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

[Ketil Johnsen <ketil.johnsen@arm.com>]

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