arch: arm: rom_start relocation configuration

In order to support Linux rproc loading, some SOCs require
the boot-vector and irq-vectors to be placed into a defined
memory area for the mcu to boot.

This is necessary for NXP's IMX SOCs for instance but
can be leveraged by other SOCs that have multiple
zephyr,flash choices.

Signed-off-by: Jérémy LOCHE - MAKEEN Energy <jlh@makeenenergy.com>

arch/arm/Kconfig

@@ -34,6 +34,71 @@ config ARM_CUSTOM_INTERRUPT_CONTROLLER
 	  is assumed that the custom interrupt control interface implementation
 	  assumes responsibility for handling the NVIC.
 
+config ROMSTART_RELOCATION_ROM
+	bool
+	default n
+	help
+	  Relocates the rom_start region containing the boot-vector data and
+	  irq vectors to the region specified by configurations:
+	  ROMSTART_REGION_ADDRESS and ROMSTART_REGION_SIZE
+
+	  This is useful for the Linux Remoteproc framework that uses the elf-loader
+	  such that it is able to load the correct boot-vector (contained in rom_start)
+	  into the correct memory location independent of the chosen zephyr,flash
+	  ROM region.
+
+	  Most SOCs include an alias for the boot-vector at address 0x00000000
+	  so a default which might be supported by the corresponding Linux rproc driver.
+	  If it is not, additionnal options allows to specify the addresses.
+
+	  In general this option should be chosen if the zephyr,flash chosen node
+	  is not placed into the boot-vector memory area.
+
+	  While this aims at generating a correct zephyr.elf file, it has the side
+	  effect of enlarging the bin file. If the zephyr.bin file is used to boot the
+	  secondary core, this option should be disabled.
+
+	  Example:
+		on IMX7D, the chosen zephyr,flash can be OCRAM/OCRAM_S/TCM/DDR memories
+		for code location. But the boot-vector must be placed into OCRAM_S for the
+		CORTEX-M to boot (alias 0, real 0x00180000/32K available).
+
+if ROMSTART_RELOCATION_ROM
+
+	config ROMSTART_REGION_ADDRESS
+		hex
+		default 0x00000000
+		help
+		  Start address of the rom_start region.
+		  This setting can be derived from a DT node reg property or specified directly.
+
+		  A default value of 0x00000000 might work in most cases as SOCs have an alias
+		  to the right memory region of the boot-vector.
+
+		  Examples:
+		  -IMX7D the boot-vector is OCRAM_S (0x00180000, aliased at 0x0).
+		  -IMX6SX the boot-vector is TCML (0x007F8000, aliased at 0x0).
+		  -IMX8MQ the boot-vector is TCML (0x007E0000, aliased at 0x0).
+		  -IMX8MN the boot-vector is ITCM (0x007E0000, aliased at 0x0).
+
+		  Example of DT definition:
+		  $(dt_nodelabel_reg_addr_hex,ocram_s_sys)
+
+	config ROMSTART_REGION_SIZE
+		hex
+		default 1
+		help
+		  Size of the rom_start region in KB.
+
+		  Default is 1KB which is enough to store the boot and irq vectors.
+
+		  This setting can be derived from a DT node reg property or specified directly.
+
+		  Example for IMX7D that needs the boot-vector into OCRAM_S (0x00180000):
+		  $(dt_nodelabel_reg_size_hex,ocram_s_sys,0,K)
+
+endif
+
 config CODE_DATA_RELOCATION_SRAM
 	bool "Relocate code/data sections to SRAM"
 	depends on CPU_CORTEX_M

include/zephyr/arch/arm/cortex_m/scripts/linker.ld

@@ -26,6 +26,14 @@
 #endif
 #define RAMABLE_REGION RAM
 
+/* Region of the irq vectors and boot-vector SP/PC */
+#if defined(CONFIG_ROMSTART_RELOCATION_ROM)
+#define ROMSTART_ADDR CONFIG_ROMSTART_REGION_ADDRESS
+#define ROMSTART_SIZE (CONFIG_ROMSTART_REGION_SIZE * 1K)
+#else
+#define ROMSTART_REGION ROMABLE_REGION
+#endif
+
 #if !defined(CONFIG_XIP) && (CONFIG_FLASH_SIZE == 0)
 #define ROM_ADDR RAM_ADDR
 #else
@@ -85,6 +93,9 @@ _region_min_align = 4;
 
 MEMORY
     {
+#if defined(CONFIG_ROMSTART_RELOCATION_ROM)
+    ROMSTART_REGION (rx) : ORIGIN = ROMSTART_ADDR, LENGTH = ROMSTART_SIZE
+#endif
     FLASH (rx) : ORIGIN = ROM_ADDR, LENGTH = ROM_SIZE
     RAM   (wx) : ORIGIN = RAM_ADDR, LENGTH = RAM_SIZE
 #if defined(CONFIG_LINKER_DEVNULL_MEMORY)
@@ -129,7 +140,7 @@ SECTIONS
  */
 #include <snippets-rom-start.ld>
 
-	} GROUP_LINK_IN(ROMABLE_REGION)
+	} GROUP_LINK_IN(ROMSTART_REGION)
 
 #ifdef CONFIG_CODE_DATA_RELOCATION