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).

src/rp2_common/pico_standard_link/CMakeLists.txt

@@ -39,6 +39,18 @@ if (NOT TARGET pico_standard_link)
         pico_add_link_depend(${TARGET} ${LDSCRIPT})
     endfunction()
 
+    function(pico_customize_linker_script TARGET LDSCRIPT_INCLUDE)
+        set(LDSCRIPT_INPUT ${CMAKE_CURRENT_FUNCTION_LIST_DIR}/memmap.ld.in)
+        set(LDSCRIPT_GENERATED ${TARGET}.ld)
+        message("generating linker script at ${LDSCRIPT_GENERATED} from ${LDSCRIPT_INPUT} and ${LDSCRIPT_INCLUDE}")
+
+        add_custom_command(TARGET ${TARGET} DEPENDS ${LDSCRIPT_INCLUDE} PRE_BUILD
+            COMMAND ${CMAKE_C_COMPILER} -E -x c -CC ${CFLAGS} -DLD_INCLUDE=\"${LDSCRIPT_INCLUDE}\" ${LDSCRIPT_INPUT} | sed -e "s@^#.*@/*&*/@" | tr "\\\\" "\\n" > ${LDSCRIPT_GENERATED}
+            VERBATIM
+        )
+        set_target_properties(${TARGET} PROPERTIES PICO_TARGET_LINKER_SCRIPT ${LDSCRIPT_GENERATED})
+    endfunction()
+
     function(pico_set_binary_type TARGET TYPE)
         set_target_properties(${TARGET} PROPERTIES PICO_TARGET_BINARY_TYPE ${TYPE})
     endfunction()

