In u-boot compile, linker script's sdram start and length are set by CONFIG_SPL_BSS_START_ADDR and CONFIG_SPL_BSS_MAX_SIZE values, why? - embedded

Question

I'm trying to build u-boot for our simple test board. (arm64)
After setting in include/configs/ab21m.h (our board),
#define CONFIG_SPL_BSS_START_ADDR 0x4f00000
#define CONFIG_SPL_BSS_MAX_SIZE SZ_32K
when I compile it, it gives me error while linking u-boot-spl. The error message is like this.
===================== WARNING ======================
This board does not use CONFIG_DM_ETH (Driver Model
for Ethernet drivers). Please update the board to use
CONFIG_DM_ETH before the v2020.07 release. Failure to
update by the deadline may result in board removal.
See doc/driver-model/migration.rst for more info.
====================================================
UPD include/generated/timestamp_autogenerated.h
CFGCHK u-boot.cfg
CC cmd/version.o
AR cmd/built-in.o
LD u-boot
CC spl/common/spl/spl.o
OBJCOPY u-boot.srec
OBJCOPY u-boot-nodtb.bin
SYM u-boot.sym
RELOC u-boot-nodtb.bin
COPY u-boot.bin
MKIMAGE u-boot.img
LD u-boot.elf
AR spl/common/spl/built-in.o
LD spl/u-boot-spl
aarch64-none-elf-ld.bfd: invalid length for memory region .sdram
make[1]: *** [scripts/Makefile.spl:509: spl/u-boot-spl] Error 1
make: *** [Makefile:1984: spl/u-boot-spl] Error 2
make: *** Waiting for unfinished jobs....
By the way, the linker script for spl starts like this after the build.
MEMORY { .sram : ORIGIN = 0x4000000,
LENGTH = (14*1024*1024) }
MEMORY { .sdram : ORIGIN = 0x4f00000,
LENGTH = SZ_32K }
OUTPUT_FORMAT("elf64-littleaarch64", "elf64-littleaarch64", "elf64-littleaarch64")
OUTPUT_ARCH(aarch64)
ENTRY(_start)
SECTIONS
{
This is strange because this 0x4f00000 and SZ_32K is what I gave for CONFIG_SPL_BSS_START_ADDR and CONFIG_SPL_BSS_MAX_SIZE. I placed this range in the on-chip RAM area some enough space above CONFIG_SPL_TEXT_BASE and below CONFIG_SPL_STACK with enough stack space. (I referenced imx8mm_evk board). What should I correct?
BTW, I found CONFIG_SYS_SDRAM_BASE, CONFIG_SYS_INIT_RAM_ADDR are all set to 0x40000000in imx8mm_evk, which is the DRAM start addrss. But CONFIG_SYS_INIT_RAM_SIZE is set to 0x200000 (2MB) where there is actually 3072MB DDR. Why is this value set to small size?
I asked two qeustions. Any help will be very much appreciated.

Answer 1

So, your first problem is a literal one. You have SZ_32K as the value for CONFIG_SPL_BSS_MAX_SIZE but since you're likely lacking #include <linux/sizes.h> in include/configs/ab21m.h you're not getting that constant evaluated.
As for what this is all doing, and why you should likely use something more like 2MB as seen on other platforms and place it in SDRAM rather than much smaller on-chip memory, if you look at arch/arm/cpu/armv8/u-boot-spl.lds you can see we're defining where the BSS should reside and that's likely larger than 32KB (and you'll get an overflow error when linking, if so).