soc/ite/it8xxx2/kconfig: add warning for enable ram code with LTO

[RuibinChang]

Because CONFIG_LTO default is y,
CONFIG_SOC_IT8XXX2_SERIAL_IN_RAM=y
CONFIG_SOC_IT8XXX2_KERNEL_IN_RAM=y
CONFIG_SOC_IT8XXX2_ZEPHYR_IN_RAM=y
These three configs are not effective.

So I add a warning if we enable CONFIG_LTO and put code to EC ram at the same time.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[keith-zephyr]

Can you explain why CONFIG_LTO and moving these drivers into RAM cannot be used at the same time?

[RuibinChang]

Can you explain why CONFIG_LTO and moving these drivers into RAM cannot be used at the same time?

In upstream ilm.c, we would copy text section from __ilm_flash_start ~ __ilm_flash_end to __ilm_ram_start ~ __ilm_ram_end (ilm_ram section ), so we can run ram code for better performance.

In downstream linker.ld, we arrange libdrivers__serial, libkernel and libzephyr within __ilm_flash_start/end, if CONFIG_LTO=n, we can see those three .object in our assign region for each functions and text length, but after CONFIG_LTO=y, those three .object in our assign region don't have any bytes, which means nothing will be copied to ilm_ram section.

Below are build with
CONFIG_LTO=y:

 0x80035000 __ilm_flash_start = . ... .exception.other.__soc_handle_irq 0x80035b64 0x6 zephyr/libzephyr.a(soc_irq.S.obj) 0x80035b64 __soc_handle_irq *libdrivers__serial.a:*(SORT_BY_ALIGNMENT(.text) SORT_BY_ALIGNMENT(.text.*)) *libdrivers__serial.a:*(SORT_BY_ALIGNMENT(.rodata) SORT_BY_ALIGNMENT(.rodata.*)) *libdrivers__serial.a:*(SORT_BY_ALIGNMENT(.srodata) SORT_BY_ALIGNMENT(.srodata.*)) *libkernel.a:*(SORT_BY_ALIGNMENT(.text) SORT_BY_ALIGNMENT(.text.*)) *libkernel.a:*(SORT_BY_ALIGNMENT(.rodata) SORT_BY_ALIGNMENT(.rodata.*)) *libkernel.a:*(SORT_BY_ALIGNMENT(.srodata) SORT_BY_ALIGNMENT(.srodata.*)) *libzephyr.a:*(SORT_BY_ALIGNMENT(.text) SORT_BY_ALIGNMENT(.text.*)) *libzephyr.a:*(SORT_BY_ALIGNMENT(.rodata) SORT_BY_ALIGNMENT(.rodata.*)) *libzephyr.a:*(SORT_BY_ALIGNMENT(.srodata) SORT_BY_ALIGNMENT(.srodata.*)) 0x80035b6a __ilm_flash_end = . 

CONFIG_LTO=n:

 0x80030000 __ilm_flash_start = . ... .exception.other.__soc_handle_irq 0x80030cc8 0x6 zephyr/libzephyr.a(soc_irq.S.obj) 0x80030cc8 __soc_handle_irq *libdrivers__serial.a:*(SORT_BY_ALIGNMENT(.text) SORT_BY_ALIGNMENT(.text.*)) .text.z_impl_gpio_pin_interrupt_configure 0x80030cce 0x12c zephyr/drivers/serial/libdrivers__serial.a(uart_ite_it8xxx2.c.obj) .text.uart1_wui_isr 0x80030dfa 0x4a zephyr/drivers/serial/libdrivers__serial.a(uart_ite_it8xxx2.c.obj) 0x80030dfa uart1_wui_isr .text.uart2_wui_isr 0x80030e44 0x4 zephyr/drivers/serial/libdrivers__serial.a(uart_ite_it8xxx2.c.obj) 0x80030e44 uart2_wui_isr ... 0x8003cc0b __ilm_flash_end = .