[Qemu-devel] [PATCH v2] device_tree: Add support for reading device tree properties

[Peter Maydell]

Add support for reading device tree properties (both generic
and single-cell ones) to QEMU's convenience wrapper layer.

Signed-off-by: Peter Maydell <address@hidden>
---
Here's a v2:
 * added qemu_devtree_get_one_cell_from_prop() which reads a single cell from
   a property which is an array of cells
 * NB that qemu_devtree_getprop() isn't implemented in terms of this because
   that would give worse error handling.

I still think that having this new function is misguided:
 * nobody's using it
 * it breaks the current model where functions at the qemu_devtree and
   libfdt levels deal only with entire properties and do not look inside
   them to operate on only part of the property value

But here's a patch so we can argue about something concrete.

 device_tree.c |   49 +++++++++++++++++++++++++++++++++++++++++++++++++
 device_tree.h |    6 ++++++
 2 files changed, 55 insertions(+), 0 deletions(-)

diff --git a/device_tree.c b/device_tree.c
index b366fdd..3a8ff13 100644
--- a/device_tree.c
+++ b/device_tree.c
@@ -178,6 +178,55 @@ int qemu_devtree_setprop_string(void *fdt, const char 
*node_path,
     return r;
 }
 
+const void *qemu_devtree_getprop(void *fdt, const char *node_path,
+                                 const char *property, int *lenp)
+{
+    int len;
+    const void *r;
+    if (!lenp) {
+        lenp = &len;
+    }
+    r = fdt_getprop(fdt, findnode_nofail(fdt, node_path), property, lenp);
+    if (!r) {
+        fprintf(stderr, "%s: Couldn't get %s/%s: %s\n", __func__,
+                node_path, property, fdt_strerror(*lenp));
+        exit(1);
+    }
+    return r;
+}
+
+uint32_t qemu_devtree_get_one_cell_from_prop(void *fdt, const char *node_path,
+                                             const char *property, int idx)
+{
+    int len;
+    const uint32_t *p = qemu_devtree_getprop(fdt, node_path, property, &len);
+    if (len % 4 != 0) {
+        fprintf(stderr, "%s: %s/%s not multiple of 4 bytes long "
+                "(not cells?)\n",
+                __func__, node_path, property);
+        exit(1);
+    }
+    if (len < (idx + 1) * 4) {
+        fprintf(stderr, "%s: %s/%s wrong length to contain %d cells\n",
+                __func__, node_path, property, idx + 1);
+        exit(1);
+    }
+    return be32_to_cpu(p[idx]);
+}
+
+uint32_t qemu_devtree_getprop_cell(void *fdt, const char *node_path,
+                                   const char *property)
+{
+    int len;
+    const uint32_t *p = qemu_devtree_getprop(fdt, node_path, property, &len);
+    if (len != 4) {
+        fprintf(stderr, "%s: %s/%s not 4 bytes long (not a cell?)\n",
+                __func__, node_path, property);
+        exit(1);
+    }
+    return be32_to_cpu(*p);
+}
+
 uint32_t qemu_devtree_get_phandle(void *fdt, const char *path)
 {
     uint32_t r;
diff --git a/device_tree.h b/device_tree.h
index 2244270..86669ea 100644
--- a/device_tree.h
+++ b/device_tree.h
@@ -28,6 +28,12 @@ int qemu_devtree_setprop_string(void *fdt, const char 
*node_path,
 int qemu_devtree_setprop_phandle(void *fdt, const char *node_path,
                                  const char *property,
                                  const char *target_node_path);
+const void *qemu_devtree_getprop(void *fdt, const char *node_path,
+                                 const char *property, int *lenp);
+uint32_t qemu_devtree_getprop_cell(void *fdt, const char *node_path,
+                                   const char *property);
+uint32_t qemu_devtree_get_one_cell_from_prop(void *fdt, const char *node_path,
+                                             const char *property, int idx);
 uint32_t qemu_devtree_get_phandle(void *fdt, const char *path);
 uint32_t qemu_devtree_alloc_phandle(void *fdt);
 int qemu_devtree_nop_node(void *fdt, const char *node_path);
-- 
1.7.1