src/rp2_common/pico_standard_link/memmap.ld.in

@@ -0,0 +1,272 @@
+/* Based on GCC ARM embedded samples.
+   Defines the following symbols for use by code:
+    __exidx_start
+    __exidx_end
+    __etext
+    __data_start__
+    __preinit_array_start
+    __preinit_array_end
+    __init_array_start
+    __init_array_end
+    __fini_array_start
+    __fini_array_end
+    __data_end__
+    __bss_start__
+    __bss_end__
+    __end__
+    end
+    __HeapLimit
+    __StackLimit
+    __StackTop
+    __stack (== StackTop)
+*/
+
+/* defaults that can be redefined */
+#define MAIN_FLASH_LENGTH 2048k
+#define MAIN_RAM_LENGTH 256k
+#define MAIN_RAM_ORIGIN 0x20000000
+#define MAIN_ROM_REGION FLASH
+
+#define ADDITIONAL_FLASH_REGIONS
+#define ADDITIONAL_RAM_REGIONS
+#define ADDITIONAL_SECTIONS
+
+#ifdef LD_INCLUDE
+#include LD_INCLUDE
+#endif
+
+MEMORY
+{
+    FLASH(rx) : ORIGIN = 0x10000000, LENGTH = MAIN_FLASH_LENGTH
+    ADDITIONAL_FLASH_REGIONS
+    RAM(rwx) : ORIGIN = MAIN_RAM_ORIGIN, LENGTH = MAIN_RAM_LENGTH
+    ADDITIONAL_RAM_REGIONS
+    SCRATCH_X(rwx) : ORIGIN = 0x20040000, LENGTH = 4k
+    SCRATCH_Y(rwx) : ORIGIN = 0x20041000, LENGTH = 4k
+}
+
+ENTRY(_entry_point)
+
+SECTIONS
+{
+    /* Second stage bootloader is prepended to the image. It must be 256 bytes big
+       and checksummed. It is usually built by the boot_stage2 target
+       in the Raspberry Pi Pico SDK
+    */
+
+    .flash_begin : {
+        __flash_binary_start = .;
+    } > FLASH
+
+    .boot2 : {
+        __boot2_start__ = .;
+        KEEP (*(.boot2))
+        __boot2_end__ = .;
+    } > FLASH
+
+    ASSERT(__boot2_end__ - __boot2_start__ == 256,
+        "ERROR: Pico second stage bootloader must be 256 bytes in size")
+
+    /* The second stage will always enter the image at the start of .text.
+       The debugger will use the ELF entry point, which is the _entry_point
+       symbol if present, otherwise defaults to start of .text.
+       This can be used to transfer control back to the bootrom on debugger
+       launches only, to perform proper flash setup.
+    */
+
+    .vectors : {
+        __logical_binary_start = .;
+        KEEP (*(.vectors))
+    } > MAIN_ROM_REGION
+
+    .text : {
+        KEEP (*(.binary_info_header))
+        __binary_info_header_end = .;
+        KEEP (*(.reset))
+        /* TODO revisit this now memset/memcpy/float in ROM */
+        /* bit of a hack right now to exclude all floating point and time critical (e.g. memset, memcpy) code from
+         * FLASH ... we will include any thing excluded here in .data below by default */
+        *(.init)
+        *(EXCLUDE_FILE(*libgcc.a: *libc.a:*lib_a-mem*.o *libm.a:) .text*)
+        *(.fini)
+        /* Pull all c'tors into .text */
+        *crtbegin.o(.ctors)
+        *crtbegin?.o(.ctors)
+        *(EXCLUDE_FILE(*crtend?.o *crtend.o) .ctors)
+        *(SORT(.ctors.*))
+        *(.ctors)
+        /* Followed by destructors */
+        *crtbegin.o(.dtors)
+        *crtbegin?.o(.dtors)
+        *(EXCLUDE_FILE(*crtend?.o *crtend.o) .dtors)
+        *(SORT(.dtors.*))
+        *(.dtors)
+
+        *(.eh_frame*)
+        . = ALIGN(4);
+    } > MAIN_ROM_REGION
+
+ADDITIONAL_SECTIONS
+
+    .rodata : {
+        *(EXCLUDE_FILE(*libgcc.a: *libc.a:*lib_a-mem*.o *libm.a:) .rodata*)
+        . = ALIGN(4);
+        *(SORT_BY_ALIGNMENT(SORT_BY_NAME(.flashdata*)))
+        . = ALIGN(4);
+    } > MAIN_ROM_REGION
+
+    .ARM.extab :
+    {
+        *(.ARM.extab* .gnu.linkonce.armextab.*)
+    } > MAIN_ROM_REGION
+
+    __exidx_start = .;
+    .ARM.exidx :
+    {
+        *(.ARM.exidx* .gnu.linkonce.armexidx.*)
+    } > MAIN_ROM_REGION
+    __exidx_end = .;
+
+    /* Machine inspectable binary information */
+    . = ALIGN(4);
+    __binary_info_start = .;
+    .binary_info :
+    {
+        KEEP(*(.binary_info.keep.*))
+        *(.binary_info.*)
+    } > MAIN_ROM_REGION
+    __binary_info_end = .;
