Add Memory scrubbing for ECC

[bsousi5]

Scrub memory based of what available in the lds files, ie

      PROVIDE( metal_itim_memory_start = ADDR(.itim) );
      PROVIDE( metal_itim_memory_end = ADDR(.itim) + SIZEOF(.itim) );
      PROVIDE( metal_dtim_memory_start = ADDR(.dtim) );
      PROVIDE( metal_dtim_memory_end = ADDR(.dtim) + SIZEOF(.dtim) );
      PROVIDE( metal_main_memory_start = ADDR(.ram) );
      PROVIDE( metal_main_memory_end = ADDR(.ram) + SIZEOF(.ram) );

And have scrub.S implement the __metal_before_start() which call metal_memory_scrub()
User/Customer have the option to override with their own if desire.

The intention is for metal_init() to enable the BEU and disable ECC suppression

[keith-packard]

you'll need to save and restore 'ra' across this function.

[bsousi5]

This get called using jal though. It is a single instruction to jump and save return address: jump and link (jal)

[keith-packard]

Your general approach looks good; we'll want to make this optional as it will have a pretty significant impact on startup time.

[nategraff-sifive]

This looks fine to me.

As far as startup cost is concerned, maybe disabling this could be as simple as the linker script generator setting the start and end symbols equal to each other when it knows this isn't necessary?

[bsousi5]

Making the start = end would be a quick work-around. I did make the function a weak type, so user may include their empty function. But have not tested that yet

[keith-packard]

This looks fine to me.

As far as startup cost is concerned, maybe disabling this could be as simple as the linker script generator setting the start and end symbols equal to each other when it knows this isn't necessary?

If we can get to compiling freedom-metal for each project, instead of creating a library, then we can put conditional compilation based on project configuration which will make things like this really easy.