rp2/memmap_mp.ld: Allow a board to reserve memory for the C heap.

Since c80e7c14 changed the GC heap to use
all unused RAM, there is no longer any RAM available for the traditional C
heap (which is not used by default in MicroPython but may be used by C
extensions).  This commit adds a provision for a board to reserve RAM for
the C heap, by defining MICROPY_C_HEAP_SIZE.
Signed-off-by: Damien George <damien@micropython.org>

ports/rp2/CMakeLists.txt

-cmake_minimum_required(VERSION 3.12)
+cmake_minimum_required(VERSION 3.13)
 
 # Set build type to reduce firmware size
 if(NOT CMAKE_BUILD_TYPE)
@@ -45,6 +45,12 @@ if(NOT PICO_BOARD)
     string(TOLOWER ${MICROPY_BOARD} PICO_BOARD)
 endif()
 
+# Set the amount of C heap, if it's not already set.
+# If a board uses malloc then it must set this to at least 4096.
+if(NOT MICROPY_C_HEAP_SIZE)
+    set(MICROPY_C_HEAP_SIZE 0)
+endif()
+
 # Enable extmod components that will be configured by extmod.cmake.
 # A board may also have enabled additional components.
 set(MICROPY_SSL_MBEDTLS ON)
@@ -365,6 +371,10 @@ target_compile_options(${MICROPY_TARGET} PRIVATE
     -Werror
 )
 
+target_link_options(${MICROPY_TARGET} PRIVATE
+    -Wl,--defsym=__micropy_c_heap_size__=${MICROPY_C_HEAP_SIZE}
+)
+
 set_source_files_properties(
     ${PICO_SDK_PATH}/src/rp2_common/pico_double/double_math.c
     ${PICO_SDK_PATH}/src/rp2_common/pico_float/float_math.c

ports/rp2/memmap_mp.ld

@@ -243,19 +243,19 @@ SECTIONS
     } > FLASH
 
     /* stack limit is poorly named, but historically is maximum heap ptr */
-    __StackLimit = ORIGIN(RAM) + LENGTH(RAM);
+    __StackLimit = __bss_end__ + __micropy_c_heap_size__;
+
+    /* Define start and end of GC heap */
+    __GcHeapStart = __StackLimit; /* after the C heap (sbrk limit) */
+    __GcHeapEnd = ORIGIN(RAM) + LENGTH(RAM);
+
+    /* Define memory for the C stack */
     __StackOneTop = ORIGIN(SCRATCH_X) + LENGTH(SCRATCH_X);
     __StackTop = ORIGIN(SCRATCH_Y) + LENGTH(SCRATCH_Y);
     __StackOneBottom = __StackOneTop - SIZEOF(.stack1_dummy);
     __StackBottom = __StackTop - SIZEOF(.stack_dummy);
-    /* Define start and end of GC heap */
-    __GcHeapStart = __bss_end__;
-    __GcHeapEnd   = __StackLimit;
     PROVIDE(__stack = __StackTop);
 
-    /* Check if data + heap + stack exceeds RAM limit */
-    ASSERT(__StackLimit >= __HeapLimit, "region RAM overflowed")
-
     /* Check GC heap is at least 128 KB */
     /* On a RP2040 using all SRAM this should always be the case. */
     ASSERT((__GcHeapEnd - __GcHeapStart) > 128*1024, "GcHeap is too small")
@@ -263,4 +263,3 @@ SECTIONS
     ASSERT( __binary_info_header_end - __logical_binary_start <= 256, "Binary info must be in first 256 bytes of the binary")
     /* todo assert on extra code */
 }
-