drm/panthor: Make sure the tiler initial/max chunks are consistent

It doesn't make sense to have a maximum number of chunks smaller than
the initial number of chunks attached to the context.

Fix the uAPI header to reflect the new constraint, and mention the
undocumented "initial_chunk_count > 0" constraint while at it.

v3:
- Add R-b

v2:
- Fix the check

Fixes: 9cca48fa ("drm/panthor: Add the heap logical block")
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240502165158.1458959-3-boris.brezillon@collabora.com

drivers/gpu/drm/panthor/panthor_heap.c

@@ -281,6 +281,9 @@ int panthor_heap_create(struct panthor_heap_pool *pool,
 	if (initial_chunk_count == 0)
 		return -EINVAL;
 
+	if (initial_chunk_count > max_chunks)
+		return -EINVAL;
+
 	if (hweight32(chunk_size) != 1 ||
 	    chunk_size < SZ_256K || chunk_size > SZ_2M)
 		return -EINVAL;

include/uapi/drm/panthor_drm.h

@@ -895,13 +895,17 @@ struct drm_panthor_tiler_heap_create {
 	/** @vm_id: VM ID the tiler heap should be mapped to */
 	__u32 vm_id;
 
-	/** @initial_chunk_count: Initial number of chunks to allocate. */
+	/** @initial_chunk_count: Initial number of chunks to allocate. Must be at least one. */
 	__u32 initial_chunk_count;
 
 	/** @chunk_size: Chunk size. Must be a power of two at least 256KB large. */
 	__u32 chunk_size;
 
-	/** @max_chunks: Maximum number of chunks that can be allocated. */
+	/**
+	 * @max_chunks: Maximum number of chunks that can be allocated.
+	 *
+	 * Must be at least @initial_chunk_count.
+	 */
 	__u32 max_chunks;
 
 	/**