arm: implement safe reads by intercepting the data abort handler.

Implement functions to read from a memory location and indicate
failure in case this is not possible. Since we do not have a MMU,
intercept the data abort handler and simply return when the abort
comes from the safe read routines.

Change-Id: I08f2e59898dcac893319a8150d4cf626f3adabbd
Reviewed-on: http://gerrit.rockbox.org/207
Reviewed-by: Marcin Bukat <marcin.bukat@gmail.com>

lib/unwarminder/safe_read.h

@@ -0,0 +1,33 @@
+/***************************************************************************
+ *             __________               __   ___.
+ *   Open      \______   \ ____   ____ |  | _\_ |__   _______  ___
+ *   Source     |       _//  _ \_/ ___\|  |/ /| __ \ /  _ \  \/  /
+ *   Jukebox    |    |   (  <_> )  \___|    < | \_\ (  <_> > <  <
+ *   Firmware   |____|_  /\____/ \___  >__|_ \|___  /\____/__/\_ \
+ *                     \/            \/     \/    \/            \/
+ * $Id$
+ *
+ * Copyright (C) 2012 by Amaury Pouly
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
+ * KIND, either express or implied.
+ *
+ ****************************************************************************/
+#ifndef __SAFE_READ__
+#define __SAFE_READ__
+
+#include "system.h"
+
+/* Try to read an X-bit unsigned integer. If the address is not readable,
+ * returns false. Otherwise returns true and store the result in *value
+ * if value is not NULL */
+bool safe_read8(uint8_t *addr, uint8_t *value);
+bool safe_read16(uint16_t *addr, uint16_t *value);
+bool safe_read32(uint32_t *addr, uint32_t *value);
+
+#endif /* __SAFE_READ__ */