *rb)
> >    */
> >   #define WQ_SIZE			(PAGE_SIZE / 2)
> >   #define WQ_OFFSET		(PAGE_SIZE - WQ_SIZE)
> > -static u32 __get_process_desc_offset(struct intel_context *ce)
> > +
> > +struct parent_page {
> > +	struct guc_process_desc pdesc;
> > +
> > +	u32 child_go_memory;
> > +	u8 unused0[CACHELINE_BYTES - sizeof(u32)];
> > +
> > +	struct {
> > +		u32 child_join_memory;
> > +		u8 unused1[CACHELINE_BYTES - sizeof(u32)];
> > +	} join[MAX_ENGINE_INSTANCE + 1];
> Could have a common structure for these. Call the u32 'semaphore_memory' or
> something then just have:
>   struct sync_semaphore go;
>   struct sync_semaphore go[MAX + 1];
> 

Sure.

> > +
> > +	u8 unused2[(WQ_OFFSET - sizeof(struct guc_process_desc) -
> > +		    CACHELINE_BYTES * (MAX_ENGINE_INSTANCE + 2))];
> And this bit could be 'sizeof(struct sync_semaphore) * MAX + 2' to be
> clearer what it refers to.
> 
> And to be totally paranoid about it, could also add
> 'BUILD_BUG_ON(sizeof(struct sync_semaphore) != CACHELINE_BYTES'.
> 
> And BUILD_BUG_ON(sizeof(parent_page) == PARENT_PAGE_SIZE)'.
>

Sure.
 
> > +
> > +	u32 wq[WQ_SIZE / sizeof(u32)];
> > +};
> > +
> > +static u32 __get_parent_page_offset(struct intel_context *ce)
> >   {
> >   	GEM_BUG_ON(!ce->parallel.guc.parent_page);
> > @@ -388,23 +413,35 @@ static u32 __get_process_desc_offset(struct intel_context *ce)
> >   static u32 __get_wq_offset(struct intel_context *ce)
> >   {
> > -	return __get_process_desc_offset(ce) + WQ_OFFSET;
> > +	BUILD_BUG_ON(offsetof(struct parent_page, wq) != WQ_OFFSET);
> > +
> > +	return __get_parent_page_offset(ce) + WQ_OFFSET;
> >   }
> > -static struct guc_process_desc *
> > -__get_process_desc(struct intel_context *ce)
> > +static struct parent_page *
> > +__get_parent_page(struct intel_context *ce)
> >   {
> > +	BUILD_BUG_ON(sizeof(struct parent_page) != PAGE_SIZE);
> > +
> >   	/*
> >   	 * Need to subtract LRC_STATE_OFFSET here as the
> >   	 * parallel.guc.parent_page is the offset into ce->state while
> >   	 * ce->lrc_reg_reg is ce->state + LRC_STATE_OFFSET.
> >   	 */
> > -	return (struct guc_process_desc *)
> > +	return (struct parent_page *)
> >   		(ce->lrc_reg_state +
> > -		 ((__get_process_desc_offset(ce) -
> > +		 ((__get_parent_page_offset(ce) -
> >   		   LRC_STATE_OFFSET) / sizeof(u32)));
> >   }
> > +static struct guc_process_desc *
> > +__get_process_desc(struct intel_context *ce)
> > +{
> > +	struct parent_page *pp = __get_parent_page(ce);
> > +
> > +	return &pp->pdesc;
> > +}
> > +
> >   static u32 *get_wq_pointer(struct guc_process_desc *desc,
> >   			   struct intel_context *ce,
> >   			   u32 wqi_size)
> > @@ -424,8 +461,7 @@ static u32 *get_wq_pointer(struct guc_process_desc *desc,
> >   	}
> >   #undef AVAILABLE_SPACE
> > -	return ((u32 *)__get_process_desc(ce)) +
> > -		((WQ_OFFSET + ce->parallel.guc.wqi_tail) / sizeof(u32));
> > +	return &__get_parent_page(ce)->wq[ce->parallel.guc.wqi_tail / sizeof(u32)];
> >   }
> >   static struct guc_lrc_desc *__get_lrc_desc(struct intel_guc *guc, u32 index)
> > @@ -1829,6 +1865,26 @@ static int deregister_context(struct intel_context *ce, u32 guc_id)
> >   	return __guc_action_deregister_context(guc, guc_id);
> >   }
> > +static inline void clear_children_join_go_memory(struct intel_context *ce)
> > +{
> > +	u32 *mem = (u32 *)(&__get_parent_page(ce)->child_go_memory);
> > +	u8 i;
> > +
> > +	for (i = 0; i < ce->parallel.number_children + 1; ++i)
> > +		mem[i * (CACHELINE_BYTES / sizeof(u32))] = 0;
> Can't this be written as:
>   pp->child_go_memory = 0;
>   for(i = 0 to number_children)
>     pp->child_join_memory = 0;
> 
> Seems like that would be much clearer than this magic casting and
> offsetting. I mean, that was the whole point of creating the parent_page
> structure.
> 

