[PATCH v3 4/5] drm/panthor: Fix an off-by-one in the heap context retrieval logic

[Boris Brezillon]

The heap ID is used to index the heap context pool, and allocating
in the [1:MAX_HEAPS_PER_POOL] leads to an off-by-one. This was
originally to avoid returning a zero heap handle, but given the handle
is formed with (vm_id << 16) | heap_id, with vm_id > 0, we already can't
end up with a valid heap handle that's zero.

v3:
- Allocate in the [0:MAX_HEAPS_PER_POOL-1] range

v2:
- New patch

Fixes: 9cca48fa4f89 ("drm/panthor: Add the heap logical block")
Reported-by: Eric Smith <eric.smith at collabora.com>
Signed-off-by: Boris Brezillon <boris.brezillon at collabora.com>
Tested-by: Eric Smith <eric.smith at collabora.com>
---
 drivers/gpu/drm/panthor/panthor_heap.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/panthor/panthor_heap.c b/drivers/gpu/drm/panthor/panthor_heap.c
index 683bb94761bc..252332f5390f 100644
--- a/drivers/gpu/drm/panthor/panthor_heap.c
+++ b/drivers/gpu/drm/panthor/panthor_heap.c
@@ -323,7 +323,8 @@ int panthor_heap_create(struct panthor_heap_pool *pool,
 	if (!pool->vm) {
 		ret = -EINVAL;
 	} else {
-		ret = xa_alloc(&pool->xa, &id, heap, XA_LIMIT(1, MAX_HEAPS_PER_POOL), GFP_KERNEL);
+		ret = xa_alloc(&pool->xa, &id, heap,
+			       XA_LIMIT(0, MAX_HEAPS_PER_POOL - 1), GFP_KERNEL);
 		if (!ret) {
 			void *gpu_ctx = panthor_get_heap_ctx(pool, id);
 
-- 
2.44.0