+    . = ALIGN(4);
+
+    .ram_vector_table (NOLOAD): {
+        *(.ram_vector_table)
+    } > RAM
+
+    .uninitialized_data (NOLOAD): {
+        . = ALIGN(4);
+        *(.uninitialized_data*)
+    } > RAM
+
+    .data : {
+        __data_start__ = .;
+        *(vtable)
+
+        *(.time_critical*)
+
+        /* remaining .text and .rodata; i.e. stuff we exclude above because we want it in RAM */
+        *(.text*)
+        . = ALIGN(4);
+        *(.rodata*)
+        . = ALIGN(4);
+
+        *(.data*)
+
+        . = ALIGN(4);
+        *(.after_data.*)
+        . = ALIGN(4);
+        /* preinit data */
+        PROVIDE_HIDDEN (__mutex_array_start = .);
+        KEEP(*(SORT(.mutex_array.*)))
+        KEEP(*(.mutex_array))
+        PROVIDE_HIDDEN (__mutex_array_end = .);
+
+        . = ALIGN(4);
+        /* preinit data */
+        PROVIDE_HIDDEN (__preinit_array_start = .);
+        KEEP(*(SORT(.preinit_array.*)))
+        KEEP(*(.preinit_array))
+        PROVIDE_HIDDEN (__preinit_array_end = .);
+
+        . = ALIGN(4);
+        /* init data */
+        PROVIDE_HIDDEN (__init_array_start = .);
+        KEEP(*(SORT(.init_array.*)))
+        KEEP(*(.init_array))
+        PROVIDE_HIDDEN (__init_array_end = .);
+
+        . = ALIGN(4);
+        /* finit data */
+        PROVIDE_HIDDEN (__fini_array_start = .);
+        *(SORT(.fini_array.*))
+        *(.fini_array)
+        PROVIDE_HIDDEN (__fini_array_end = .);
+
+        *(.jcr)
+        . = ALIGN(4);
+        /* All data end */
+        __data_end__ = .;
+    } > RAM AT> MAIN_ROM_REGION
+    /* __etext is (for backwards compatibility) the name of the .data init source pointer (...) */
+    __etext = LOADADDR(.data);
+
+    /* Start and end symbols must be word-aligned */
+    .scratch_x : {
+        __scratch_x_start__ = .;
+        *(.scratch_x.*)
+        . = ALIGN(4);
+        __scratch_x_end__ = .;
+    } > SCRATCH_X AT > MAIN_ROM_REGION
+    __scratch_x_source__ = LOADADDR(.scratch_x);
+
+    .scratch_y : {
+        __scratch_y_start__ = .;
+        *(.scratch_y.*)
+        . = ALIGN(4);
+        __scratch_y_end__ = .;
+    } > SCRATCH_Y AT > MAIN_ROM_REGION
+    __scratch_y_source__ = LOADADDR(.scratch_y);
+
+    .bss  : {
+        . = ALIGN(4);
+        __bss_start__ = .;
+        *(SORT_BY_ALIGNMENT(SORT_BY_NAME(.bss*)))
+        *(COMMON)
+        . = ALIGN(4);
+        __bss_end__ = .;
+    } > RAM
+
+    .heap (NOLOAD):
+    {
+        __end__ = .;
+        end = __end__;
+        KEEP(*(.heap*))
+        __HeapLimit = .;
+    } > RAM
+
+    /* .stack*_dummy section doesn't contains any symbols. It is only
+     * used for linker to calculate size of stack sections, and assign
+     * values to stack symbols later
+     *
+     * stack1 section may be empty/missing if platform_launch_core1 is not used */
+
+    /* by default we put core 0 stack at the end of scratch Y, so that if core 1
+     * stack is not used then all of SCRATCH_X is free.
+     */
+    .stack1_dummy (NOLOAD):
+    {
+        *(.stack1*)
+    } > SCRATCH_X
+    .stack_dummy (NOLOAD):
+    {
+        KEEP(*(.stack*))
+    } > SCRATCH_Y
+
+    .flash_end : {
+        PROVIDE(__flash_binary_end = .);
+    } > MAIN_ROM_REGION
+
+    /* stack limit is poorly named, but historically is maximum heap ptr */
+    __StackLimit = ORIGIN(RAM) + LENGTH(RAM);
+    __StackOneTop = ORIGIN(SCRATCH_X) + LENGTH(SCRATCH_X);
+    __StackTop = ORIGIN(SCRATCH_Y) + LENGTH(SCRATCH_Y);
+    __StackOneBottom = __StackOneTop - SIZEOF(.stack1_dummy);
+    __StackBottom = __StackTop - SIZEOF(.stack_dummy);
+    PROVIDE(__stack = __StackTop);
+
+    /* Check if data + heap + stack exceeds RAM limit */
+    ASSERT(__StackLimit >= __HeapLimit, "region RAM overflowed")
+
+    ASSERT( __binary_info_header_end - __logical_binary_start <= 256, "Binary info must be in first 256 bytes of the binary")
+    /* todo assert on extra code */
+}
+