Re: [PATCH 1/3] drm/sa: Scan all holes when fences are disabled

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

On Wed, 2026-03-11 at 15:52 +0000, Satyanarayana K V P wrote:
> The suballocator algorithm tracks a hole cursor at the last
> allocation
> and tries to allocate after it. This is optimized for fence-ordered
> progress, where older allocations are expected to become reusable
> first.
> 
> In fence-enabled mode, that ordering assumption holds. In fence-
> disabled
> mode, allocations may be freed in arbitrary order, so limiting
> allocation
> to the current hole window can miss valid free space and fail
> allocations
> despite sufficient total space.
> 
> Update the fence-disabled path to walk through all holes and select
> any
> fitting range, while keeping the existing fence-based behavior
> unchanged.
> 
> Signed-off-by: Satyanarayana K V P <satyanarayana.k.v.p@intel.com>
> Cc: Matthew Brost <matthew.brost@intel.com>
> Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
> Cc: Maarten Lankhorst <dev@lankhorst.se>
> Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
> Cc: Christian König <christian.koenig@amd.com>
> Cc: dri-devel@lists.freedesktop.org

Hmm.

The amount of changes we discover are needed for this use-case makes me
wonder whether we are actually using the incorrect subsystems and
should have coded up a separate utility for this: The utility holds the
HW and shadow BOs, and we then embed (or possibly point to) a struct
drm_mm_node in the bos.

Perhaps should've Claude code that up and see what it looks like?

