[support/ld] Add linker support for reconfigurable heap.

- common_header.inc: Declare EXTERN symbols _lm_heap_start and
  _lm_heap_end.

- common_rom.inc: Check for _lm_heap_start and _lm_heap_end.  If they
  are defined, preserve their values.  Otherwise, _lm_heap_start is
  starts after .bss, and _lm_heap_end is the end of SRAM.

  This allows existing linker scripts to continue using the old heap
  scheme, but allows for customizability elsewhere.

- syscalls.c: Respect the addresses of _lm_heap_start and _lm_heap_end
  as the boundaries of the heap in _sbrk().

3 files changed, 21 insertions, 15 deletions

diff --git a/libmaple/syscalls.c b/libmaple/syscalls.c
index f28202c..a3f12f1 100644
--- a/libmaple/syscalls.c
+++ b/libmaple/syscalls.c
@@ -29,20 +29,12 @@
 #include <sys/stat.h>
 #include <errno.h>
 
-/* Set by the linker script */
-extern int _end;
+/* Set by the linker */
+extern char _lm_heap_start;
+extern char _lm_heap_end;
 
-/* FIXME these should be determined by the linker script.
- *
- * Doing so will allow the heap to be configured on a per-board basis.
- * Current values are just stopgaps for a heap in built-in SRAM.
- *
- * STACK_RESERVED_BYTES is just a hack to ensure a minimum stack size.
- * It should probably go away as well. */
-#define STACK_RESERVED_BYTES 1024
-#define HEAP_START           ((caddr_t)&_end)
-#define HEAP_END             ((caddr_t)((uint32)STM32_SRAM_END -        \
-                                        STACK_RESERVED_BYTES))
+#define HEAP_START           ((caddr_t)&_lm_heap_start)
+#define HEAP_END             ((caddr_t)&_lm_heap_end)
 
 /*
  * _sbrk -- Increment the program break.
diff --git a/support/ld/common_header.inc b/support/ld/common_header.inc
index 36ed2b1..1d11ba0 100644
--- a/support/ld/common_header.inc
+++ b/support/ld/common_header.inc
@@ -32,5 +32,6 @@ PROVIDE(__cs3_stack = __cs3_region_start_ram + LENGTH(ram));
 EXTERN(_start)
 PROVIDE(__cs3_reset = _start);
 
-/* Beginning of the heap */
-PROVIDE(__cs3_heap_start = _end);
+/* Heap boundaries, for libmaple */
+EXTERN(_lm_heap_start);
+EXTERN(_lm_heap_end);
diff --git a/support/ld/common_rom.inc b/support/ld/common_rom.inc
index 7284492..7182a4d 100644
--- a/support/ld/common_rom.inc
+++ b/support/ld/common_rom.inc
@@ -112,6 +112,19 @@ SECTIONS
 
         *(.got.plt) *(.got)
         *(.data .data.* .gnu.linkonce.d.*)
+
+
+        /*
+         * Heap: Linker scripts may choose a custom heap by overriding
+         * _lm_heap_start and _lm_heap_end. Otherwise, the heap is in
+         * internal SRAM, beginning after .bss, and growing towards
+         * the stack.
+         *
+         * I'm shoving these here naively; there's probably a cleaner way
+         * to go about this. [mbolivar]
+         */
+        _lm_heap_start = DEFINED(_lm_heap_start) ? _lm_heap_start : _end;
+        _lm_heap_end   = DEFINED(_lm_heap_end) ? _lm_heap_end : __cs3_stack;
         . = ALIGN (8);
         _edata = .;
       } > REGION_DATA AT> REGION_TEXT