Re: [PATCH v2] drm/panthor: Avoid adding of kernel BOs to extobj list

[Boris Brezillon <boris.brezillon@collabora.com>]

On Thu, 20 Nov 2025 17:21:18 +0000
Akash Goel <akash.goel@arm.com> wrote:

> The kernel BOs unnecessarily got added to the external objects list
> of drm_gpuvm, when mapping to GPU, which would have resulted in few
> extra CPU cycles being spent at the time of job submission as
> drm_exec_until_all_locked() loop iterates over all external objects.
> 
> Kernel BOs are private to a VM and so they share the dma_resv object of
> the dummy GEM object created for a VM. Use of DRM_EXEC_IGNORE_DUPLICATES
> flag ensured the recursive locking of the dummy GEM object was ignored.
> Also no extra space got allocated to add fences to the dma_resv object
> of dummy GEM object. So no other impact apart from few extra CPU cycles.
> 
> This commit sets the pointer to dma_resv object of GEM object of
> kernel BOs before they are mapped to GPU, to prevent them from
> being added to external objects list.
> 
> v2: Add R-bs and fixes tags
> 
> Fixes: 8a1cc07578bf ("drm/panthor: Add GEM logical block")
> Signed-off-by: Akash Goel <akash.goel@arm.com>
> Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
> Reviewed-by: Steven Price <steven.price@arm.com>

Queued to drm-misc-next.

> ---
>  drivers/gpu/drm/panthor/panthor_gem.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/panthor/panthor_gem.c b/drivers/gpu/drm/panthor/panthor_gem.c
> index f369cc3e2a5f..10d255cccc09 100644
> --- a/drivers/gpu/drm/panthor/panthor_gem.c
> +++ b/drivers/gpu/drm/panthor/panthor_gem.c
> @@ -145,6 +145,9 @@ panthor_kernel_bo_create(struct panthor_device *ptdev, struct panthor_vm *vm,
>  	bo = to_panthor_bo(&obj->base);
>  	kbo->obj = &obj->base;
>  	bo->flags = bo_flags;
> +	bo->exclusive_vm_root_gem = panthor_vm_root_gem(vm);
> +	drm_gem_object_get(bo->exclusive_vm_root_gem);
> +	bo->base.base.resv = bo->exclusive_vm_root_gem->resv;
>  
>  	if (vm == panthor_fw_vm(ptdev))
>  		debug_flags |= PANTHOR_DEBUGFS_GEM_USAGE_FLAG_FW_MAPPED;
> @@ -168,9 +171,6 @@ panthor_kernel_bo_create(struct panthor_device *ptdev, struct panthor_vm *vm,
>  		goto err_free_va;
>  
>  	kbo->vm = panthor_vm_get(vm);
> -	bo->exclusive_vm_root_gem = panthor_vm_root_gem(vm);
> -	drm_gem_object_get(bo->exclusive_vm_root_gem);
> -	bo->base.base.resv = bo->exclusive_vm_root_gem->resv;
>  	return kbo;
>  
>  err_free_va: