bsp(lpc11u37): move the main stack to the USB SRAM bank

The 8 KB main bank is packed tightly enough that only ~280 bytes remained above
.bss, and interrupt frames overflowed into the topmost task stack - a hard fault
in cdc_msc_freertos. Put the MSP at the top of the 2 KB USB SRAM bank, which
nothing else uses in either build system, so the stack no longer shrinks as .bss
grows. The Make build's CFG_TUSB_MEM_SECTION placement of endpoint buffers into
that bank is dropped so both build systems agree on the layout.

The headroom assert is written as an addition rather than a subtraction, since
linker script arithmetic is unsigned and an overflowing bank would underflow the
difference into a huge positive value and pass silently.

2 files changed, 17 insertions, 2 deletions

diff --git a/hw/bsp/lpc11/boards/lpcxpresso11u37/board.mk b/hw/bsp/lpc11/boards/lpcxpresso11u37/board.mk
index fdc17374b..718c46bbf 100644
--- a/hw/bsp/lpc11/boards/lpcxpresso11u37/board.mk
+++ b/hw/bsp/lpc11/boards/lpcxpresso11u37/board.mk
@@ -4,8 +4,7 @@ MCU_DRV = 11xx
 CFLAGS += \
   -DCORE_M0 \
   -DCFG_EXAMPLE_MSC_READONLY \
-  -DCFG_EXAMPLE_VIDEO_READONLY \
-  -DCFG_TUSB_MEM_SECTION='__attribute__((section(".data.$$RAM2")))'
+  -DCFG_EXAMPLE_VIDEO_READONLY
 
 # mcu driver cause following warnings
 CFLAGS += \
diff --git a/hw/bsp/lpc11/boards/lpcxpresso11u37/lpc11u37.ld b/hw/bsp/lpc11/boards/lpcxpresso11u37/lpc11u37.ld
index 8e0a4e4c6..b7237a3ec 100644
--- a/hw/bsp/lpc11/boards/lpcxpresso11u37/lpc11u37.ld
+++ b/hw/bsp/lpc11/boards/lpcxpresso11u37/lpc11u37.ld
@@ -172,6 +172,22 @@ SECTIONS
          . = ALIGN(4) ;
         _end_noinit = .;
     } > RamLoc8
+    /* Main (MSP/ISR) stack lives at the top of the USB SRAM bank: the 8K main bank is packed so
+       tight that only ~280 B remained above .bss, and ISR frames overflowed into the topmost task
+       stack (cdc_msc_freertos hard fault). Nothing else is placed in this bank in either build
+       system, so the stack owns all 2 KB; the ASSERT is future-proofing in case USB buffers are
+       ever mapped here again.
+
+       This bank is clocked by SYSAHBCLKCTRL[27] (USBRAM enable), and the stack is used from the
+       first instruction of the reset handler - long before any TinyUSB or BSP code could turn a
+       clock on. It works because the boot ROM hands over with that bit already set. Anything that
+       gates the USB RAM clock to save power will hard fault at reset, not at USB init. */
+    __user_stack_top = ORIGIN(RamUsb2) + LENGTH(RamUsb2);
+    /* Stated as an addition, not a subtraction: ld arithmetic is unsigned, so an overflowing
+       bank would underflow the difference into a huge positive value and pass silently. */
+    ASSERT(ADDR(.noinit_RAM2) + SIZEOF(.noinit_RAM2) + 0x200 <= __user_stack_top,
+           "main stack headroom in RamUsb2 below 512 bytes")
+
     PROVIDE(_pvHeapStart = DEFINED(__user_heap_base) ? __user_heap_base : .);
     PROVIDE(_vStackTop = DEFINED(__user_stack_top) ? __user_stack_top : __top_RamLoc8 - 0);