Will rewrite.

> 
> > +}
> > +
> > +static inline u32 get_children_go_value(struct intel_context *ce)
> > +{
> > +	return __get_parent_page(ce)->child_go_memory;
> > +}
> > +
> > +static inline u32 get_children_join_value(struct intel_context *ce,
> > +					  u8 child_index)
> > +{
> > +	return __get_parent_page(ce)->join[child_index].child_join_memory;
> > +}
> > +
> >   static void guc_context_policy_init(struct intel_engine_cs *engine,
> >   				    struct guc_lrc_desc *desc)
> >   {
> > @@ -1888,7 +1944,7 @@ static int guc_lrc_desc_pin(struct intel_context *ce, bool loop)
> >   		ce->parallel.guc.wqi_head = 0;
> >   		desc->process_desc = i915_ggtt_offset(ce->state) +
> > -			__get_process_desc_offset(ce);
> > +			__get_parent_page_offset(ce);
> >   		desc->wq_addr = i915_ggtt_offset(ce->state) +
> >   			__get_wq_offset(ce);
> >   		desc->wq_size = WQ_SIZE;
> > @@ -1910,6 +1966,8 @@ static int guc_lrc_desc_pin(struct intel_context *ce, bool loop)
> >   			desc->context_flags = CONTEXT_REGISTRATION_FLAG_KMD;
> >   			guc_context_policy_init(engine, desc);
> >   		}
> > +
> > +		clear_children_join_go_memory(ce);
> >   	}
> >   	/*
> > @@ -2976,6 +3034,31 @@ static const struct intel_context_ops virtual_child_context_ops = {
> >   	.get_sibling = guc_virtual_get_sibling,
> >   };
> > +/*
> > + * The below override of the breadcrumbs is enabled when the user configures a
> > + * context for parallel submission (multi-lrc, parent-child).
> > + *
> > + * The overridden breadcrumbs implements an algorithm which allows the GuC to
> > + * safely preempt all the hw contexts configured for parallel submission
> > + * between each BB. The contract between the i915 and GuC is if the parent
> > + * context can be preempted, all the children can be preempted, and the GuC will
> > + * always try to preempt the parent before the children. A handshake between the
> > + * parent / children breadcrumbs ensures the i915 holds up its end of the deal
> > + * creating a window to preempt between each set of BBs.
> > + */
> > +static int emit_bb_start_parent_no_preempt_mid_batch(struct i915_request *rq,
> > +						     u64 offset, u32 len,
> > +						     const unsigned int flags);
> > +static int emit_bb_start_child_no_preempt_mid_batch(struct i915_request *rq,
> > +						    u64 offset, u32 len,
> > +						    const unsigned int flags);
> > +static u32 *
> > +emit_fini_breadcrumb_parent_no_preempt_mid_batch(struct i915_request *rq,
> > +						 u32 *cs);
> > +static u32 *
> > +emit_fini_breadcrumb_child_no_preempt_mid_batch(struct i915_request *rq,
> > +						u32 *cs);
> > +
> >   static struct intel_context *
> >   guc_create_parallel(struct intel_engine_cs **engines,
> >   		    unsigned int num_siblings,
> > @@ -3011,6 +3094,20 @@ guc_create_parallel(struct intel_engine_cs **engines,
> >   		}
> >   	}
> > +	parent->engine->emit_bb_start =
> > +		emit_bb_start_parent_no_preempt_mid_batch;
> > +	parent->engine->emit_fini_breadcrumb =
> > +		emit_fini_breadcrumb_parent_no_preempt_mid_batch;
> > +	parent->engine->emit_fini_breadcrumb_dw =
> > +		12 + 4 * parent->parallel.number_children;
> > +	for_each_child(parent, ce) {
> > +		ce->engine->emit_bb_start =
> > +			emit_bb_start_child_no_preempt_mid_batch;
> > +		ce->engine->emit_fini_breadcrumb =
> > +			emit_fini_breadcrumb_child_no_preempt_mid_batch;
> > +		ce->engine->emit_fini_breadcrumb_dw = 16;
> > +	}
> > +
> >   	kfree(siblings);
> >   	return parent;
> > @@ -3840,6 +3937,17 @@ void intel_guc_submission_print_context_info(struct intel_guc *guc,
> >   			drm_printf(p, "\t\tWQI Status: %u\n\n",
> >   				   READ_ONCE(desc->wq_status));
> > +			if (ce->engine->emit_bb_start ==
> > +			    emit_bb_start_parent_no_preempt_mid_batch) {
> > +				u8 i;
> > +
> > +				drm_printf(p, "\t\tChildren Go: %u\n\n",
> > +					   get_children_go_value(ce));
> > +				for (i = 0; i < ce->parallel.number_children; ++i)
> > +					drm_printf(p, "\t\tChildren Join: %u\n",
> > +						   get_children_join_value(ce, i));
> > +			}
> > +
> >   			for_each_child(ce, child)
> >   				guc_log_context(p, child);
> >   		}
> > @@ -3847,6 +3955,208 @@ void intel_guc_submission_print_context_info(struct intel_guc *guc,
> >   	xa_unlock_irqrestore(&guc->context_lookup, flags);
> >   }
> > +static inline u32 get_children_go_addr(struct intel_context *ce)
> > +{
> > +	GEM_BUG_ON(!intel_context_is_parent(ce));
> > +	BUILD_BUG_ON(offsetof(struct parent_page, child_go_memory) !=
> > +		     sizeof(struct guc_process_desc));
> > +
> > +	return i915_ggtt_offset(ce->state) +
> > +		__get_parent_page_offset(ce) +
> > +		sizeof(struct guc_process_desc);
> Rather than relying on the BUILD_BUG to make sure that the magic calculation
> matches the structure definition, can't this just say "ggtt_offset +
> pp_offset + offsetof(pp, child_go)"?
> 

Probably.

> > +}
> > +
> > +static inline u32 get_children_join_addr(struct intel_context *ce,
> > +					 u8 child_index)
> > +{
> > +	GEM_BUG_ON(!intel_context_is_parent(ce));
> > +
> > +	return get_children_go_addr(ce) + (child_index + 1) * CACHELINE_BYTES;
> "ggtt_offset + pp_offset + offsetof(pp, child_join[i])"?
> 

Probably.

> 
> > +}
> > +
> > +#define PARENT_GO_BB			1
> > +#define PARENT_GO_FINI_BREADCRUMB	0
> > +#define CHILD_GO_BB			1
> > +#define CHILD_GO_FINI_BREADCRUMB	0
> > +static int emit_bb_start_parent_no_preempt_mid_batch(struct i915_request *rq,
> > +						     u64 offset, u32 len,
> > +						     const unsigned int flags)
> > +{
> > +	struct intel_context *ce = rq->context;
> > +	u32 *cs;
> > +	u8 i;
> > +
> > +	GEM_BUG_ON(!intel_context_is_parent(ce));
> > +
> > +	cs = intel_ring_begin(rq, 10 + 4 * ce->parallel.number_children);
> > +	if (IS_ERR(cs))
> > +		return PTR_ERR(cs);
> > +
> > +	/* Wait on children */
> > +	for (i = 0; i < ce->parallel.number_children; ++i) {
> > +		*cs++ = (MI_SEMAPHORE_WAIT |
> > +			 MI_SEMAPHORE_GLOBAL_GTT |
> > +			 MI_SEMAPHORE_POLL |
> > +			 MI_SEMAPHORE_SAD_EQ_SDD);
> > +		*cs++ = PARENT_GO_BB;
> > +		*cs++ = get_children_join_addr(ce, i);
> > +		*cs++ = 0;
> > +	}
> > +
> > +	/* Turn off preemption */
> > +	*cs++ = MI_ARB_ON_OFF | MI_ARB_DISABLE;
> > +	*cs++ = MI_NOOP;
> > +
> > +	/* Tell children go */
> > +	cs = gen8_emit_ggtt_write(cs,
> > +				  CHILD_GO_BB,
> > +				  get_children_go_addr(ce),
> > +				  0);
> > +
> > +	/* Jump to batch */
> > +	*cs++ = MI_BATCH_BUFFER_START_GEN8 |
> > +		(flags & I915_DISPATCH_SECURE ? 0 : BIT(8));
> > +	*cs++ = lower_32_bits(offset);
> > +	*cs++ = upper_32_bits(offset);
> > +	*cs++ = MI_NOOP;
> > +
> > +	intel_ring_advance(rq, cs);
> > +
> > +	return 0;
> > +}
> > +
> > +static int emit_bb_start_child_no_preempt_mid_batch(struct i915_request *rq,
> > +						    u64 offset, u32 len,
> > +						    const unsigned int flags)
> > +{
