On Wed, 2009-10-07 at 01:07 +0200, Wolfgang Denk wrote:
Dear Peter Tyser,
In message 1254862383.24664.2742.camel@localhost.localdomain you wrote:
What's the advantage of having the bss not be located next to U-Boot?
One advantage is that we might chose the same address for all boards, and eventually for all Power processor families.
We could achieve this wherever we end up putting the bss. eg if people think putting the bss right after the u-boot image is best, we can update the 44x linker script, etc to do the same thing. I think this discussion is applicable to most any PPC board.
One disadvantage is that we need to relocate it separately, or we will have a gap in the RAm memory map which is IMO not acceptable.
What does "relocating the bss separately" entail?
The big disadvantage of picking an arbitrary address for the bss is that there's now 1 more magical section of SDRAM that the user needs to know shouldn't be used. I already field enough question from people that
Why should it not be used? You seem to be pretty fixed on that idea, which is wrong. No code will ever be written to RAM at list location.
When I say user, I'm refering to an end user, eg a customer. Not a developer.
For arguments sake, lets say we developers put the bss at a "fixed (random, non-zero) address" of 0x80000. A user tftps an image to 0x80000 and suddenly their board starts acting up.
In the current setup, we don't write any code to RAM at 0x0 either.
Right, and this limitation causes headaches. I personally get lots of questions from customers about why their board hangs when they tftp an image to 0x0. In a perfect world we'd only have 1 reserved section of memory which contained the interrupt vectors, text, bss, malloc, stack, etc.
corrupt their exception vectors or stack/malloc pool/u-boot code, I don't want to add more bss questions:)
Its crappy to have 2 sections of memory that a user has to know not to touch, I don't want to have 3:)
Maybe I'm not understanding your suggestion "to chose a fixed (random, non-zero) address" for the bss. That implies to me we choose an address low memory (eg 0x10000) and put the bss there. I think it'd be more plausible for someone to blow this away accidentally than high memory by U-Boot, and you also couldn't use any data stored in the bss after you blow it away, eg right before jumping to a linux kernel.
Best, Peter