linux-kernel - Re: [PATCH] drm/panthor: Evict groups before VM termination

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Message-ID: <0939a54e-dc35-434b-9dbc-06837f04e62f@arm.com>
Date: Thu, 18 Dec 2025 16:57:28 +0000
From: Steven Price <steven.price@....com>
To: Ketil Johnsen <ketil.johnsen@....com>,
 Boris Brezillon <boris.brezillon@...labora.com>,
 Liviu Dudau <liviu.dudau@....com>,
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>,
 David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
 Grant Likely <grant.likely@...aro.org>, Heiko Stuebner <heiko@...ech.de>
Cc: dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/panthor: Evict groups before VM termination

On 18/12/2025 16:26, Ketil Johnsen wrote:
> Ensure all related groups are evicted and suspended before VM
> destruction takes place.
> 
> This fixes an issue where panthor_vm_destroy() destroys and unmaps the
> heap context while there are still on slot groups using this.
> The FW will do a write out to the heap context when a CSG (group) is
> suspended, so a premature unmap of the heap context will cause a
> GPU page fault.
> This page fault is quite harmless, and do not affect the continued
> operation of the GPU.
> 
> Fixes: 647810ec2476 ("drm/panthor: Add the MMU/VM logical block")
> Co-developed-by: Boris Brezillon <boris.brezillon@...labora.com>
> Signed-off-by: Ketil Johnsen <ketil.johnsen@....com>
> ---
>  drivers/gpu/drm/panthor/panthor_mmu.c   |  4 ++++
>  drivers/gpu/drm/panthor/panthor_sched.c | 16 ++++++++++++++++
>  drivers/gpu/drm/panthor/panthor_sched.h |  1 +
>  3 files changed, 21 insertions(+)
> 
> diff --git a/drivers/gpu/drm/panthor/panthor_mmu.c b/drivers/gpu/drm/panthor/panthor_mmu.c
> index 74230f7199121..0e4b301a9c70e 100644
> --- a/drivers/gpu/drm/panthor/panthor_mmu.c
> +++ b/drivers/gpu/drm/panthor/panthor_mmu.c
> @@ -1537,6 +1537,10 @@ static void panthor_vm_destroy(struct panthor_vm *vm)
>  
>  	vm->destroyed = true;
>  
> +	/* Tell scheduler to stop all GPU work related to this VM */
> +	if (refcount_read(&vm->as.active_cnt) > 0)
> +		panthor_sched_prepare_for_vm_destruction(vm->ptdev);
> +
>  	mutex_lock(&vm->heaps.lock);
>  	panthor_heap_pool_destroy(vm->heaps.pool);
>  	vm->heaps.pool = NULL;
> diff --git a/drivers/gpu/drm/panthor/panthor_sched.c b/drivers/gpu/drm/panthor/panthor_sched.c
> index f680edcd40aad..fbbaab9b25efb 100644
> --- a/drivers/gpu/drm/panthor/panthor_sched.c
> +++ b/drivers/gpu/drm/panthor/panthor_sched.c
> @@ -2930,6 +2930,22 @@ void panthor_sched_report_mmu_fault(struct panthor_device *ptdev)
>  		sched_queue_delayed_work(ptdev->scheduler, tick, 0);
>  }
>  
> +void panthor_sched_prepare_for_vm_destruction(struct panthor_device *ptdev)
> +{
> +	/* FW can write out internal state, like the heap context, during CSG
> +	 * suspend. It is therefore important that the scheduler has fully
> +	 * evicted any pending and related groups before VM destruction can
> +	 * safely continue. Failure to do so can lead to GPU page faults.
> +	 * A controlled termination of a Panthor instance involves destroying
> +	 * the group(s) before the VM. This means any relevant group eviction
> +	 * has already been initiated by this point, and we just need to
> +	 * ensure that any pending tick_work() has been completed.
> +	 */
> +	if (ptdev->scheduler) {
> +		flush_work(&ptdev->scheduler->tick_work.work);
> +	}

NIT: braces not needed.

But I'm also struggling to understand in what situation ptdev->scheduler
would be NULL?

Thanks,
Steve

> +}
> +
>  void panthor_sched_resume(struct panthor_device *ptdev)
>  {
>  	/* Force a tick to re-evaluate after a resume. */
> diff --git a/drivers/gpu/drm/panthor/panthor_sched.h b/drivers/gpu/drm/panthor/panthor_sched.h
> index f4a475aa34c0a..9a8692de8aded 100644
> --- a/drivers/gpu/drm/panthor/panthor_sched.h
> +++ b/drivers/gpu/drm/panthor/panthor_sched.h
> @@ -50,6 +50,7 @@ void panthor_sched_suspend(struct panthor_device *ptdev);
>  void panthor_sched_resume(struct panthor_device *ptdev);
>  
>  void panthor_sched_report_mmu_fault(struct panthor_device *ptdev);
> +void panthor_sched_prepare_for_vm_destruction(struct panthor_device *ptdev);
>  void panthor_sched_report_fw_events(struct panthor_device *ptdev, u32 events);
>  
>  void panthor_fdinfo_gather_group_samples(struct panthor_file *pfile);