> > +	struct intel_context *ce = rq->context;
> > +	struct intel_context *parent = intel_context_to_parent(ce);
> > +	u32 *cs;
> > +
> > +	GEM_BUG_ON(!intel_context_is_child(ce));
> > +
> > +	cs = intel_ring_begin(rq, 12);
> > +	if (IS_ERR(cs))
> > +		return PTR_ERR(cs);
> > +
> > +	/* Signal parent */
> > +	cs = gen8_emit_ggtt_write(cs,
> > +				  PARENT_GO_BB,
> > +				  get_children_join_addr(parent,
> > +							 ce->parallel.child_index),
> > +				  0);
> > +
> > +	/* Wait on parent for go */
> > +	*cs++ = (MI_SEMAPHORE_WAIT |
> > +		 MI_SEMAPHORE_GLOBAL_GTT |
> > +		 MI_SEMAPHORE_POLL |
> > +		 MI_SEMAPHORE_SAD_EQ_SDD);
> > +	*cs++ = CHILD_GO_BB;
> > +	*cs++ = get_children_go_addr(parent);
> > +	*cs++ = 0;
> > +
> > +	/* Turn off preemption */
> > +	*cs++ = MI_ARB_ON_OFF | MI_ARB_DISABLE;
> > +
> > +	/* Jump to batch */
> > +	*cs++ = MI_BATCH_BUFFER_START_GEN8 |
> > +		(flags & I915_DISPATCH_SECURE ? 0 : BIT(8));
> > +	*cs++ = lower_32_bits(offset);
> > +	*cs++ = upper_32_bits(offset);
> > +
> > +	intel_ring_advance(rq, cs);
> > +
> > +	return 0;
> > +}
> > +
> > +static u32 *
> > +emit_fini_breadcrumb_parent_no_preempt_mid_batch(struct i915_request *rq,
> > +						 u32 *cs)
> > +{
> > +	struct intel_context *ce = rq->context;
> > +	u8 i;
> > +
> > +	GEM_BUG_ON(!intel_context_is_parent(ce));
> > +
> > +	/* Wait on children */
> > +	for (i = 0; i < ce->parallel.number_children; ++i) {
> > +		*cs++ = (MI_SEMAPHORE_WAIT |
> > +			 MI_SEMAPHORE_GLOBAL_GTT |
> > +			 MI_SEMAPHORE_POLL |
> > +			 MI_SEMAPHORE_SAD_EQ_SDD);
> > +		*cs++ = PARENT_GO_FINI_BREADCRUMB;
> > +		*cs++ = get_children_join_addr(ce, i);
> > +		*cs++ = 0;
> > +	}
> > +
> > +	/* Turn on preemption */
> > +	*cs++ = MI_ARB_ON_OFF | MI_ARB_ENABLE;
> > +	*cs++ = MI_NOOP;
> > +
> You mentioned possibly needing to add an MI_ARB_CHECK in here but I'm not
> seeing it. Did the testing happen? I don't see that it should be necessary.
> Once you execute the MI_ARB_ENABLE, the CS can preempt anywhere, I thought?