Thanks,
Thomas



> ---
>  drivers/gpu/drm/drm_suballoc.c | 90
> ++++++++++++++++++++++++++++++++++
>  include/drm/drm_suballoc.h     |  5 ++
>  2 files changed, 95 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_suballoc.c
> b/drivers/gpu/drm/drm_suballoc.c
> index b74277bbc14b..307da191a855 100644
> --- a/drivers/gpu/drm/drm_suballoc.c
> +++ b/drivers/gpu/drm/drm_suballoc.c
> @@ -78,12 +78,33 @@ void drm_suballoc_manager_init(struct
> drm_suballoc_manager *sa_manager,
>  	sa_manager->size = size;
>  	sa_manager->align = align;
>  	sa_manager->hole = &sa_manager->olist;
> +	sa_manager->fence_disable = false;
>  	INIT_LIST_HEAD(&sa_manager->olist);
>  	for (i = 0; i < DRM_SUBALLOC_MAX_QUEUES; ++i)
>  		INIT_LIST_HEAD(&sa_manager->flist[i]);
>  }
>  EXPORT_SYMBOL(drm_suballoc_manager_init);
>  
> +/**
> + * drm_suballoc_manager_fence_disable() - Enable or disable fence
> tracking.
> + * @sa_manager: Pointer to the suballocation manager.
> + * @fence_disable: Whether to disable fence tracking for free
> suballocations.
> + *
> + * When @fence_disable is true allocation scans all holes without
> waiting on
> + * fences. When false, the manager tracks free suballocations behind
> fences and
> + * reuses them only after the fence signals.
> + *
> + * This should be called immediately after creating the suballocator
> and before
> + * any allocations are made. The behaviour is undefined if this is
> called after
> + * allocations have been made.
> + */
> +void drm_suballoc_manager_fence_disable(struct drm_suballoc_manager
> *sa_manager,
> +					bool fence_disable)
> +{
> +	sa_manager->fence_disable = fence_disable;
> +}
> +EXPORT_SYMBOL(drm_suballoc_manager_fence_disable);
> +
>  /**
>   * drm_suballoc_manager_fini() - Destroy the drm_suballoc_manager
>   * @sa_manager: pointer to the sa_manager
> @@ -110,6 +131,7 @@ void drm_suballoc_manager_fini(struct
> drm_suballoc_manager *sa_manager)
>  	}
>  
>  	sa_manager->size = 0;
> +	sa_manager->fence_disable = false;
>  }
>  EXPORT_SYMBOL(drm_suballoc_manager_fini);
>  
> @@ -161,6 +183,69 @@ static size_t drm_suballoc_hole_eoffset(struct
> drm_suballoc_manager *sa_manager)
>  	return sa_manager->size;
>  }
>  
> +/**
> + * drm_suballoc_hole_soffset_eoffset() - Find a hole fitting size
> and alignment.
> + * @sa_manager: Suballocator manager.
> + * @soffset: Start offset of the selected hole.
> + * @eoffset: End offset of the selected hole.
> + * @size: Size to be allocated.
> + * @align: Allocation alignment.
> + *
> + * This function is used when fences are disabled. It scans the
> holes starting
> + * at the current hole pointer and selects the first one that can
> satisfy @size
> + * once @align padding is applied. We may need to scan the entire
> list of holes
> + * and update soffset and eoffset for each hole to find a suitable
> one, as the
> + * current hole pointer may not be the same as sa_manager->hole.
> + *
> + * Return: true if a suitable hole is found, false otherwise.
> + */
> +static bool drm_suballoc_hole_soffset_eoffset(struct
> drm_suballoc_manager *sa_manager,
> +					      size_t *soffset,
> size_t *eoffset,
> +					      size_t size, size_t
> align)
> +{
> +	struct list_head *start, *hole;
> +
> +	if (!sa_manager->fence_disable)
> +		return true;
> +
> +	start = sa_manager->hole;
> +	hole = start;
> +
> +	do {
> +		size_t s, e, wasted;
> +
> +		/*
> +		 * We can't use drm_suballoc_hole_soffset() and
> +		 * drm_suballoc_hole_eoffset() here as the hole may
> not be same
> +		 * as sa_manager->hole.
> +		 */
> +		if (hole != &sa_manager->olist)
> +			s = list_entry(hole, struct drm_suballoc,
> olist)->eoffset;
> +		else
> +			s = 0;
> +
> +		if (hole->next != &sa_manager->olist)
> +			e = list_entry(hole->next, struct
> drm_suballoc, olist)->soffset;
> +		else
> +			e = sa_manager->size;
> +
> +		if (e < s || e == s || (e - s) < size) {
> +			hole = hole->next;
> +			continue;
> +		}
> +
> +		wasted = round_up(s, align) - s;
> +		if ((e - s) >= (size + wasted)) {
> +			*soffset = s;
> +			*eoffset = e;
> +			sa_manager->hole = hole;
> +			return true;
> +		}
> +	} while (hole != start);
> +
> +	return false;
> +}
> +
>  static bool drm_suballoc_try_alloc(struct drm_suballoc_manager
> *sa_manager,
>  				   struct drm_suballoc *sa,
>  				   size_t size, size_t align)
> @@ -169,6 +254,11 @@ static bool drm_suballoc_try_alloc(struct
> drm_suballoc_manager *sa_manager,
>  
>  	soffset = drm_suballoc_hole_soffset(sa_manager);
>  	eoffset = drm_suballoc_hole_eoffset(sa_manager);
> +
> +	if (!drm_suballoc_hole_soffset_eoffset(sa_manager, &soffset,
> +					       &eoffset, size,
> align))
> +		return false;
> +
>  	wasted = round_up(soffset, align) - soffset;
>  
>  	if ((eoffset - soffset) >= (size + wasted)) {
> diff --git a/include/drm/drm_suballoc.h b/include/drm/drm_suballoc.h
> index 29befdda35d2..066412196f8a 100644
> --- a/include/drm/drm_suballoc.h
> +++ b/include/drm/drm_suballoc.h
> @@ -20,6 +20,7 @@
>   * @flist: Array[fence context hash] of queues of fenced allocated
> ranges.
>   * @size: Size of the managed range.
>   * @align: Default alignment for the managed range.
> + * @fence_disable: Fences are disabled for this SA manager.
>   */
>  struct drm_suballoc_manager {
>  	wait_queue_head_t wq;
> @@ -28,6 +29,7 @@ struct drm_suballoc_manager {
>  	struct list_head flist[DRM_SUBALLOC_MAX_QUEUES];
>  	size_t size;
>  	size_t align;
> +	bool fence_disable;
>  };
>  
>  /**
> @@ -51,6 +53,9 @@ struct drm_suballoc {
>  void drm_suballoc_manager_init(struct drm_suballoc_manager
> *sa_manager,
>  			       size_t size, size_t align);
>  
> +void drm_suballoc_manager_fence_disable(struct drm_suballoc_manager
> +					*sa_manager, bool
> fence_disable);
> +
>  void drm_suballoc_manager_fini(struct drm_suballoc_manager
> *sa_manager);
>  
>  struct drm_suballoc *drm_suballoc_alloc(gfp_t gfp);