west build generates large EMPTY partitions

[maarten v]

I am trying to make a build with a slightly modified copy of the Zephyr button example on a nrf9160 board with a upgradable bootloader.

The following command can build our firmware:

west build --build-dir build/SG1500-board-tester --board ${BOARD}_ns SG1500-board-tester --pristine -- \
-DCONFIG_SECURE_BOOT=y \
-DCONFIG_BOOTLOADER_MCUBOOT=y \
-DCONFIG_BUILD_S1_VARIANT=y \
-Db0_CONF_FILE=prj_minimal.conf \
-Dmcuboot_CONF_FILE=${HOME}/ncs/v2.0.2/bootloader/mcuboot/boot/zephyr/prj_minimal.conf \
-DCONF_FILE:STRING="${BUILD_ROOT}/SG1500-board-tester/prj.conf"

I added 2 partitions by making a pm_static.yml:

data_partition_flash:
  address: 0xf0000
  end_address: 0x100000
  region: flash_primary
  size: 0x10000
data_partition_sram:
  address: 0x20020000
  end_address: 0x20040000
  region: sram_primary
  size: 0x20000

Compiling this result in BO size:

Memory region         Used Size  Region Size  %age Used
           FLASH:       12240 B        32 KB     37.35%
            SRAM:       37088 B      47896 B     77.43%
        IDT_LIST:          0 GB         2 KB      0.00%

For mcuboot this results in:

Memory region         Used Size  Region Size  %age Used
           FLASH:       18916 B        32 KB     57.73%
            SRAM:       18120 B      47896 B     37.83%
        IDT_LIST:          0 GB         2 KB      0.00%

This seems to work fine but the partition_manager_report reserved show that a lot of memeory is wasted: too much space is reserved for the bootloaders and it has very large EMPTY_X regions:

+--------------------------------------------------+
+---0x0: b0_container (0x8000 - 32kB)--------------+
| 0x0: b0 (0x8000 - 32kB) |
+---0x8000: s0 (0x8200 - 32kB)---------------------+
| 0x8000: s0_pad (0x200 - 512B) |
+---0x8200: s0_image (0x8000 - 32kB)---------------+
| 0x8200: mcuboot (0x8000 - 32kB) |
+--------------------------------------------------+
| 0x10200: EMPTY_0 (0x7e00 - 31kB) |
+---0x18000: s1 (0x8200 - 32kB)--------------------+
| 0x18000: s1_pad (0x200 - 512B) |
| 0x18200: s1_image (0x8000 - 32kB) |
+--------------------------------------------------+
| 0x20200: EMPTY_1 (0x7e00 - 31kB) |
+---0x28000: mcuboot_primary (0x60000 - 384kB)-----+
+---0x28000: tfm_secure (0x8200 - 32kB)------------+
| 0x28000: mcuboot_pad (0x200 - 512B) |
+---0x28200: app_image (0x5fe00 - 383kB)-----------+
+---0x28200: mcuboot_primary_app (0x5fe00 - 383kB)-+
| 0x28200: tfm (0x8000 - 32kB) |
+---0x30200: tfm_nonsecure (0x57e00 - 351kB)-------+
| 0x30200: app (0x57e00 - 351kB) |
+--------------------------------------------------+
| 0x88000: mcuboot_secondary (0x60000 - 384kB) |
| 0xe8000: EMPTY_2 (0x8000 - 32kB) |
| 0xf0000: data_partition_flash (0x10000 - 64kB) |
+--------------------------------------------------+

otp (0x2f4 - 756B):
+------------------------------------+
| 0xff8108: provision (0x280 - 640B) |
| 0xff8388: otp (0x74 - 116B) |
+------------------------------------+

sram_primary (0x40000 - 256kB):
+---------------------------------------------------+
+---0x20000000: sram_secure (0x10000 - 64kB)--------+
| 0x20000000: tfm_sram (0x10000 - 64kB) |
+---0x20010000: sram_nonsecure (0x10000 - 64kB)-----+
+---0x20010000: nrf_modem_lib_sram (0x44e8 - 17kB)--+
| 0x20010000: nrf_modem_lib_ctrl (0x4e8 - 1kB) |
| 0x200104e8: nrf_modem_lib_tx (0x2000 - 8kB) |
| 0x200124e8: nrf_modem_lib_rx (0x2000 - 8kB) |
+---------------------------------------------------+
| 0x200144e8: sram_primary (0xbb18 - 46kB) |
| 0x20020000: data_partition_sram (0x20000 - 128kB) |
+---------------------------------------------------+

I have no former experience using zephyr so i have no clue why this happens, so please can somebody explain:

1. what are those empty reqions used for? How can they be made minimal?
2. the wasted space looks like an alignment issue.
   1. why is the reserved space for the bootloaders much larger then the bootloader code size?
   2. How can i change this so no memory will be wasted?