STM32 Define start of stack separately from heap end.

[chrismas9]

boards/*.ld Add _sstack symbol. Define stack and heap in a clearer way.
gccollect.h, main.c, pybthread.c, stm32_it.c Use _sstackk instead of _heap_end
modmachine.c machine.info() print _sstack.

[chrismas9]

@dpgeorge This format clearly shows the stack and heap size and highlights some anomalies. For example some parts with 256k RAM have 16k stack and other have 32k. H7 with 512k RAM has 16k stack.
Could you please review and adjust stack sizes as you see fit?

[chrismas9]

i would like to clean up the MEMORY section of the linker scripts to be more consistent and make it more obvious what the RAM block sizes are and what they are used for. The reason is to make it easier to configure custom ports that need variations of the linker script, eg to use small cache for external FS, move stack to CCRAM, etc. It won't change any memory allocations. Should that be another commit, or a separate PR?

[dpgeorge]

The reason is to make it easier to configure custom ports that need variations of the linker script, eg to use small cache for external FS, move stack to CCRAM, etc.

A given board can easily use its own linker script to declare the MEMORY section, it doesn't need to use any of the shared provided ones. See #4043 for a related discussion.

Should that be another commit, or a separate PR?

Let's do it a step at a time, so please make it a separate PR.

[dpgeorge]

Note that the STM32F769DISC board doesn't build with this patch.

[chrismas9]

All ports compile error free. micropython.mem_info() and mahine.info() report the correct (same) memory ranges after this change on NUCLEO_L432KC and PYBV3

[chrismas9]

Please hold off merging this yet. I have an improved layout in the STM32L452 port:

`MEMORY
{
FLASH (rx) : ORIGIN = 0x08000000, LENGTH = 384K /* sectors 0-191 /
FLASH_FS (r) : ORIGIN = 0x08060000, LENGTH = 128K / sectors 192-255 /
RAM (xrw) : ORIGIN = 0x20000000, LENGTH = 160K / SRAM1, 128K + SRAM2, 32K */
}

/* produce a link error if there is not this amount of RAM for these sections */
_minimum_stack_size = 2K;
_minimum_heap_size = 16K;

/* Define the top end of the stack. The stack is full descending so begins just
above last byte of RAM, or bottom of FS cache. Note that EABI requires the stack to be
8-byte aligned for a call. */

/* RAM extents for the garbage collector */
_ram_end = ORIGIN(RAM) + LENGTH(RAM);
_ram_start = ORIGIN(RAM);

_ram_fs_cache_end = _ram_end;
_ram_fs_cache_start = _ram_fs_cache_end - 2K; /* fs cache = 2K */

_estack = _ram_fs_cache_start;
_sstack = _estack - 16K; /* stack = 16K */

_heap_end = _sstack; /* heap = 142K, tunable by adjusting stack size /
_heap_start = _ebss; / heap starts just after statically allocated memory */

_flash_fs_start = ORIGIN(FLASH_FS);
_flash_fs_end = ORIGIN(FLASH_FS) + LENGTH(FLASH_FS);
`

There are are two changes that improve clarity:

1. In the MEMORY section contiguous blocks are merged into one region. This is currently inconsistent between boards.
2. The memory regions are defined in descending order and the top of each region is defined as the bottom of the region above if they are contiguous. It is much easier to visualise the regions and see or change their sizes.

Would you like me to remove the minimum heap and stack size checks. These don't work when the heap or stack are moved into other memory regions (CCRAM, DRAM, etc)?

[dpgeorge]

Would you like me to remove the minimum heap and stack size checks. These don't work when the heap or stack are moved into other memory regions (CCRAM, DRAM, etc)?

I'll have to have a good look at it first. I don't want to make too many changes at once.

[dpgeorge]

This PR was merged in 14cf91f