[PATCH 6/8] drm/panthor: Explicit expansion of locked VM region

Ketil Johnsen
2026-05-05T14:05:57Z

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 | 69 +++++++++++++++++++--------
1 file changed, 50 insertions(+), 19 deletions(-)

diff --git a/drivers/gpu/drm/panthor/panthor_mmu.c b/drivers/gpu/drm/panthor/panthor_mmu.c
index fc930ee158a52..07f54176ec1bf 100644
--- a/drivers/gpu/drm/panthor/panthor_mmu.c
+++ b/drivers/gpu/drm/panthor/panthor_mmu.c
@@ -1701,15 +1701,36 @@ 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) {
+ /* Lock the region that needs to be updated */
+ gpu_write64(ptdev, AS_LOCKADDR(vm->as.id),
+ 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);
+ }

- /* 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_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
@@ -1717,16 +1738,24 @@ 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;

- start = min(start, vm->locked_region.start);
- size = end - start;
- }
+ /* 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;
+
+ 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) {
@@ -2252,11 +2281,13 @@ 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.
+ * atomicity.
*/
if (!unmap_vma->evicted) {
- panthor_vm_lock_region(vm, unmap_start, unmap_range);
+ ret = panthor_vm_expand_region(vm, unmap_start, unmap_range);
+ if (ret)
+ return ret;
+
panthor_vm_unmap_pages(vm, unmap_start, unmap_range);
}

--
2.43.0