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

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

On Thu, 18 Dec 2025 16:57:28 +0000
Steven Price <steven.price@arm.com> wrote:

> 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@collabora.com>
> > Signed-off-by: Ketil Johnsen <ketil.johnsen@arm.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?

I thought it could happen if the FW initialization fails in the middle,
and the FW VM is destroyed before the scheduler had a chance to
initialize, but it turns out the FW logic never calls
panthor_vm_destroy().

> 
> 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);  
>