[PATCH v4 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.

v4:
- s/XA_FLAGS_ALLOC1/XA_FLAGS_ALLOC/

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.sm...@collabora.com>
Signed-off-by: Boris Brezillon <boris.brezil...@collabora.com>
Tested-by: Eric Smith <eric.sm...@collabora.com>
---
 drivers/gpu/drm/panthor/panthor_heap.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/panthor/panthor_heap.c 
b/drivers/gpu/drm/panthor/panthor_heap.c
index b0fc5b9ee847..95a1c6c9f35e 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);
 
@@ -543,7 +544,7 @@ panthor_heap_pool_create(struct panthor_device *ptdev, 
struct panthor_vm *vm)
        pool->vm = vm;
        pool->ptdev = ptdev;
        init_rwsem(&pool->lock);
-       xa_init_flags(&pool->xa, XA_FLAGS_ALLOC1);
+       xa_init_flags(&pool->xa, XA_FLAGS_ALLOC);
        kref_init(&pool->refcount);
 
        pool->gpu_contexts = panthor_kernel_bo_create(ptdev, vm, bosize,
-- 
2.44.0