Overwrite _sbrk() to support external SRAM

[ccli8]

Hi,
I am porting mbed classic to Nuvoton's Nu-mbed-NUC472 board. There is external SRAM on this board. To locate heap on the external SRAM, I need to overwrite _sbrk(). But I meet 'multiply defined' error on GCC_ARM target. The _sbrk() implementation in retarget.cpp cannot meet my requirement (heap and stack are not contiguous in my case). Could _sbrk() in retarget.cpp declare to be weak to overwrite for like my case?
https://github.com/mbedmicro/mbed/blob/master/hal/common/retarget.cpp

[0xc0170]

cc @niklas-arm

[niklarm]

Short of modifying the code to attribute that function to be weak, you may be able to wrap this function using the GNU linker by -Wl,--wrap,_sbrk.

Please note, that dlmalloc assumes a linear heap address space, meaning with _sbrk you can only have one heap section and it must be in continous memory.
On Cortex-A this isn't an issue, because the MMU maps non-continous memory (ie. pages) to appear linear, but on Cortex-M you do not have this functionality, and the simple _sbrk approach does not work anymore.

For you this means, you'd have to relocate your entire heap to the external memory, you cannot use the internal SRAM anymore, because that would require you to have two memory pools, and _sbrk and dlmalloc cannot deal with that.

Also note that external RAM may consist of several banks and it may not be possible to do unaligned access across bank address boundaries. Though I'd expect the external memory peripheral to take care of splitting unaligned accesses into two aligned accesses, but that depends on the vendor implementation.
In the worst case, your external heap _sbrk is limited to use only one bank of the external memory, due to this issue.

[niklarm]

For clarity:

you cannot use the internal SRAM anymore

for heap usage. Of course, it's still usable for stack and static data and whatever else you need.

[ccli8]

@niklas-arm In my scenario, there is just one bank of external SRAM in the Nu-mubed-NUC472 board. I can relocate the whole heap to the external SRAM now per your suggestion. Thanks for your information.

[ccli8]

@niklas-arm In original workspace_tools/targets.py, I can wrap _sbrk (-Wl,--wrap,_sbrk) by hook.hook_cmdline_linker. But after target definition is moved from workspace_tools/targets.py to hal/targets.json, I find no way to adding such hook. Do you have a plan to support such hook function?

class NUMBED_NUC472(Target):
    def __init__(self):
        Target.__init__(self)
        self.core = "Cortex-M4F"
        self.is_disk_virtual = True
        self.extra_labels = ['Nuvoton', 'NUC472', 'NUMBED_NUC472']
        self.macros = ['TARGET_NUVOTON', 'TARGET_NUC472', 'TARGET_NUMBED', 'TARGET_NUC472HI8AE']
        self.supported_toolchains = ["GCC_ARM", "ARM", "uARM", "IAR"]
        self.default_toolchain = "ARM"
        # NOTE: Pin mappings of Arduino form factor are inconsistent with test samples spi_master/spi_slave, so cancel it.
        #self.supported_form_factors = ["ARDUINO"]
        self.detect_code = ["1301"]
        self.progen = {
            "target": "numbed-nuc472",
        }

    def init_hooks(self, hook, toolchain_name):
        if toolchain_name in ['GCC_ARM']:
            hook.hook_cmdline_linker(self.cmdline_linker_hook)

    def cmdline_linker_hook(self, toolchain_name, cmdline):
        if toolchain_name in ['GCC_ARM']:
            # Wrap calls to _sbrk() to relocate heap on external SRAM.
            cmdline.append(r'-Wl,--wrap,_sbrk')
        return cmdline

[niklarm]

cc @0xc0170 @bogdanm @sg- for workspace tools

[ciarmcom]

ARM Internal Ref: IOTMORF-178

[ccli8]

On mbed OS 5. the wrap mechanism cannot work. I overwrite _sbrk with conditional compile in retarget.cpp. This modification has merged and so I close the issue.