[Qemu-devel] [PATCH] device_tree: Add qemu_fdt_totalsize function

[Michael Clark]

Currently the device-tree create_device_tree function
returns the size of the allocated device tree buffer
however there is no way to get the actual amount of
buffer space used by the device-tree.

14ec3cbd7c1e31dca4d23f028100c8f43e156573 increases
the FDT_MAX_SIZE to 1 MiB. This creates an issue
for boards that have less than 1 MiB in their ROM
for device tree. While cpu_physical_memory_write
will not write past the end of a buffer there is
and a board is aware of its ROM buffer size, so
can use min(fdt_size,rom_size); this provides no
indication as to whether the device-tree may be
truncated. qemu_fdt_totalsize allows a board to
check that a dynamically created device tree will
fit within its alloted ROM space.

Add qemu_fdt_totalsize which uses the logic and
public APIs from libfdt to calculate the device
size: struct_offset + struct_size + strings_size
+ terminator.

Cc: Peter Crosthwaite <address@hidden>
Cc: Alexander Graf <address@hidden>
Cc: Alistair Francis <address@hidden>
Cc: Peter Maydell <address@hidden>
---
 device_tree.c                | 6 ++++++
 include/sysemu/device_tree.h | 6 ++++++
 2 files changed, 12 insertions(+)

diff --git a/device_tree.c b/device_tree.c
index 52c3358a5583..3a2166d61f37 100644
--- a/device_tree.c
+++ b/device_tree.c
@@ -215,6 +215,12 @@ void *load_device_tree_from_sysfs(void)
 
 #endif /* CONFIG_LINUX */
 
+size_t qemu_fdt_totalsize(void *fdt)
+{
+    return fdt_off_dt_struct(fdt) + fdt_size_dt_struct(fdt) +
+           fdt_size_dt_strings(fdt) + sizeof(uint32_t) /* terminator */;
+}
+
 static int findnode_nofail(void *fdt, const char *node_path)
 {
     int offset;
diff --git a/include/sysemu/device_tree.h b/include/sysemu/device_tree.h
index e22e5bec9c3f..4af232dfdc65 100644
--- a/include/sysemu/device_tree.h
+++ b/include/sysemu/device_tree.h
@@ -26,6 +26,12 @@ void *load_device_tree_from_sysfs(void);
 #endif
 
 /**
+ * qemu_fdt_total_size: returns the size required to store the current
+ * device tree versus the buffer size returned by create_device_tree
+ */
+size_t qemu_fdt_totalsize(void *fdt);
+
+/**
  * qemu_fdt_node_path: return the paths of nodes matching a given
  * name and compat string
  * @fdt: pointer to the dt blob
-- 
2.7.0