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 <[email protected]>
> Cc: Matthew Brost <[email protected]>
> Cc: Thomas Hellström <[email protected]>
> Cc: Maarten Lankhorst <[email protected]>
> Cc: Michal Wajdeczko <[email protected]>
> Cc: Christian König <[email protected]>
> Cc: [email protected]
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);
