Re: [PATCH v2 3/7] drm/panthor: Explicit expansion of locked VM region

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

On Sun, 12 Jul 2026 15:54:35 +0200
Ketil Johnsen <ketil.johnsen@arm.com> wrote:

> Currently the panthor_vm_lock_region() function will implicitly expand
> an already locked VM region. This can be problematic because the caller
> do not reliably know if it needs to call panthor_vm_unlock_region()
> or not.
> 
> Worth noting, there is currently no known issues with this as the code
> is written today.
> 
> This change introduces panthor_vm_expand_region() which will only work
> if there is already a locked VM region. This again means that the
> original lock and unlock functions can work as a pair. This pairing is
> needed for subsequent protected memory changes.
> 
> Signed-off-by: Ketil Johnsen <ketil.johnsen@arm.com>
> ---
>  drivers/gpu/drm/panthor/panthor_mmu.c | 87 +++++++++++++++++++--------
>  1 file changed, 62 insertions(+), 25 deletions(-)
> 
> diff --git a/drivers/gpu/drm/panthor/panthor_mmu.c b/drivers/gpu/drm/panthor/panthor_mmu.c
> index 1f6b9242279c2..b82b01013611c 100644
> --- a/drivers/gpu/drm/panthor/panthor_mmu.c
> +++ b/drivers/gpu/drm/panthor/panthor_mmu.c
> @@ -1774,20 +1774,49 @@ static const char *access_type_name(struct panthor_device *ptdev,
>  	}
>  }
>  
> +static int panthor_vm_apply_as_lock(struct panthor_vm *vm, u64 region)
> +{
> +	struct panthor_device *ptdev = vm->ptdev;
> +
> +	lockdep_assert_held(&ptdev->mmu->as.slots_lock);
> +
> +	gpu_write64(ptdev->mmu->iomem, AS_LOCKADDR(vm->as.id), region);
> +	return as_send_cmd_and_wait(ptdev, vm->as.id, AS_COMMAND_LOCK);
> +}
> +
>  static int panthor_vm_lock_region(struct panthor_vm *vm, u64 start, u64 size)
>  {
>  	struct panthor_device *ptdev = vm->ptdev;
>  	int ret = 0;
>  
> -	/* sm_step_remap() can call panthor_vm_lock_region() to account for
> -	 * the wider unmap needed when doing a partial huge page unamp. We
> -	 * need to ignore the lock if it's already part of the locked region.
> -	 */
> -	if (start >= vm->locked_region.start &&
> -	    start + size <= vm->locked_region.start + vm->locked_region.size)
> -		return 0;
> +	if (drm_WARN_ON(&ptdev->base, vm->locked_region.size))
> +		return -EINVAL;
> +
> +	mutex_lock(&ptdev->mmu->as.slots_lock);
> +	if (vm->as.id >= 0 && size) {
> +		u64 region = pack_region_range(ptdev, &start, &size);
> +
> +		ret = panthor_vm_apply_as_lock(vm, region);
> +	}
>  
> -	/* sm_step_remap() may need a locked region that isn't a strict superset
> +	if (!ret) {
> +		vm->locked_region.start = start;
> +		vm->locked_region.size = size;
> +	}
> +	mutex_unlock(&ptdev->mmu->as.slots_lock);
> +
> +	return ret;
> +}
> +
> +static int panthor_vm_expand_locked_region(struct panthor_vm *vm,
> +					   u64 start, u64 size)
> +{
> +	struct panthor_device *ptdev = vm->ptdev;
> +	u64 end;
> +	int ret = 0;
> +
> +	/* This function is here to handle the following case:
> +	 * sm_step_remap() may need a locked region that isn't a strict superset
>  	 * of the original one because of having to extend unmap boundaries beyond
>  	 * it to deal with partial unmaps of transparent huge pages. What we want
>  	 * in those cases is to lock the union of both regions. The new region must
> @@ -1795,25 +1824,30 @@ static int panthor_vm_lock_region(struct panthor_vm *vm, u64 start, u64 size)
>  	 * boundaries in a remap operation can only shift up or down respectively,
>  	 * but never otherwise.
>  	 */
> -	if (vm->locked_region.size) {
> -		u64 end = max(vm->locked_region.start + vm->locked_region.size,
> -			      start + size);
>  
> -		drm_WARN_ON_ONCE(&vm->ptdev->base, (start + size <= vm->locked_region.start) ||
> -				 (start >= vm->locked_region.start + vm->locked_region.size));
> +	/* This function can only expand an already locked region */
> +	if (drm_WARN_ON(&ptdev->base, !vm->locked_region.size))
> +		return -EINVAL;
> +
> +	/* Early out if requested range is already locked */
> +	if (start >= vm->locked_region.start &&
> +	    start + size <= vm->locked_region.start + vm->locked_region.size)
> +		return 0;
>  
> -		start = min(start, vm->locked_region.start);
> -		size = end - start;
> -	}
> +	end = max(vm->locked_region.start + vm->locked_region.size,
> +		  start + size);
> +
> +	drm_WARN_ON_ONCE(&ptdev->base, (start + size <= vm->locked_region.start) ||
> +			 (start >= vm->locked_region.start + vm->locked_region.size));
> +
> +	start = min(start, vm->locked_region.start);
> +	size = end - start;
>  
>  	mutex_lock(&ptdev->mmu->as.slots_lock);
>  	if (vm->as.id >= 0 && size) {
> -		/* Lock the region that needs to be updated */
> -		gpu_write64(ptdev->mmu->iomem, AS_LOCKADDR(vm->as.id),
> -			    pack_region_range(ptdev, &start, &size));
> +		u64 region = pack_region_range(ptdev, &start, &size);
>  
> -		/* If the lock succeeded, update the locked_region info. */
> -		ret = as_send_cmd_and_wait(ptdev, vm->as.id, AS_COMMAND_LOCK);
> +		ret = panthor_vm_apply_as_lock(vm, region);
>  	}
>  
>  	if (!ret) {
> @@ -2367,11 +2401,14 @@ static int panthor_gpuva_sm_step_remap(struct drm_gpuva_op *op,
>  		 */
>  		unmap_hugepage_align(&op->remap, &unmap_start, &unmap_range);
>  
> -		/* If the range changed, we might have to lock a wider region to guarantee
> -		 * atomicity. panthor_vm_lock_region() bails out early if the new region
> -		 * is already part of the locked region, so no need to do this check here.
> +		/* If the range changed, we might have to lock a wider region to
> +		 * guarantee atomicity.
>  		 */
> -		panthor_vm_lock_region(vm, unmap_start, unmap_range);
> +		ret = panthor_vm_expand_locked_region(vm, unmap_start,
> +						      unmap_range);

Could we add the panthor_vm_lock_region() error check in a commit coming
before this one, and maybe flag it for backport, with a Fixes tag?

> +		if (ret)
> +			return ret;
> +
>  		panthor_vm_unmap_pages(vm, unmap_start, unmap_range);
>  	}
>