Fix malloc, using OCRAM rather than DTCM

teensy4/imxrt.ld

 	_sbss = ADDR(.bss);
 	_ebss = ADDR(.bss) + SIZEOF(.bss);
 
+	_sdmamem = ADDR(.bss.dma);
+	_edmamem = ADDR(.bss.dma) + SIZEOF(.bss.dma);
+	_emallocmem = ORIGIN(RAM) + LENGTH(RAM);
+
 	_flashimagelen = SIZEOF(.text.progmem) + SIZEOF(.text) + SIZEOF(.data);
 	_teensy_model_identifier = 0x23;
 

teensy4/libc.c

 #include <errno.h>
+#include "debug/printf.h"
 
-extern unsigned long _ebss;
-
-char *__brkval = (char *)&_ebss; // TODO: put heap into OCRAM, not DTCM
-#define STACK_MARGIN  8192
+// from the linker script
+extern unsigned long _edmamem;
+extern unsigned long _emallocmem;
 
+char *__brkval = (char *)&_edmamem;
 
 void * _sbrk(int incr)
 {
-        char *prev, *stack;
+	char *prev;
+
+	printf("sbrk incr = %d\n", incr);
 
-        prev = __brkval;
-        if (incr != 0) {
-                __asm__ volatile("mov %0, sp" : "=r" (stack) ::);
-                if (prev + incr >= stack - STACK_MARGIN) {
-                        errno = ENOMEM;
-                        return (void *)-1;
-                }
-                __brkval = prev + incr;
-        }
-        return prev;
+	prev = __brkval;
+	if (incr != 0) {
+		if (prev + incr > (char *)&_emallocmem) {
+			errno = ENOMEM;
+			return (void *)-1;
+		}
+		__brkval = prev + incr;
+	}
+	return prev;
 }