[PATCH v2 1/2] drm/panthor: Extend VM locked region for remap case to be a superset

[Adrián Larumbe]

In the event of an sm_step_remap() that leads to a partial unmap of a
transparent huge page, the new locked region required by an extended unmap
might not be a superset of the original one. Then, if it leaves a portion
of the initially requested one out, the ensuing map will trigger a warning.

Signed-off-by: Adrián Larumbe <adrian.larumbe@xxxxxxxxxxxxx>
Fixes: 8e7460eac786 ("drm/panthor: Support partial unmaps of huge pages")
Reviewed-by: Boris Brezillon <boris.brezillon@xxxxxxxxxxxxx>
---
 drivers/gpu/drm/panthor/panthor_mmu.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/drivers/gpu/drm/panthor/panthor_mmu.c b/drivers/gpu/drm/panthor/panthor_mmu.c
index fa8b31df85c9..bd41c892beb7 100644
--- a/drivers/gpu/drm/panthor/panthor_mmu.c
+++ b/drivers/gpu/drm/panthor/panthor_mmu.c
@@ -1709,6 +1709,25 @@ static int panthor_vm_lock_region(struct panthor_vm *vm, u64 start, u64 size)
 	    start + size <= vm->locked_region.start + vm->locked_region.size)
 		return 0;
 
+	/* 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
+	 * always overlap with the original one, because the upper and lower unmap
+	 * 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));
+
+		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 */
-- 
2.53.0