Re: [PATCH v5 5/6] drm/xe: Disallow input fences on zero batch execs and zero binds

["Thomas Hellström" <thomas.hellstrom@linux.intel.com>]

On Wed, 2025-10-29 at 13:57 -0700, Matthew Brost wrote:
> Prevent input fences from being installed on zero batch execs or zero
> binds, which were originally added to support queue idling in Mesa
> via
> output fences. Although input fence support was introduced for
> interface
> consistency, it leads to incorrect behavior due to chained composite
> fences, which are disallowed.
> 
> Avoid the complexity of fixing this by removing support, as input
> fences
> for these cases are not used in practice.
> 
> Signed-off-by: Matthew Brost <matthew.brost@intel.com>

Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>

> ---
>  drivers/gpu/drm/xe/xe_sync.c | 101 +++++++++++++--------------------
> --
>  1 file changed, 36 insertions(+), 65 deletions(-)
> 
> diff --git a/drivers/gpu/drm/xe/xe_sync.c
> b/drivers/gpu/drm/xe/xe_sync.c
> index df7ca349398b..ff74528ca0c6 100644
> --- a/drivers/gpu/drm/xe/xe_sync.c
> +++ b/drivers/gpu/drm/xe/xe_sync.c
> @@ -301,84 +301,55 @@ xe_sync_in_fence_get(struct xe_sync_entry
> *sync, int num_sync,
>  
>  	lockdep_assert_held(&vm->lock);
>  
> -	/* Count in-fences */
> -	for (i = 0; i < num_sync; ++i) {
> -		if (sync[i].fence) {
> -			++num_fence;
> -			fence = sync[i].fence;
> -		}
> -	}
> -
> -	/* Easy case... */
> -	if (!num_fence) {
> -		if (q->flags & EXEC_QUEUE_FLAG_VM) {
> -			struct xe_exec_queue *__q;
> -			struct xe_tile *tile;
> -			u8 id;
> -
> -			for_each_tile(tile, vm->xe, id)
> -				num_fence += (1 +
> XE_MAX_GT_PER_TILE);
> -
> -			fences = kmalloc_array(num_fence,
> sizeof(*fences),
> -					       GFP_KERNEL);
> -			if (!fences)
> -				return ERR_PTR(-ENOMEM);
> -
> +	/* Reject in fences */
> +	for (i = 0; i < num_sync; ++i)
> +		if (sync[i].fence)
> +			return ERR_PTR(-EOPNOTSUPP);
> +
> +	if (q->flags & EXEC_QUEUE_FLAG_VM) {
> +		struct xe_exec_queue *__q;
> +		struct xe_tile *tile;
> +		u8 id;
> +
> +		for_each_tile(tile, vm->xe, id)
> +			num_fence += (1 + XE_MAX_GT_PER_TILE);
> +
> +		fences = kmalloc_array(num_fence, sizeof(*fences),
> +				       GFP_KERNEL);
> +		if (!fences)
> +			return ERR_PTR(-ENOMEM);
> +
> +		fences[current_fence++] =
> +			xe_exec_queue_last_fence_get(q, vm);
> +		for_each_tlb_inval(i)
> +			fences[current_fence++] =
> +				xe_exec_queue_tlb_inval_last_fence_g
> et(q, vm, i);
> +		list_for_each_entry(__q, &q->multi_gt_list,
> +				    multi_gt_link) {
>  			fences[current_fence++] =
> -				xe_exec_queue_last_fence_get(q, vm);
> +				xe_exec_queue_last_fence_get(__q,
> vm);
>  			for_each_tlb_inval(i)
>  				fences[current_fence++] =
> -
> 					xe_exec_queue_tlb_inval_last_fence_get(q, vm, i);
> -			list_for_each_entry(__q, &q->multi_gt_list,
> -					    multi_gt_link) {
> -				fences[current_fence++] =
> -
> 					xe_exec_queue_last_fence_get(__q, vm);
> -				for_each_tlb_inval(i)
> -					fences[current_fence++] =
> -
> 						xe_exec_queue_tlb_inval_last_fence_get(__q, vm, i);
> -			}
> -
> -			xe_assert(vm->xe, current_fence ==
> num_fence);
> -			cf = dma_fence_array_create(num_fence,
> fences,
> -						   
> dma_fence_context_alloc(1),
> -						    1, false);
> -			if (!cf)
> -				goto err_out;
> -
> -			return &cf->base;
> +					xe_exec_queue_tlb_inval_last
> _fence_get(__q, vm, i);
>  		}
>  
> -		fence = xe_exec_queue_last_fence_get(q, vm);
> -		return fence;
> -	}
> +		xe_assert(vm->xe, current_fence == num_fence);
> +		cf = dma_fence_array_create(num_fence, fences,
> +					   
> dma_fence_context_alloc(1),
> +					    1, false);
> +		if (!cf)
> +			goto err_out;
>  
> -	/*
> -	 * Create composite fence - FIXME - the below code doesn't
> work. This is
> -	 * unused in Mesa so we are ok for the moment. Perhaps we
> just disable
> -	 * this entire code path if number of in fences != 0.
> -	 */
> -	fences = kmalloc_array(num_fence + 1, sizeof(*fences),
> GFP_KERNEL);
> -	if (!fences)
> -		return ERR_PTR(-ENOMEM);
> -	for (i = 0; i < num_sync; ++i) {
> -		if (sync[i].fence) {
> -			dma_fence_get(sync[i].fence);
> -			fences[current_fence++] = sync[i].fence;
> -		}
> +		return &cf->base;
>  	}
> -	fences[current_fence++] = xe_exec_queue_last_fence_get(q,
> vm);
> -	cf = dma_fence_array_create(num_fence, fences,
> -				    dma_fence_context_alloc(1), 1,
> false);
> -	if (!cf)
> -		goto err_out;
>  
> -	return &cf->base;
> +	fence = xe_exec_queue_last_fence_get(q, vm);
> +	return fence;
>  
>  err_out:
>  	while (current_fence)
>  		dma_fence_put(fences[--current_fence]);
>  	kfree(fences);
> -	kfree(cf);
>  
>  	return ERR_PTR(-ENOMEM);
>  }