Commits on Nov 17, 2023

1. rp2_common/pico_standard_link: implement customizable linker script u… …

   …sing C preprocessor
   
   Background:
   
   For cases where modifications to the linker script are required, pico-sdk only
   provides the option to provide an entire linker script using
   pico_set_linker_script. This patch adds a function pico_customize_linker_script
   to create a linker script in the build process, based on a template provided by
   pico-sdk and local settings provided by the user.
   
   Use cases for linker script modification include:
   - using the linker script as a rudimentary file system for binary blobs
   - sharing WiFi firmware blobs between bootloader and application (picowota)
   - reducing the number of code duplication in linker scripts (e.g.
     memmap_blocked_ram.ld which differs in only one line from the default)
   
   In such cases, deriving the linker script from a template may/should lead to
   code that is easier to maintain.
   
   Implementation:
   
   The template is memmap.ld.in, the user input is provided by specifying a
   customization file to be included, and the linker script is generated by the C
   preprocessor.
   
   The template exposes a few settings by #defining them before including the
   customization file, and provides a few hooks to add elements to the template.
   
   Examples and hints for use, based on a working example where cyw43 firmware
   lives in a separate region in flash:
   
   in CMakeLists.txt:
   
       pico_customize_linker_script(my_target tweaks.h)
   
   tweaks.h:
   
       #undef MAIN_FLASH_LENGTH
       #define MAIN_FLASH_LENGTH 256k
   
       #undef ADDITIONAL_FLASH_REGIONS
       #define ADDITIONAL_FLASH_REGIONS \
           FLASH_CYWFW(rx):            ORIGIN = 0x10040000, LENGTH = 256k
   
       #undef ADDITIONAL_SECTIONS
       #define ADDITIONAL_SECTIONS \
           .fw_cyw43 : {               \\
               . = ALIGN(4k);          \\
               KEEP(*(.fw_cyw43.meta)) \\
               . = ALIGN(512);         \\
               *(.fw_cyw43.blob)       \\
           } > FLASH_CYWFW
   
   Details:
   
   - The linker script will be a build product named ${TARGET}.ld
   - When using \\ at the end of lines, newlines are inserted in the resulting
     linker script in a postprocessing step. This is done to improve readability
     of the resulting linker script.
   - Lines starting with # need to be removed from the output; they are now turned
     into /*comments*/
   - Values that are to be overridden need to be #undeffed before redefining them,
     which is a bit awkward; another option is to use #ifdef for each and every
     variable in the template; yet another option (for integers) is to use linker
     variables (i.e. simply setting MAIN_FLASH_LENGTH=256k).

   

   mvds00 committed Nov 17, 2023
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