u-boot : Relocation

问题:

This one is a basic question related to u-boot.

Why does the u-boot code relocate itself ?

Ok, it makes sense if u-boot is executing from NOR-flash or boot ROM space but if it runs from SDRAM already why does it have to relocate itself once again ?

回答1:

This question comes up frequently. Good answers sometimes too.

I agree it is handy to load the build to SDRAM during development. That works for me, I do it all the time. I have some special boot code in flash which does not enable MMU/cache. For my u-boot builds I switch CONFIG_SYS_TEXT_BASE between flash and ram builds. I run my development builds that way routinely.

As a practical matter, handling re-initialization of MMU/cache would be a nontrivial matter. And U-Boot benefits IMO from simplicity, as result of leaving out things like that.

The tech lead at Denx has expressed his opinion. IIRC his other posts are more strongly worded than that one. I get the impression that he does not like to repeat himself.

update: why relocate. Memory access is faster from RAM than from ROM, this will matter particularly if target has no instruction cache. Executing from RAM allows flash reprogramming; also (more minor) it allows software breakpoints with "trap" instructions; also it is more like the target's normal mode of operation, so if e.g. burst reads from RAM are iffy the failure will be seen at early boot.

回答2:

U-boot has to reserve 3 regions in memory that stores: 1) u-boot itself, 2) uImage (compressed kernel), and 3) uncompressed kernel. These 3 regions must be carefully placed in u-boot to prevent conflict.

However, the previous stage boot-loader, (BL2 or BL1) that brings u-boot into DRAM memory don\t know u-boot's planing on these 3 regions. So it can only loads u-boot onto a lower address in DRAM memory and jump to it. Then, after u-boot execute some basic initialization and detect current PC is not in planed location, u-boot call relocate function that move u-boot to the planned location and jump to it.

回答3:

The code of NOR flash must initialize the SDRAM, Then the copy code from Nor Flash to SDRAM, The process will copy itself, because you could enable MMU, we will start Virtual address mapping.