struct devices should be allocated in ROM, not RAM

[mbolivar-nordic]

Somebody please prove me wrong by showing me I'm missing something obvious...

Pull request #24873 marked all devices const as a fix for issue #27399. The problem description says:

At this point all device instance could be made constant to save RAM.

However, despite being const, devices are still linked into RAM, at least on Arm Cortex-M:

$ west build -b nrf52840dk_nrf52840 samples/hello_world/ 

$ arm-none-eabi-gdb build/zephyr/zephyr.elf -q -ex 'x __device_start' -ex q
Reading symbols from build/zephyr/zephyr.elf...
0x20000040 <__device_dts_ord_26>:	0x0000406a

Note 0x20000000 is the base address of SRAM in the SoC memory map.

It seems that #24873 did not actually save any RAM for devices.

Since the device related linker section is in include/linker/common-ram.h:

zephyr/include/linker/common-ram.ld

Line 25 in efccfcb

SECTION_DATA_PROLOGUE(devices,,)

I'm guessing this bug applies to most if not all targets.

Describe the solution you'd like

I'm not sure if this is just an oversight or if there's something else blocking the fix, but I moved the linker section to ROM and it seems to work in basic testing, and produces the RAM savings I expected: #36032.

[dleach02]

Generic linker scripts usually put .text and .ro together in the same section or ROM in this case. Looking at your PR, I'm guessing our linking is overriding that because of where these are defined. I wonder how many other const data items are ending up in RAM?

[mbolivar-nordic]

Looking at your PR, I'm guessing our linking is overriding that because of where these are defined.

Yes, that's correct.

I wonder how many other const data items are ending up in RAM?

They would have to be const and specifically placed into a linker section that ends up in RAM. "Normal" global const data without an explicit section attribute ends up in ROM.

You'd have to audit the linker scripts to find the answer, which is probably nontrivial due to Zephyr's linker snippets CMake API.