MCUXpresso: Update ARM linker files to eliminate reserving RAM for stack & heap

[mmahadevan108]

Description

Update ARM linker files to eliminate reserving RAM for stack & heap. Heap and stack size is determined via the RTOS. This should fix the below issue:
#3763

Status

**READY

• Tests
  Ran & passed mbed-os tests.

[mmahadevan108]

cc @0xc0170 @sg- @maclobdell

[0xc0170]

@mmahadevan108 Again, this is fine for mbed 5 ,but some of these targets that you are changing are enabled for mbed 2 (non rtos applications).

[mmahadevan108]

Is there a way to do this for mbed 5 only. We are wasting memory as this is handled in the RTOS as well.

[sg-]

What about using MBED_CONF_RTOS_PRESENT given the linkers now support preprocessing?

@theotherjimmy @0xc0170 @c1728p9

[c1728p9]

This change is relevant to both mbed 2 and 5. Even when the rtos is not present the heap and stack are determined at runtime, similar to with the rtos. These sizes can be decreased, but it would probably be good to set them to non-zero values so you get a compile time error, rather than runtime, on both 2 and 5 if your program is too big. Something like ~1k for both should be sufficient.

[sg-]

Even when the rtos is not present the heap and stack are determined at runtime, similar to with the rto

What happens to the symbols defined for the heap in the linker scripts, are they not used ie wasted space?

[c1728p9]

These defines aren't used to set the size of a dedicated heap region. Instead they are subtracted off the max size of the RW data section so you have at least __stack_size__ + __heap_size__ of free space. At run-time both mbed 2 and 5 setup the stack and heap based on this free space.

[mmahadevan108]

Thanks Russ. Why do we need this free space?

[c1728p9]

This space is for the stack and the heap. If it isn't big enough then the stack will grow into valid data and clobber it leading to a crash.

[mmahadevan108]

I am confused. I thought the stack & heap space is reserved in the RTOS file for both mbed 2 & 5.

[c1728p9]

With the ARM compiler the free space starts at the end of RW_IRAM1 and goes to the end of the RAM bank. This is the case both for mbed 2 and mbed 5.

• mbed 5 uses this space for the interrupt stack and for the heap. The code to do this is located here
• mbed 2 uses this space for the main stack and for the heap. The code to do this for ARM is target specific at the moment and can be found here for the K66F.

[mmahadevan108]

Thanks Russ. Some of the platform linker files for ARM compiler do not have __stack & __heap defined and no space is not reserved. Setting these to 0 would result in a similar environment.

[c1728p9]

It works with the size set to zero, you just get a runtime error rather than a compile time error if your program is too big. If the scatter file has support for this extra level of checking why not use it?

[mmahadevan108]

I understand. I will reduce the stack & heap size but not make them 0.

[mmahadevan108]

@0xc0170 @sg- @c1728p9 I have pushed in an updated patch. Space reserved for stack & heap has been reduced but is not 0.

[c1728p9]

Thanks for the update @mmahadevan108. This PR looks good to me.

[sg-]

/morph test-nightly

[mbed-bot]

Result: SUCCESS

Your command has finished executing! Here's what you wrote!

/morph test-nightly

Output

mbed Build Number: 34

All builds and test passed!