No, it can only preempt on certain instructions - e.g. MI_ARB_CHECK or a
semaphore.

> Even if it can't there should be an MI_ARB_CHECK added at the next level up
> after the breadcrumb code. Or do we not have those in between batches any
> more?
>

Right. A MI_ARB_CHECK before writing the fini breadcrumbs would be wrong
as we could preeempt after the batch is complete but before the fini
breadcrumbs are written so the i915 still thinks the batch (request)
isn't done. The code is fine as is. The emit BB code for the parent has
semaphore instructions where is can be preempted.

Matt
 
> John.
> 
> 
> > +	/* Tell children go */
> > +	cs = gen8_emit_ggtt_write(cs,
> > +				  CHILD_GO_FINI_BREADCRUMB,
> > +				  get_children_go_addr(ce),
> > +				  0);
> > +
> > +	/* Emit fini breadcrumb */
> > +	cs = gen8_emit_ggtt_write(cs,
> > +				  rq->fence.seqno,
> > +				  i915_request_active_timeline(rq)->hwsp_offset,
> > +				  0);
> > +
> > +	/* User interrupt */
> > +	*cs++ = MI_USER_INTERRUPT;
> > +	*cs++ = MI_NOOP;
> > +
> > +	rq->tail = intel_ring_offset(rq, cs);
> > +
> > +	return cs;
> > +}
> > +
> > +static u32 *
> > +emit_fini_breadcrumb_child_no_preempt_mid_batch(struct i915_request *rq, u32 *cs)
> > +{
> > +	struct intel_context *ce = rq->context;
> > +	struct intel_context *parent = intel_context_to_parent(ce);
> > +
> > +	GEM_BUG_ON(!intel_context_is_child(ce));
> > +
> > +	/* Turn on preemption */
> > +	*cs++ = MI_ARB_ON_OFF | MI_ARB_ENABLE;
> > +	*cs++ = MI_NOOP;
> > +
> > +	/* Signal parent */
> > +	cs = gen8_emit_ggtt_write(cs,
> > +				  PARENT_GO_FINI_BREADCRUMB,
> > +				  get_children_join_addr(parent,
> > +							 ce->parallel.child_index),
> > +				  0);
> > +
> > +	/* Wait parent on for go */
> > +	*cs++ = (MI_SEMAPHORE_WAIT |
> > +		 MI_SEMAPHORE_GLOBAL_GTT |
> > +		 MI_SEMAPHORE_POLL |
> > +		 MI_SEMAPHORE_SAD_EQ_SDD);
> > +	*cs++ = CHILD_GO_FINI_BREADCRUMB;
> > +	*cs++ = get_children_go_addr(parent);
> > +	*cs++ = 0;
> > +
> > +	/* Emit fini breadcrumb */
> > +	cs = gen8_emit_ggtt_write(cs,
> > +				  rq->fence.seqno,
> > +				  i915_request_active_timeline(rq)->hwsp_offset,
> > +				  0);
> > +
> > +	/* User interrupt */
> > +	*cs++ = MI_USER_INTERRUPT;
> > +	*cs++ = MI_NOOP;
> > +
> > +	rq->tail = intel_ring_offset(rq, cs);
> > +
> > +	return cs;
> > +}
> > +
> >   static struct intel_context *
> >   guc_create_virtual(struct intel_engine_cs **siblings, unsigned int count,
> >   		   unsigned long flags)
>