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

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Message-ID: <20251113201506.76a1af69@fedora>
Date: Thu, 13 Nov 2025 20:15:06 +0100
From: Boris Brezillon <boris.brezillon@...labora.com>
To: Akash Goel <akash.goel@....com>
Cc: liviu.dudau@....com, steven.price@....com,
 dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
 maarten.lankhorst@...ux.intel.com, mripard@...nel.org, tzimmermann@...e.de,
 airlied@...il.com, daniel@...ll.ch, nd@....com
Subject: Re: [PATCH] drm/panthor: Avoid adding of kernel BOs to extobj list

On Thu, 13 Nov 2025 18:45:44 +0000
Akash Goel <akash.goel@....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@....com>

With a proper Fixes tag, this is

Reviewed-by: Boris Brezillon <boris.brezillon@...labora.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: