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

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

[Akash Goel]

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.

Signed-off-by: Akash Goel <akash.goel@arm.com>
---
 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:
-- 
2.25.1

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

[Boris Brezillon]

On Thu, 13 Nov 2025 18:45:44 +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.
> 
> Signed-off-by: Akash Goel <akash.goel@arm.com>

With a proper Fixes tag, this is

Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>

> ---
>  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:

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

[Steven Price]

On 13/11/2025 18:45, Akash Goel 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.
> 
> Signed-off-by: Akash Goel <akash.goel@arm.com>

Reviewed-by: Steven Price <steven.price@arm.com>

As Boris says it would be good to have a Fixes tag - can you dig out an
appropriate commit then I can merge?

Thanks,
Steve

> ---
>  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: