commit 84d35492b145cebc000f8fd72818eb25b8e65c04
author Stewart Miles <smiles@google.com> Thu May 01 09:03:27 2014 -0700
committer Stewart Miles <smiles@google.com> Mon May 12 12:35:37 2014 -0700
tree 0a7adfd3f93de27984377aa1e210685540891ff8
parent 4c847f2b79d99b1fecf487b167dd5bbe3ea7da06

Described restrictions for common HAL object methods.

Inheritance of HAL object is performed by composing a child structure of a
single parent structure located at offset 0 followed by new data members
and function pointers in the child structure.

For example,

struct child {
  struct parent common;
  int a_data_member;
  void (*a_method)(struct child *c, int v);
};

HAL code assumes this layout when accessing child structures given a pointer
to a parent structure such that users write code like the following...

void child_method(struct *parent, int v) {
  struct child * c = (struct child*)parent;
  // do stuff with c
}

Code above will break if a member is added before "common" in "struct child".

This change adds comments that describe the restriction on the location of
parent HAL objects within a derived HAL object.  HAL objects that already
have comments that describe the required location of parent objects are not
modified.

Change-Id: Ibe4300275286ef275b2097534c84f1029d761d87

include/hardware/keymaster.h

diff --git a/include/hardware/keymaster.h b/include/hardware/keymaster.h
index 12158bf..8c5ff14 100644
--- a/include/hardware/keymaster.h
+++ b/include/hardware/keymaster.h
@@ -83,6 +83,12 @@
 };
 
 struct keystore_module {
+    /**
+     * Common methods of the keystore module.  This *must* be the first member of
+     * keystore_module as users of this structure will cast a hw_module_t to
+     * keystore_module pointer in contexts where it's known the hw_module_t references a
+     * keystore_module.
+     */
     hw_module_t common;
 };
 
@@ -166,6 +172,12 @@
  * The parameters that can be set for a given keymaster implementation.
  */
 struct keymaster_device {
+    /**
+     * Common methods of the keymaster device.  This *must* be the first member of
+     * keymaster_device as users of this structure will cast a hw_device_t to
+     * keymaster_device pointer in contexts where it's known the hw_device_t references a
+     * keymaster_device.
+     */
     struct hw_device_t common;
 
     /**
@@ -282,4 +294,3 @@
 __END_DECLS
 
 #endif  // ANDROID_HARDWARE_KEYMASTER_H
-